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Sony A9 vs Sony A7R mk IV (1 Viewer)

StuartV

Member
400mm * 2x (tc) * 1.55 = 1240mm @ 10MP or 1736mm @ 5MP
400mm * 2x (tc) * 2.47 = 1976mm @ 10MP or 2766mm @ 5MP

I don't understand the point of this.

Why would you care about comparing images that have the same pixel count, instead of images that were shot with the same effective focal length?

Here, you are comparing an image with an effective focal length of 1240mm to one shot with an effective focal length of 1976mm. What is the point?

I'm here. My subject is (say) 100 feet away. I'm going to shoot with the same lens focal length, regardless of whether it's an a9 or a7rIV. Suppose that the result leaves me wanting to crop the image by 1.5X, to get the composition I want in the final product. I'm going to want to do that 1.5X crop, whether the original image is 24MP or 61MP. Are you saying that, if, for example, I shoot the image with an aperture of f/11, the result (from the 61MP source) is going to have worse image quality if the source is 61MP vs 24MP (given a 1.5X crop from either source)?

Are you saying that, if you were shooting an a9, you would get closer to the subject, in order to compensate for not having as large of a max effective focal length? I.e. you can't crop it as much, so you get closer to make up for that. If you can get closer, why wouldn't you also get closer while using an a7rIV?

I am pretty much of a beginner hobbyist photographer, so I apologize if any of this comes across as sounding really ignorant. It just does not make sense to me that you'd compare two different FF cameras by comparing the image quality after cropping to the same number of MP. It seems like you'd want to compare after cropping to the same effective focal length. I.e. so the final images are the SAME - not an image of a whole bird in one (i.e. 1.5X crop) and an image of just the bird's body in the other (i.e. 2.5X crop).

A friend referred me to this thread because I am also in the process of deciding between an a7r IV and an a9 II.
 

marcsantacurz

Well-known member
I don't understand the point of this.

Why would you care about comparing images that have the same pixel count, instead of images that were shot with the same effective focal length?

Here, you are comparing an image with an effective focal length of 1240mm to one shot with an effective focal length of 1976mm. What is the point?

I'm here. My subject is (say) 100 feet away. I'm going to shoot with the same lens focal length, regardless of whether it's an a9 or a7rIV. Suppose that the result leaves me wanting to crop the image by 1.5X, to get the composition I want in the final product. I'm going to want to do that 1.5X crop, whether the original image is 24MP or 61MP. Are you saying that, if, for example, I shoot the image with an aperture of f/11, the result (from the 61MP source) is going to have worse image quality if the source is 61MP vs 24MP (given a 1.5X crop from either source)?

Are you saying that, if you were shooting an a9, you would get closer to the subject, in order to compensate for not having as large of a max effective focal length? I.e. you can't crop it as much, so you get closer to make up for that. If you can get closer, why wouldn't you also get closer while using an a7rIV?

I am pretty much of a beginner hobbyist photographer, so I apologize if any of this comes across as sounding really ignorant. It just does not make sense to me that you'd compare two different FF cameras by comparing the image quality after cropping to the same number of MP. It seems like you'd want to compare after cropping to the same effective focal length. I.e. so the final images are the SAME - not an image of a whole bird in one (i.e. 1.5X crop) and an image of just the bird's body in the other (i.e. 2.5X crop).

A friend referred me to this thread because I am also in the process of deciding between an a7r IV and an a9 II.

Stuart,

My calculation assumes that you want the image quality of either 10 MP or 5 MP, minimum. I.e., that is the maximum you would want to crop to. So, what's the equivalent focal length at that maximum crop? Usually shooting birds, it's always a question of needing more focal length, so I wanted to know what the maximum is for 10MP and 5MP output.

So, assuming you've gotten as close as you can, and you shoot with the same lens, you'll get 1.6x more magnification from the A7RIV than the A9. The examples I gave with the 400mm + 2x TC were just to give some concrete numbers to what 1.6x more means.

Your point of view -- which is also valid -- would give the same number, but in terms of MP instead of focal length. If you get 1.6x more magnification, you get 2.56x more pixels (1.6^2) on the higher res camera for the composition. But you already knew that must by dividing 61/24. If you have the same lens and same distance and crop both images to the same composition, you'll still have 61/24 more pixels on the A7RIV, as both cameras are the same sensor aspect ratio (I believe that's true, I didn't double check it).

marc
 

StuartV

Member
Stuart,

My calculation assumes that you want the image quality of either 10 MP or 5 MP, minimum. I.e., that is the maximum you would want to crop to. So, what's the equivalent focal length at that maximum crop? Usually shooting birds, it's always a question of needing more focal length, so I wanted to know what the maximum is for 10MP and 5MP output.

