Some digiscoping experiments with a dslr (1 Viewer)

[gowerboy]

Hello all, just saw this thread!
I started digiscoping this year, and find it a very steep learning curve.
Paul, with your 300mm/ scope comparison did you take into account the distance of the sensor from the scope including the mount? The 300mm will have this taken into account.
JGobiel,you say you are going to try the 100-400mm lens, I have read that the 400mm end is actually around 370mm on 1.6x sensor cameras.
I use a 1.4x Canon convertor on a 20D and Swarovski ATS80HD with 800mm attatchment, and mine works fine with just tape over the contacts between the camera and converter.
I will be following this thread with interest, I find 'scoping a bit of a "black art"

www.flickr.com/photos/mosesdavies

Clive

 

[maxxxx]

How do you work out the X magnification on a digital slr. On a 35mm camera it's roughly 50mm, although I read it's nearer 44mm if such a lens was available. I read on a dslr that a 28mm lens is considered 1X.

Nice photo btw. Did you spacer the barlow away from the camera to increase the power?

Paul.

A 1.5 crop with a APS-C sensor gives you 3x per 100mm of focal length and a conventional 35mm 2x per 100mm of focal length.

So a 500mm gives a 35mm 10x while it gives a DSLR with a ACP-C size sensor 15x. So my 1000mm scope at prime focus gives me 30x. Put in a 2x barlow and it doubles the FL to 2000mm. Thus 3x20 gives me 60x. At least that is how I understand it.

No I keep the barlow as close to the sensor as possible.

BTW I've been seeing what my Pentax K20D can do. This camera has a very high resolution sensor of 14.6 mp (4672x3140px) so I thought I'd see how well it stands up to cropping.

All taken at highest quality jpg settings and downsized and cropped as jpgs so it's not the best way but should give a rough idea.

Taken through the Astor-Tech 80mm at prime focus distance about 25 feet.
Full frame and crop.

 

[JGobeil]

Paul,

I tested my 100-400 against the 560mm scope.

20D + 100-400 @ 400mm + 1.4X Canon TC = 896 mm

20D + 560mm Astro-Tech AT80ED 80mm f/7 ED doublet refractor = 896mm

I had to increase by exactlly 20% the picture taken with the 100-4000 to make it the same size as the one taken with the scope.

However, the scope had a Kenko set of spacers (68mm) in line to allow it to focus on the target full screen. Could the spacers increase the magnification by 20% ?

 

[maxxxx]

Paul,

I tested my 100-400 against the 560mm scope.

20D + 100-400 @ 400mm + 1.4X Canon TC = 896 mm

20D + 560mm Astro-Tech AT80ED 80mm f/7 ED doublet refractor = 896mm

I had to increase by exactlly 20% the picture taken with the 100-4000 to make it the same size as the one taken with the scope.

However, the scope had a Kenko set of spacers (68mm) in line to allow it to focus on the target full screen. Could the spacers increase the magnification by 20% ?

Jules perhaps I misunderstand what you are doing but given what you state above I would think both would have a focal length of 560mm

1. 400mm x 1.4 = 560mm
2. 560mm x 1.0 = 560mm

Am I missing something?

 

[Paul Corfield]

Jules perhaps I misunderstand what you are doing but given what you state above I would think both would have a focal length of 560mm

1. 400mm x 1.4 = 560mm
2. 560mm x 1.0 = 560mm

Am I missing something?

Plus the 1.6X crop factor equals 896mm.

We were testing why there is a difference between the two. Photo taken through scope seems to be bigger than comparable photo taken through a camera lens. I found mine to be around 35% bigger while Jules was 20% bigger with the scope.

Paul.

 

[Paul Corfield]

Paul,

I tested my 100-400 against the 560mm scope.

20D + 100-400 @ 400mm + 1.4X Canon TC = 896 mm

20D + 560mm Astro-Tech AT80ED 80mm f/7 ED doublet refractor = 896mm

I had to increase by exactlly 20% the picture taken with the 100-4000 to make it the same size as the one taken with the scope.