So, assuming you've gotten as close as you can, and you shoot with the same lens, you'll get 1.6x more magnification from the A7RIV than the A9. The examples I gave with the 400mm + 2x TC were just to give some concrete numbers to what 1.6x more means.

Your point of view -- which is also valid -- would give the same number, but in terms of MP instead of focal length. If you get 1.6x more magnification, you get 2.56x more pixels (1.6^2) on the higher res camera for the composition. But you already knew that must by dividing 61/24. If you have the same lens and same distance and crop both images to the same composition, you'll still have 61/24 more pixels on the A7RIV, as both cameras are the same sensor aspect ratio (I believe that's true, I didn't double check it).

marc

I feel like the issue here is that, basically, your earlier posts translated in my mind as saying that the a9 will give better results for the OP than the a7rIV. But, that doesn't seem to be supported.

You quoted Lloyd Chambers, but I think you left out a key part of what he said. You said:

Lloyd Chambers puts it like this: compared to the A7RII you should stop down 1 stop more for similar DoF

But, what Lloyd actually said is (emphasis added):

demand another stop of depth of field versus the Sony A7R III pixel density for the same per-pixel depth of field

He's talking about the same PER-PIXEL DoF and you're just talking about absolute or overall (apparently) DoF.

With the a7rIV having more pixels (in the same image/composition), won't the overall DoF work out to be the same between the a7rIV and the a9? Isn't overall DoF really only dependent on focal length, aperture, and overall sensor size?

To circle it back to the OP's (and my) question: You're in your "spot". Your subject is where it is. You have the same lens. Which camera will give you a better result? It seems to me that (assuming correct focus and exposure), the a7rIV would be the clear winner.

If the final composition you want is cropped so much that an a7rIV source produces a 5MP file, that tells me that the a9 version of that same original is not even going to be usable. I.e. when cropped to the same composition, it will be way less than the 5MP you need.

In other words, if the subject at the limit of the reach of an a7rIV, it's out reach of the a9, so the a7rIV is the obvious winner.

If the subject is in reach of the a9, and we suppose that the final cropped image produces a 5MP image from the original a9 source, then that same photo shot with an a7rIV is going to have considerably more (2.5X?) pixels worth of detail. And, while the per-pixel DoF might be less, the overall DoF that the eye actually sees will be the same, won't it?

In this scenario, you MIGHT argue that the extra detail in the a7rIV image brings no benefit - it's a wash between the 2 cameras (for image quality). You got the 5MP image you needed, either way - one, achieved only by cropping, the other by cropping and then reducing the image size (down-scaling the resolution). But, I am still unclear on how you would support the notion that the a9 result would be actually better?
 

StuartV

Member
One other issue that I don't think was mentioned is this: It seems as though it is somewhat common for some lenses to have their best sharpness in the center of the image and to be a little less sharp as you get to the corners or edges.

So, if you have a lower MP camera, you likely have to attempt to compose the shot in the frame so that it is pretty close to the final composition that you want. But often, you don't want your subject to be in the exact center of the composition. You have to trade off some sharpness in your actual subject to having the subject off center.

With a higher MP sensor, you potentially have the option to be zoomed out a bit and shoot for the subject to be dead center, then crop the image to the composition you want. This gives you maximum sharpness in the subject, and the composition you want. No real tradeoff, as compared to using a lower MP sensor.

As I said, I'm a basic and somewhat new hobbyist photographer, so this is just what seems to make sense to me, in my head. If there is something in the real world that shreds this "pro" of higher MPs, I hope someone can take the time to explain it to me.

Maybe this whole point is moot. If the subject fills the frame enough to have to compose it with the subject off-center, then you're not likely to end up cropping it to the point where lens softness at the edges becomes an issue. And if the subject is so far away that you will have to crop enough for lens edge softness to be an issue, then you will have the freedom to shoot on center and crop to your desired composition, no matter which sensor you have. ???
 

njlarsen

Gallery Moderator
Opus Editor
Supporter
Barbados
your last paragraph looks correct to me. I cannot remember every post in this thread but at least the last few posts assume that the product in the two cameras will be pin sharp at the level of the individual pixel. That might not be true necessarily, especially for the high resolution sensor. The quality of the image depends on the quality of the glass (you need higher quality for the higher resolution sensor) and it depends on whether you are getting into diffraction deteriorating the image. It also depends on the technique used: handheld versus a tripod, and how stable is the tripod relative to the shutter speed used.

Niels
 

marcsantacurz

Well-known member
I feel like the issue here is that, basically, your earlier posts translated in my mind as saying that the a9 will give better results for the OP than the a7rIV. But, that doesn't seem to be supported.

You quoted Lloyd Chambers, but I think you left out a key part of what he said. You said:



But, what Lloyd actually said is (emphasis added):



He's talking about the same PER-PIXEL DoF and you're just talking about absolute or overall (apparently) DoF.

With the a7rIV having more pixels (in the same image/composition), won't the overall DoF work out to be the same between the a7rIV and the a9? Isn't overall DoF really only dependent on focal length, aperture, and overall sensor size?

To circle it back to the OP's (and my) question: You're in your "spot". Your subject is where it is. You have the same lens. Which camera will give you a better result? It seems to me that (assuming correct focus and exposure), the a7rIV would be the clear winner.

If the final composition you want is cropped so much that an a7rIV source produces a 5MP file, that tells me that the a9 version of that same original is not even going to be usable. I.e. when cropped to the same composition, it will be way less than the 5MP you need.

In other words, if the subject at the limit of the reach of an a7rIV, it's out reach of the a9, so the a7rIV is the obvious winner.

If the subject is in reach of the a9, and we suppose that the final cropped image produces a 5MP image from the original a9 source, then that same photo shot with an a7rIV is going to have considerably more (2.5X?) pixels worth of detail. And, while the per-pixel DoF might be less, the overall DoF that the eye actually sees will be the same, won't it?

In this scenario, you MIGHT argue that the extra detail in the a7rIV image brings no benefit - it's a wash between the 2 cameras (for image quality). You got the 5MP image you needed, either way - one, achieved only by cropping, the other by cropping and then reducing the image size (down-scaling the resolution). But, I am still unclear on how you would support the notion that the a9 result would be actually better?

Stuart -- Let me address these items 1 by 1. They are good points.

Note: Below when talking about DoF, I use "sensor size" to mean the MP assuming a fixed physical size. In your post you said sensor size in terms of physical size. I have tried to fix my wording below.

0) High MP overall.

I did mention in my post a couple times that I went to the d850 so I would have a lot more MP and said that I would likely choose the A7RIV over the A9 because I tend to need to crop a lot. The caveat is that one needs to know that more MP has some downsides, such as a smaller effective f-stop range.

In my opinion, the main issues with the A7RIV are (a) small 68 exposure buffer @ 10 FPS jpeg, (b) need to stay around f/5.6 maybe f/8 max to optimize DoF vs diffraction, (c) shake and vibration will need better IS or faster shutter speeds (assuming you want the same per-pixel sharpness as 24MP sensor). (b) and (c) are only significant issues if you are concerned about big enlargements or heavy crops or high micro contrast. If you have pixels to spare, you can likely sharpen your way around them.

If you can work with those limitations, the A7RIV is a great camera and will give stunning results. And you can crop it 1.6x more for the same MP output & IQ output assuming you control for the (b) and (c).

For me (a) is not too much of a problem as I rarely shoot long bursts. What I have run into with the A7RIV when I borrowed one is that with an older sd card shooting BIF, it would take a long time to write out the buffer when I was shooting raw. So, even though I was shooting maybe 10-shot bursts, I was shooting them frequently enough that I still backlogged the buffer. I assume that a modern super fast sd card would handle this a lot better. One just needs to budget for a few of those cards with the camera.

To reiterate, I would likely choose the A7RIV over the A9 for my shooting style. The reasons to choose the A9 are faster fps, deeper buffer, better high ISO performance, better performance at higher f-stops (assuming you are pushing the pixel resolution), maybe better AF.

1) Composition vs. maximum crop at a given MP.

The OP specifically asked about heavily cropping photos, so I was trying to answer his question in terms of what is the maximum crop you could get, assuming the same MP output at the end.

Your aspect of having a specific composition and comparing the two cameras is different. In that case, the A7RIV will always have 2.5x more MP and assuming (b) and (c) are not issues, will almost always give better results. My reading of the OP's question, however, is that he wanted to use those extra pixels for more cropping.

I agree with your example of the A7RIV with a 5MP composition being far superior to the A9 with the same composition. In terms of my post, the A7RIV is in the 1736mm - 2766mm effective focal length range (at 5MP), whereas the A9 has topped out at 1736mm. This is the point I was trying to make with the max equivalent focal lengths.

Really the view of which is better in a given composition vs which has a longer effective focal length are two sides of the same coin. If you are within the effective focal lengths of each camera, the A7RIV will have 2.5x more MP for the same composition. If you are outside the A9's effective focal length, the A7RIV will become increasingly and increasingly better than the A9.

I like to think in terms of effective focal length, as I can then immediately have an idea of the distance to subject size that I can achieve with a given rig. So that is why I focus on the 1.6x more focal length than the 2.5x more pixels. But they are really expressing the same thing: 1.6 = sqrt(2.5).

2) DoF. Depth of field is caused by how much a point of light is blurred over a circle by the lens. The size of the circle is independent of the sensor. So, the A9 and A7RIV, for the same lens and f-stop will have the same circle of confusion. Because the A7RIV has smaller sensor sites, the circle covers more pixels that it does on the A9. The circles themselves are the same size on the sensor. So, if you have a big enlargement or do a lot of cropping, the A7RIV will show worse DoF (more blurring) when you enlarge it, on a pixel-by-pixel basis. Of course, with the A7RIV you get to enlarge more or crop more to get to that point.