However, the scope had a Kenko set of spacers (68mm) in line to allow it to focus on the target full screen. Could the spacers increase the magnification by 20% ?

I don't think the spacers come into it because that is all factored into the focal length of the scope. A simple and fairly accurate way to check the focal length of the scope is to put the camera on the back of the scope and focus on something that is as far away as possible. Then take the camera off the scope and hold a sheet of paper or card behind the back of the scope without altering the focus. Move the paper close to the back of the scope until you see an image form and come into sharp focus. This is where the image would have been in focus on the camera ccd. Measure the distance from the scopes objective lens to paper with the image in focus.

I just did this test and it was pretty much dead on 600mm which is what my scopes focal length is. This shows that the extension tubes don't alter the magnification, they just make focus possible at the correct point.

Also remember that this type of scopes focuser is meant to be used with a 90° mirror or prism and that wouldn't need the extension tube to achieve focus. This is why the focuser doesn't have the amount of travel that we need when mounting the camera without the erector mirror/prism. We have to create that extra travel with the extension tube.

Paul.

 

[Derry]

Paul, a couple items for further discussion,,

I just called TeleVue and could not believe it but was talking with Al Nagler (owner and inventor of TeleVue) and asked him about our F L question,,,

Al stated all scopes F L are made at an infinity setting with the diagonal and any time you add to that length you are increasing the F L of the scope,, he offered me some calcualtion (over my head) to figure out the "NEW" F L when using additional tubes to allow closer focus,,

a prime example of this would be my Questar,, it will focus to about 10 feet and to accomplish this I am adjusting or moving the intermal mirror which is also increasing the scopes focal length to accomodate the closer focus,, it also increases the f stop (smaller) which can clearly be seen in the Questar with light fall off when going this close,, guess we need some math understanding on this one,,


I still am not sold on how the cameras X factor is being figured,, this factor is always being used as a multiplier in power when in reality it is to identify the FOV adjustment compared to a equal 35mm lens, not the size the subject will be magnified,,

a 200mm lens is a 4X magnification and offers a 12.13 degrees fov,, on a body with a 1.5 factor it offers an 8.24 degrees fov so the camera is only capturing the central portion of that image but it is still at 4X,,

appreciate your thoughts,,

Derry

 

[Paul Corfield]

The method I posted in the post before yours Derry will give you a fairly accurate way to check a scopes focal length. Although not quite at infinity, as long as you focus on a subject a few miles away it will give a good idea. When I checked with this method, using my camera and 60mm spacer it was as near as dead on 600mm focal length. I suppose as soon as you don't focus on something at infinity then the scopes focal length is changing?

With the X factor you are right that the image isn't being magnified as such. In your example of the 200mm lens you would need to use a 300mm lens on a 35mm camera to end up with the same result as a 200mm lens on a 1.5X crop factor camera.

Paul.

 

[Derry]

agree Paul,, was trying to figure the difference between my 2" diagonal focus point and when using two extension tubes and my T mount,, I know even with the diagonal in and trying to focus at 30 feet I need the 50mm extension tube to reach focus,, not all of teh 50mm but close to it,,

as for the cameras X factor should we be using it as a multiplier in the F L calculation,,???

would think we should just be using what the scope and extension tubes are since the cameras X factor is just altering the FOV and not truly increasing or multiplying the magnification,,

Derry

 

[Paul Corfield]

I don't mind either way about the crop factor. If we didn't include it on the test results in the other thread then it would have to be stated in the part where the focal length is written that the crop factor has not been used to calculate the final focal length.

Something I was reading about focal length just now stated that the focal length is the distance from the objective lens to the point where the image is in focus on the film or ccd. That's quite obvious I suppose but it means that every photo will have a different focal length when at prime focus. God knows what's happening once a few teleconverters are stacked up. I'm getting brain ache. :eek!:

Paul.

 

[Derry]

Jules, you reading this thread,,??

think we should not include the cameras X (crop) factor in the MM length calculation,, it is misleading others into thinking we are shooting at extreeme MMs when we are not,, it only has impact on the fov the camera is able to capture and not really a magnification factor,,

Derry

 

[Paul Corfield]

Jules, you reading this thread,,??

think we should not include the cameras X (crop) factor in the MM length calculation,, it is misleading others into thinking we are shooting at extreeme MMs when we are not,, it only has impact on the fov the camera is able to capture and not really a magnification factor,,

Derry

The magnifications are still fairly extreme. I'm at 3000mm without the crop factor. The trouble with digital is that everything gets compared to 35mm.

Paul.

 

[maxxxx]

I still am not sold on how the cameras X factor is being figured,, this factor is always being used as a multiplier in power when in reality it is to identify the FOV adjustment compared to a equal 35mm lens, not the size the subject will be magnified,,

I wonder if we may be getting hung up on semantics. To me there is no such thing as "power" or "magnification" only the angle of view that an optical system presents to the eye or a camera sensor.

For instance my Pentax K20D has a 14.6mp (4672x3104px) sensor and my Pentax DL a 6mp (3008x2000px) sensor.

If I take a picture at 14.6mp on the K20D and do a 100 percent crop down to the same size in pixels as the DL and then resample them to the same size this is "magnification" just the same as if I had done it optically by using a longer FL optic on the DL. See pic.

Also I find it confusing to express the focal length of a lens using a crop factor. All prime lens' have one and only one native physical FL regardless of which camera body happens to hanging off the back end of it.

For instance this combination of a 800mm FL lens + adaptor to a 35mm k mount body intended for the Pentax 67 medium format camera is a 800mm FL lens whether used on a medium format camera or adapted to a 35mm K mount body...

http://www.bhphotovideo.com/c/product/40784-REG/Pentax_29434_SMCT_67_800mm_f_4_Lens.html

http://www.bhphotovideo.com/c/product/40817-REG/Pentax_37954_67_Lens_to_Pentax.html

 

[JGobeil]

Jules, you reading this thread,,??

think we should not include the cameras X (crop) factor in the MM length calculation,, it is misleading others into thinking we are shooting at extreeme MMs when we are not,, it only has impact on the fov the camera is able to capture and not really a magnification factor,,

Derry

Humm... this is a difficult question - so my answer will be quite long. :smoke:

On the pro side, let's not forget that we are trying to evaluate digiscoping SETUPS. Not only do we evalutate the scope, but we also test the camera, the stability of the whole setup and the expertise of the photographer. A camera without a scope is useless, and vice versa. The camera crop factor increases effective magnification. The bird in a photograph will be bigger with a 1.6 crop factor than it is with a camera that has no crop factor.

On the negative side, let's not forget that cameras have a crop factor because they have a smaller sensor than 24x36mm. Smaller sensor means lesser image quality/precision/sharpness.

Let's study that difference in more detail:

A Canon 20D has a 1.6 crop factor because of a 8.2mpx 22.2x14mm sensor. A Canon 1Ds Mark III has a 1.0 crop factor but a 21.1mpx full size 24x36mm sensor.

On a 20D, a 500mm lens will behave like a 800mm lens for 16X while the 1Ds will stay at 500mm for 10X. This difference is in terms of magnification but not in terms of quality/precision/sharpness of image. However, one will be able to crop the 1Ds image by 2.57X (21.1/8.1) to bring it to the level of the 20D, so its lens will behave like a 1286mm lens for 26X.

In the protol, we have so far defined EFFECTIVE RESOLUTION as:
(scope+TC+barlow+spacer magnification in X) x (crop factor) x (resolution number from the target) x (distance in meters)

IMO, this formula is wrong. I think it should consider the megapixel count of the sensor instead of the crop factor. I would suggest we use this factor:
(megapixel count) / 10
I suggest 10 as it is probably the current "normal" mpx count for the average DSLR. This would give my 20D a multiplier of 0.82 - Paul's 450D would get 1.22 - the 1Ds MkIII would get 2.11.