If you are not heavily cropping, then one likely would not notice the DoF different or the diffraction difference. But when you want to crop to the limits of the sensor, you will start to notice the per-pixel effects. The 1.6x more cropping you can do on the A7RIV will only be as good as the DoF and diffraction allow. If you are not cropping down that much you won't notice it.

If you take the approach that I want the same composition (not maximum cropping) for a scene, the A7RIV will most often give the better results. I think the only times it would not is when you don't have enough light and need to either increase the f-stop or ISO in to the ranges where the A9 would have a clear advantage.

The OP also specifically asks about a 400mm f/2.8 with a 2x TC for 800mm optical focal length. That is another reason I harped on the DoF issue. It gets really small at that optical focal length unless you stop down, but stopping down will start to hit your diffraction limit sooner on the A7RIV than the A9 -- assuming you want to crop heavily, which the OP does. The DoF I listed predict that at f/8 the A7RIV at 800mm at 40m is 0.28m and the A9 is 0.47m. That might be really significant for you if shooting a large bird if flight and you want the whole wing on your side to be sharp.

If you are shooting at shorter optical focal lengths (say 400 - 500), it will not be such an issue and the A9 will lose that advantage (or at least it becomes less significant for bird-sized things).

3) The lloyd chambers quote.

As I tried to explain in #2, DoF is a multi-pixel effect. A single pixel does not have DoF. If you are looking at a full-res image at a modest enlargement (screen or print), you will not notice the DoF difference. But if you enlarge greatly or heavily crop, you will start to see those pixel-level blurring. If you are not pushing the detail to see the benefits of all those pixels, then the DoF different will not matter much, but if you want to exploit all those pixels it does matter.

For example, some people use super-high resolution prints, so even at modest enlargements they get very high detail and micro contrast. It adds a lot of pop and liveness to the print. In that case, you need to be aware of DoF and diffraction as they will rob you of that fine detail at the pixel level. If you are doing a 300 dpi print to an 8x10, it will not matter.

Marc
 

StuartV

Member
1) Composition vs. maximum crop at a given MP.

The OP specifically asked about heavily cropping photos, so I was trying to answer his question in terms of what is the maximum crop you could get, assuming the same MP output at the end.

2) DoF. Depth of field is caused by how much a point of light is blurred over a circle by the lens. The size of the circle is independent of the sensor. So, the A9 and A7RIV, for the same lens and f-stop will have the same circle of confusion. Because the A7RIV has smaller sensor sites, the circle covers more pixels that it does on the A9. The circles themselves are the same size on the sensor. So, if you have a big enlargement or do a lot of cropping, the A7RIV will show worse DoF (more blurring) when you enlarge it, on a pixel-by-pixel basis. Of course, with the A7RIV you get to enlarge more or crop more to get to that point.

If you are not heavily cropping, then one likely would not notice the DoF different or the diffraction difference. But when you want to crop to the limits of the sensor, you will start to notice the per-pixel effects. The 1.6x more cropping you can do on the A7RIV will only be as good as the DoF and diffraction allow. If you are not cropping down that much you won't notice it.

If you take the approach that I want the same composition (not maximum cropping) for a scene, the A7RIV will most often give the better results. I think the only times it would not is when you don't have enough light and need to either increase the f-stop or ISO in to the ranges where the A9 would have a clear advantage.

The OP also specifically asks about a 400mm f/2.8 with a 2x TC for 800mm optical focal length. That is another reason I harped on the DoF issue. It gets really small at that optical focal length unless you stop down, but stopping down will start to hit your diffraction limit sooner on the A7RIV than the A9 -- assuming you want to crop heavily, which the OP does. The DoF I listed predict that at f/8 the A7RIV at 800mm at 40m is 0.28m and the A9 is 0.47m. That might be really significant for you if shooting a large bird if flight and you want the whole wing on your side to be sharp.

Thank you so much for your patience and bearing with me.

I believe I am starting to get a handle on the point of view that you are speaking from.

I was starting from "I'm here, subject it here, I have this lens. Which body will give me the best results?"

You are starting from "I have this lens. If I'm willing to crop down to a 5MP final composition, which body will give me the best results?" In that case, the answer is, at its limit, the a9 will give better results than the a7rIV at its limit. But, the extra "reach" of the a7rIV would still mean that for some distance past the a9's limit and before the a7rIV's limit, the a7rIV will give better results, thanks to it starting with 2.5X more pixels at the a9's limit and then tapering down from there.