According to this suggestion, we would have to modify 2 elements of the protocol:
1- The focal length of the setup (scope+ TC + barlow + spacer) would not be multiplied anymore by the crop factor of the camera.
2- The EFFECTIVE RESOLUTION of the camera + scope setup would become:
(scope+TC+barlow+spacer magnification in X) x (MPX count of camera)/10 x (resolution number from the target) x (distance in meters)

What do you think ?

 

[Derry]

Jules,, what I am finding to read on the net keeps a lens at the same magnification,, 500mm = 10x on any camera body,, when placed on a DSLR with a crop factor it reduces the FOV but is still only magnifying the image 10x,, will see if I can locate the thread where a pro was viewing through full frame and crop factor view finders with the same mm length lens mounted to each camera,, the image he was seeing remaind the same size in X factor but the width of what he was seeing changed,, this FOV has changed and how they are printed also makes adjustments,,

the DSLR cameras do not magnify the image they are seeing but they are adjusting the size (width and height,mm) of lens captured area,,

a standard 35mm lens placed on a small chip DSLR actually floods the area beyond the sensors size with image not captured by the smaller sensor,, this sensors crop factor is not magnifying that image past the mm length of the lens,,

am I missing something in everything I am reading,,???

Derry

 

[JGobeil]

Jules,, what I am finding to read on the net keeps a lens at the same magnification,, 500mm = 10x on any camera body,, when placed on a DSLR with a crop factor it reduces the FOV but is still only magnifying the image 10x,, will see if I can locate the thread where a pro was viewing through full frame and crop factor view finders with the same mm length lens mounted to each camera,, the image he was seeing remaind the same size in X factor but the width of what he was seeing changed,, this FOV has changed and how they are printed also makes adjustments,,

the DSLR cameras do not magnify the image they are seeing but they are adjusting the size (width and height,mm) of lens captured area,,

a standard 35mm lens placed on a small chip DSLR actually floods the area beyond the sensors size with image not captured by the smaller sensor,, this sensors crop factor is not magnifying that image past the mm length of the lens,,

am I missing something in everything I am reading,,???

Derry

We are saying the same thing with different words. A camera with a crop factor does not magnify, it crops. The effect is that the resulting image will look magnified compared to one taken with a camera without crop factor.

See this:
http://en.wikipedia.org/wiki/Crop_factor

Jules

 

[theMusicMan]

I've been watching this thread with great interest.

Just a thought... some people are suggesting that smaller sensor means lower quality images. Yes, to an extent I agree, but there's also a trade off with the larger sensor - and that's the effective focal length.

Sooooo difficult to explain is this...

I appreciate that if a lens has a FL of 500mm, it has a FL of 500mm regardless on which camera it is placed on. However, if this same lens is placed on different cameras with the same number of sensor pixels [say 10MP] (but a different sensor size): then on a x2 crop factor camera the image is effectively magnified.

The image on the larger sensor camera has a larger field of view than the image on the camera with the smaller sensor... however, the image taken by the camera with the smaller sensor still covers 10MP, and if one then resizes/crops the image taken by the camera with the larger sensor to appear the same as the image taken with the camera with the larger sensor, the same image will comprise much fewer pixels.

So... which is best...? I'd say the image with the larger number of pixels i.e. smaller sensor. This is especially true for longer range FL's.

In summary: for the same image field of view; on smaller sensor camera the image is composed of 10MP, whereas on a larger sensor camera the same image field of view is composed of significantly fewer MP's.

 

[Paul Corfield]

My 2" GSO 2X ED barlow arrived today. Pushed it out to 5400mm (before the crop factor) so far with good results. If you take the crop factor of 1.6X then that equates to 8640mm. This was achieved in combination with the 2X and 1.5X teleconverters plus a 120mm long extension tube between them and the barlow to bump up the magnification. Took 2 photos, one at 600mm prime and one with the setup as posted above. Had to increase the 600mm image by 900% to get a match.