So, yes, I feel like I understand the position from which you are speaking. I nevertheless come back to thinking of it in terms of "I'm here, my subject is there, and I have this lens. Which camera body would give me the best results?" And concluding the a7rIV would be the winner (for me, anyway).

I am still curious about the DOF discussion. I think I'm getting it now, but I want to paraphrase and hope you'll tell me if I've got it, or if I'm still not understanding it completely.

If I'm shooting at 800mm focal length and f/8 focused on a distance of 40 meters, the a9 will give 47cm depth of field, where the a7rIV would only give 28cm of depth of field. I.e. with the a9 there would be a range of distance of roughly 39.75 to 40.25 meters where everything in that range would be in focus and closer and further than that would start going out of focus.

But, with the a7rIV, the only things that would be in focus would be from a distance of 39.86 to 40.14 meters. And the explanation for why the a7rIV loses that extra 19 cm of depth of field is that diffraction at the pixel level starts making things outside that range fuzzy, where on the a9 they would still be sharp.

Correct so far?

If so, then, further, the loss of DOF, for an 800mm focal length (at f/8) is approximately 0.5%? And if the focal length were less, that percentage would be reduced as well. So, it does matter if you're using really long focal lengths and small-ish apertures, at longer distances. But, it's pretty much insignificant for more "normal" shooting?
 

marcsantacurz

Well-known member
So, yes, I feel like I understand the position from which you are speaking. I nevertheless come back to thinking of it in terms of "I'm here, my subject is there, and I have this lens. Which camera body would give me the best results?" And concluding the a7rIV would be the winner (for me, anyway).

I am simply trying to answer the question, for constant image quality (which I defined as 10MP or 5MP), what is the maximum reach of the lens? That's it. I was not trying to say one is better than another for that question, just what is the maximum crop of each camera for the same lens and same MP. Because I'm looking at constant MP, one could argue that neither camera is better than the other assuming ISO and SS and Aperture are all in the goldilocks zones. The A7RIV would just have more reach, which I quantified with some specific numbers.

If I'm shooting at 800mm focal length and f/8 focused on a distance of 40 meters, the a9 will give 47cm depth of field, where the a7rIV would only give 28cm of depth of field. I.e. with the a9 there would be a range of distance of roughly 39.75 to 40.25 meters where everything in that range would be in focus and closer and further than that would start going out of focus.

But, with the a7rIV, the only things that would be in focus would be from a distance of 39.86 to 40.14 meters. And the explanation for why the a7rIV loses that extra 19 cm of depth of field is that diffraction at the pixel level starts making things outside that range fuzzy, where on the a9 they would still be sharp.

For a large aperture (small f-stop), things go out of focus because points of light get blurred by the lens into circles (or stars or hexagons). As the circles get larger than the pixel size, they start to cause softness in the details of the image. As objects get further away from the plane of focus, the circles get larger and larger causing what we call bokeh. Things in the plain of focus, or close to it, are still sharp points. This is the traditional DoF. The important thing here is that larger pixels are affected less by DoF because the circles can get larger without causing blur. Anti-alias filters and the beyer color matrix also affect the amount of blur. For example, the color matrix is usually 4 pixels (1 red, 1 blue, 2 green). If the circles bleed over a pixel boundary within a 4-cell, you can get mushy colors. If the circles cross 4-cell boundaries, you get indistinct edges. Different cameras (or photo editing software if working in raw) have different so-called de-mosaicing algorithms that they use to convert the 4-cell color matrix into single pixels with an RGB color value. These algorithms will also affect how DoF blur is rendered in the final image.

For small apertures (larger f-stop), diffraction will start to cause _all_points_ to become blurred. Diffraction only depends on the f-stop. It is caused by light passing through a small hole. It affects the in-focus points and the out-of-focus points. Again, as the blurred size exceeds the size of a pixel it becomes noticeable. Back in the days of film or early digital, the pixels (or film emulsion) were so big that one could shoot at f/11 or so, with a good lens. Diffraction degrades a lens resolution, so it basically lowers the MTF curves, or how many lines per inch (or cycles/mm or lines/mm, whatever unit one uses).

So, one can lose detail and get soft images by having too small an f-stop or too large an f-stop, but the mechanisms are different. Lower res sensors (e.g. 24MP) have a decently large range of sharp f-stops so you can get the DoF you need on the subject and not suffer from diffraction. Higher res sensors (e.g. 35 - 42 MP) have a smaller sharp range. Very high res (e.g. the 60 MP A7RIV) have even smaller ranges.

In the specific case of shooting at f/8 on the A7RIV, you might just be getting somewhat soft images no matter what -- Chambers estimated f/5.6 to be the practical diffraction limit for the A7RIV, assuming you want the best micro contrast. That f/5.6 limit does not depend on the lens (assuming the lens can resolve the 300 or so line pairs/mm of f/5.6).