The barlow itself is really nice. You take the 2" lens cell off the barlow and this then screws straight into my 2" T-mount. The body of the barlow minus the lens cell becomes a nice extension tube to mount the camera to.

One problem I had with the barlow at first was that I had some issues with internal reflections causing a bright area in the middle of the photo. I made some baffles and this cured the problem straight away and increased the contrast too. I guess as barlows are usually used at night then internal reflections wouldn't normally be a problem. I've had this problem with barlows in the past when using them in the day.

Done nothing but rain all day today so nothing really meaningful to post. Here's a Collared Dove from around 60m range taken indoors through glass. This was with the barlow combined with one of my teleconverters, can't remember which one as I tried all sorts of combinations today.

This is the whole photo, resized to 1024 pixels wide. ISO800 1/30sec very dull and wet.

edit - interesting John. So would you say it's best to add in the crop factor when stating the achieved focal length?

Paul.

 

[JGobeil]

I've been watching this thread with great interest.

Just a thought... some people are suggesting that smaller sensor means lower quality images. Yes, to an extent I agree, but there's also a trade off with the larger sensor - and that's the effective focal length.

Sooooo difficult to explain is this...

I appreciate that if a lens has a FL of 500mm, it has a FL of 500mm regardless on which camera it is placed on. However, if this same lens is placed on different cameras with the same number of sensor pixels [say 10MP] (but a different sensor size): then on a x2 crop factor camera the image is effectively magnified.

The image on the larger sensor camera has a larger field of view than the image on the camera with the smaller sensor... however, the image taken by the camera with the smaller sensor still covers 10MP, and if one then resizes/crops the image taken by the camera with the larger sensor to appear the same as the image taken with the camera with the larger sensor, the same image will comprise much fewer pixels.

So... which is best...? I'd say the image with the larger number of pixels i.e. smaller sensor. This is especially true for longer range FL's.

In summary: for the same image field of view; on smaller sensor camera the image is composed of 10MP, whereas on a larger sensor camera the same image field of view is composed of significantly fewer MP's.

This is a very difficult question with a ton of pro and con arguments. See those:
http://photo.net/nikon-camera-forum/00Pwru
http://500th.net/the-crop-factor

I think most of us understand how the crop factor works, it has been explained over and over in this thread. In fact, the question is: should we consider it in our calculations ?

Should a 600mm scope instantly become a 960mm scope when connected to a Canon 40D ? Without considering pixel count and pixel size ? Then, what about the P&S cameras that have even smaller sensors ? Does a 2.0 crop factor make the Olympus E-420 a better camera ?

Difficult question indeed |

 

[theMusicMan]

edit - interesting John. So would you say it's best to add in the crop factor when stating the achieved focal length?

Hi Paul - I would actually, and consider a larger crop factor a plus for bird photography.

This is a very difficult question with a ton of pro and con arguments. See those:
http://photo.net/nikon-camera-forum/00Pwru
http://500th.net/the-crop-factor

I think most of us understand how the crop factor works, it has been explained over and over in this thread. In fact, the question is: should we consider it in our calculations ?

Should a 600mm scope instantly become a 960mm scope when connected to a Canon 40D ? Without considering pixel count and pixel size ? Then, what about the P&S cameras that have even smaller sensors ? Does a 2.0 crop factor make the Olympus E-420 a better camera ?

Difficult question indeed |

Hi JGobeil - I don't think the 2.0x crop factor on its own necessarily makes any camera better than others - I think it is a distinct advantage. For longer range focal lengths, especially in bird photography - the trade off is between image noise and additional effective magnification. We all want to get closer to our birds, and a larger crop factor permits this. It doesn't however compensate for a larger actual focal length and high quality lens. So, even though it is becoming ever more marginal in differential quality - a 1000mm focal length lens on a 1.0x crop factor camera should still produce a better quality image than with a 500mm focal length lens on a camera with a crop factor of 2.0x.