If you need to shoot at small f-stops or large f-stops, you might do better with a lower res sensor. But, if your final work is printed/displayed in a way that needs more resolution, then you need more MP and just need to limit your f-stop to where the lens + sensor combo still has maximal sharpness. If your final work does not need those MPs, then you can be a bit sloppy on the f-stop and you likely would not see it (but if you don't need those MPs, why spend the money have have the other issues with a very high MP sensor?). In general, as long as you are not trying to crop to the limits of the sensor, having more MP and keeping the f-stop and ISO in the goldilocks zones will give better results -- you get to resolve more lines/mm assuming your lens can resolve that (a lot of lens become the limiting factor beyond the 40-50MP range).

If so, then, further, the loss of DOF, for an 800mm focal length (at f/8) is approximately 0.5%? And if the focal length were less, that percentage would be reduced as well. So, it does matter if you're using really long focal lengths and small-ish apertures, at longer distances. But, it's pretty much insignificant for more "normal" shooting?

The DoF is lost from having too small an f-stop (e.g. f/5.6 at 800mm, f/4 at 500-600mm). DoF is a loss of sharpness for the out-of-focal plane. Diffraction is an overall loss of sharpness for all of the image (in or out of focal plane) due to having too large an f-stop. What one wants is to be able to set the f-stop so the subject is all in focus and the background is pleasantly blurred away. That is a pleasing DoF.

DoF becomes less of an issue for shorter focal lengths or longer distances (i.e. lenses have a smaller DoF when the subject is close). Diffraction does not depend on focal length or distance to subject, only on f-stop. There is no pleasing diffraction -- it makes everything soft.

So, this is why very high res sensors (or small pixel size sensors) have a smaller f-stop working range, and gets back to that quote from Chambers. You want to bump up the f-stop 1 stop to reduce the DoF blur from too small an f-stop, but that is limited by diffraction that affects all pixels and only depends on the f-stop.

Again, if you are not enlarging or cropping, you might not notice these effects until they get very bad. Take a look at https://www.kenrockwell.com/tech/diffraction.htm.

Note that things get wonky when doing macro, as lenses that focus very closely have an effective aperture 1 or 2 stop smaller than the physical aperture. So, if the camera reports f/32, the physical aperture might only be f/16. Anyway, that's a big digression.

Marc
 

StuartV

Member
In the specific case of shooting at f/8 on the A7RIV, you might just be getting somewhat soft images no matter what -- Chambers estimated f/5.6 to be the practical diffraction limit for the A7RIV, assuming you want the best micro contrast. That f/5.6 limit does not depend on the lens (assuming the lens can resolve the 300 or so line pairs/mm of f/5.6).

If you need to shoot at small f-stops or large f-stops, you might do better with a lower res sensor. But, if your final work is printed/displayed in a way that needs more resolution, then you need more MP and just need to limit your f-stop to where the lens + sensor combo still has maximal sharpness. If your final work does not need those MPs, then you can be a bit sloppy on the f-stop and you likely would not see it (but if you don't need those MPs, why spend the money have have the other issues with a very high MP sensor?). In general, as long as you are not trying to crop to the limits of the sensor, having more MP and keeping the f-stop and ISO in the goldilocks zones will give better results -- you get to resolve more lines/mm assuming your lens can resolve that (a lot of lens become the limiting factor beyond the 40-50MP range).

The DoF is lost from having too small an f-stop (e.g. f/5.6 at 800mm, f/4 at 500-600mm). DoF is a loss of sharpness for the out-of-focal plane. Diffraction is an overall loss of sharpness for all of the image (in or out of focal plane) due to having too large an f-stop. What one wants is to be able to set the f-stop so the subject is all in focus and the background is pleasantly blurred away. That is a pleasing DoF.

Marc,

Thank you again so much for your patience and for taking the time to explain this so well. I see now that I did not "get it" before. But, I think I do now.

Bigger apertures limit your DoF implicitly. Because physics. Smaller apertures do not limit your DoF, per se. Smaller apertures (eventually) limit the sharpness in the whole image. The need to keep the aperture bigger than a certain size to maintain overall sharpness has the side effect of limiting DoF. Got it.

Additionally, smaller pixels are affected "sooner" by diffraction because, being smaller, light getting bent (i.e. diffraction) directs the light out of the boundary of the pixel site sooner. Got it.

On the face of it, those statements might leave me thinking "if I want more DoF, I really should buy the lower rez sensor." I.e. "if I want the DoF that I can only get with f/8, I should buy an a9 and not an a7rIV."

But, is that really a fair conclusion?

I think it is not, and here is why. Please tell me (IF you have a bit more patience for me!) if I am still missing something.

Let's say I am contemplating shooting a specific photo and all other factors are equal - my only decision is to use an a9 or an a7rIV - and the shot calls for (for example) f/8. Let's presume that the shot is close enough that the a9 will produce a usable image. I.e. will result in enough pixels to print (or view) at the desired size.

In this case, the image from the a7rIV is going to lose some sharpness due to diffraction (and my choice of f/8 for aperture). But, isn't that loss of sharpness happening at the pixel level? But, given that the final result will have 2.5X as many pixels - and even at a9 resolution, the pixel density in the final print is satisfactory - won't that loss of sharpness at the pixel level be invisible to the naked eye (when viewing the final image at its full size)? I mean, even if you are pixel-peeping, if you zoom the a9 and a7rIV images to the same zoom factor, won't the higher MP image still look just as good? I think this is basically what you were saying in your last post. I just want to be sure I have understood you correctly and completely.

I guess a related way of asking the same question is: If I take the same photo (at f/8, or f/11, or whatever) with the a9 and the a7rIV, and then I use quality photo processing software to resize the a7rIV image to the same size (i.e. same MP count) as the a9 image, shouldn't the reduced a7rIV image be just as sharp as the a9 image (at least, in practical terms)?

Thank you again.
 

marcsantacurz

Well-known member
On the face of it, those statements might leave me thinking "if I want more DoF, I really should buy the lower rez sensor." I.e. "if I want the DoF that I can only get with f/8, I should buy an a9 and not an a7rIV."

But, is that really a fair conclusion?

I think it is not, and here is why. Please tell me (IF you have a bit more patience for me!) if I am still missing something.

Let's say I am contemplating shooting a specific photo and all other factors are equal - my only decision is to use an a9 or an a7rIV - and the shot calls for (for example) f/8. Let's presume that the shot is close enough that the a9 will produce a usable image. I.e. will result in enough pixels to print (or view) at the desired size.

In this case, the image from the a7rIV is going to lose some sharpness due to diffraction (and my choice of f/8 for aperture). But, isn't that loss of sharpness happening at the pixel level? But, given that the final result will have 2.5X as many pixels - and even at a9 resolution, the pixel density in the final print is satisfactory - won't that loss of sharpness at the pixel level be invisible to the naked eye (when viewing the final image at its full size)? I mean, even if you are pixel-peeping, if you zoom the a9 and a7rIV images to the same zoom factor, won't the higher MP image still look just as good? I think this is basically what you were saying in your last post. I just want to be sure I have understood you correctly and completely.

I guess a related way of asking the same question is: If I take the same photo (at f/8, or f/11, or whatever) with the a9 and the a7rIV, and then I use quality photo processing software to resize the a7rIV image to the same size (i.e. same MP count) as the a9 image, shouldn't the reduced a7rIV image be just as sharp as the a9 image (at least, in practical terms)?

Thank you again.

Yes, now you are getting at the crux of the matter. If you are shooting near-full-frame images and then reducing them to 10-15MP prints, the A7RIV is going to give you better results and you will not see the issues with the A7RIV being affected by more diffraction. Basically, the 2.5x more pixels are compensating for them being more affected by diffraction. Remember, the diffraction is not any bigger on the sensor between the 24MP and 61MP sensors. This means it affects the same level of detail when viewed at a lower resolution. In most viewing circumstances, the human eye is only resolving at 30-40 line pairs/mm (which is why MTF charts use this for the fine detail resolution).

But, if you're usually throwing away those extra 2.5x MP, then why buy them? Why not just use a 24MP - 31 MP camera? There's other practical downsides to the MP (larger files, slower operating speeds, need for faster SD cards, etc.).

If your composition requires you to (a) crop the heck out of it or (b) print very large photos that are viewed fairly close up or (c) make ultra-fine resolution prints (e.g. Ming Thein 720ppi ultraprint), then you need to avoid unwanted Dof and diffraction. Or if you want the maximum micro-contrast, which is often overlooked.

Micro-contrast is the amount of "pop" you get from light/dark boundaries around the details of an image. Some lens seem to have "pop" or "3d" effects because they resolve very well (among other things) and give you that minute detail that seems to bring to life an image. That requires little to no diffraction and no unwanted DoF. Again, if your final output is at a substantially lower resolution than the sensor, the extra pixels can somewhat compensate for it.

There are many other things that might rob your photos of that fine detail besides diffraction. Motion blur trying to track a moving subject, vibration blurr from poor support at marginal SS, high ISO, distortion from "auto lens correction" and so on. Unless all those things are also controlled for, the lower-ends of diffraction blur will not be the dominating force.

Personally, I would tend to the A7RIV over the A9 and just keep it in the f/5.6 - f/6.3 range. The A9 at 24MP full frame is great for most field and indoor sports with a 200mm - 400mm - 600mm lens, depending on the type of shot you're doing of human-sized subjects. Personally for birds, I'd want a 400/2.8 + 2xTC or 600mm + 1.4xTC on a 24MP sensor for birds, and those lenses just are not in my foreseeable budget (I use a d850 + 500mm f/5.6e most of the time nowadays). I want to be able to get to about 1200mm equivalent focal length to get the compositions I want. So, higher res + crop is the ticket for me. Given the choice between a full frame high rez or crop sensor, I prefer the high rez FF. The important thing is to know the limitations and shoot accordingly.

Marc
 

StuartV

Member
But, if you're usually throwing away those extra 2.5x MP, then why buy them? Why not just use a 24MP - 31 MP camera? There's other practical downsides to the MP (larger files, slower operating speeds, need for faster SD cards, etc.).

Thank you again for being so patient with me and taking the time to explain things so well.

I think I’m finally on the same page with you. You input has helped me to be sure I have made the right decision (for me, of course). I bought an a7rIV yesterday. :king:

Personally, I’m probably “usually” going to be throwing all those pixels away, but sometimes it happens - I’m shooting moving subjects underwater and they end up just being that far away and/or small enough. And, there’s not time (or mental bandwidth) to zoom the lens in. So, I don’t need the extra MP all the time. But, every now and then I really want them.
 

njlarsen

Gallery Moderator
Opus Editor
Supporter
Barbados
Thank you again for being so patient with me and taking the time to explain things so well.

I think I’m finally on the same page with you. You input has helped me to be sure I have made the right decision (for me, of course). I bought an a7rIV yesterday. :king:

Personally, I’m probably “usually” going to be throwing all those pixels away, but sometimes it happens - I’m shooting moving subjects underwater and they end up just being that far away and/or small enough. And, there’s not time (or mental bandwidth) to zoom the lens in. So, I don’t need the extra MP all the time. But, every now and then I really want them.

Underwater the equation really changes. Dust and other particles in the water as well as color distortion makes it almost impossible to take good image at a distance.

Niels
 

marcsantacurz

Well-known member
Thank you again for being so patient with me and taking the time to explain things so well.

I think I’m finally on the same page with you. You input has helped me to be sure I have made the right decision (for me, of course). I bought an a7rIV yesterday. :king:

Personally, I’m probably “usually” going to be throwing all those pixels away, but sometimes it happens - I’m shooting moving subjects underwater and they end up just being that far away and/or small enough. And, there’s not time (or mental bandwidth) to zoom the lens in. So, I don’t need the extra MP all the time. But, every now and then I really want them.

A recent post on diglloyd.com [1] sums up the diffraction issue. While f/8 clearly degrades the image on the A7RIV (and the phase one he's talking about), other factors like field curvature or focus shift [2] dominate so he's OK goin a bit sub-optimal. I suspect it would be the same thing with your underwater photography -- other issues will be degrading the image at a larger scale than the loss from some added diffraction.

I'm sure you'll be very happy with the A7RIV.

Marc

[1] https://diglloyd.com/blog/2019/20191122_1430-PhaseOneIQ4.html
[2] Cameras usually focus at a larger aperture then stop down. However, some lenses change focus when they stop down, so they go slightly out of focus in the process. It depends heavily on the lens and camera system.
 

49bentley

Well-known member
Canada
Thanks for all the interesting responses. I finally went and purchased the Sony A9 ii with Sony 400mm 2.8. Went shooting birds along with a 2x tc. After a couple of weeks and 300 + pictures I must say I was not totally happy. The Sony A9 ii has an almost insane tracking lock for BIF and its high iso is very good, but the resolution with cropping was disappointing. I managed to switch the A9 ii for the Sony A7Riv. Using the full frame with 2x tc, or 1.4 tc in either full frame or crop mode gave definitely better results of birds. The tracing is still very good. The high iso is definitely not as good as the A9ii but still really good at 12,800.
 

Vollmeise

Well-known member
Thanks for all the interesting responses. I finally went and purchased the Sony A9 ii with Sony 400mm 2.8. Went shooting birds along with a 2x tc. After a couple of weeks and 300 + pictures I must say I was not totally happy. The Sony A9 ii has an almost insane tracking lock for BIF and its high iso is very good, but the resolution with cropping was disappointing. I managed to switch the A9 ii for the Sony A7Riv. Using the full frame with 2x tc, or 1.4 tc in either full frame or crop mode gave definitely better results of birds. The tracing is still very good. The high iso is definitely not as good as the A9ii but still really good at 12,800.

Hiho!

Now the thread starts to get highly interesting! As you have a comparison and field experience now, could you please post some samples made with A9 II w/ 2,8/400 w/ and w/o TC? And please some samples shot with the A7R4 w/ 2,8/400 plus TC?

I'm sure lots of the readers above will be as curious as I am :)

Cheers)

P.S.: 300+ Frames in a couple of weeks with an A9? Or did you mean 300 shot in 15 seconds - that's an A9 made for :D
 
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