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Depth of Field (1 Viewer)

Ruby

Well-known member
Hi Guys,

In 'normal' SLR photography, there is a relationship between depth of field and aperture - the smaller the aperture, the bigger the DOF..... I was wondering whether the same applies in the wacky world of digital cameras and digiscoping.

You see, I have a bit of a problem (failing eyesight!!) in telling when the image in the little LCD screen is absolutely in sharp focus - and a little bit extra DOF might help a bit.

Of course, conventional wisdom is to have the lens wide open so as to allow as much light in and to try to allow as short a shutter speed as possible.

I'm thinking that if you had (for example) a choice of going with F4@1/1000th or F8@1/250th it might be worth sacrificing the shutter speed for a bit of extra DOF.

All this is a bit of a joke at the minute, since when I was peering out of my window earlier on today, I was looking at something like F3.8@1/8th!! .... but I am dreaming of those bright, sunny days in the summer!!
 
Hi Ruby,

Same thing applies in digital as 35mm. Without getting too technical as I doubt I could anyway, depth of field does increase with the f-number.

However as you increase the focal length of a lens depth of field decreases to the point in digiscoping where it's negligible and a faster shutter speed to minimize scope and camera vibration / movement is far more important. I usually go for the lowest f-number possible.
 
I may be wrong (often am!) but I think that when you put your camera next to the scope you, in effect, create a new optical confguration, i.e. the combination of the two optical systems makes a 'new' lens.

The aperture would need to be in the correct position for this new lens to perform correctly but as it is it is in the right position only for the camera lens so cannot work in the new set-up.

Stopping the lens down will only increase the amount of vignetting, i.e. you will get a smaller and smaller image circle as you stop the lens down.

This will be most obvious on a camera that already suffers from vignetting when digiscoping
 
The problem, Ruby, is the small LCD screen (even my through the lens Fuji S602 gives you a screen to look at rather than the actual image).

There isn't an answer really. A 35mm SLR would be far easier to focus, although even there the image is quite dim. Doesn't the autofocus work as a "digiscope"? It's a bit of a devil!
 
scampo said:
Doesn't the autofocus work as a "digiscope"? It's a bit of a devil!

Too right!!

Problem with the autofocus is that it takes a while to happen, so I've been trying to pre-focus the camera on (say) a bird-feeder, press the shutter half-down to lock and then take the pic when the bird actually arrives.

This would work a good deal better if I could actually see what I'm looking at in the LCD!! I can see well enough to tell that it is more-or-less in focus, but not precisely so...I was just thinking that a bit of extra depth of field might give me some more tolerance....

I plan to take the set-up out to a hide or something asap, when I might get some co-operative subjects who will stay put for long enough for me to sort the focus problem out...
 
I don't know if your camera's lens has feet / metres distance markings on it (they all used to have years ago...); if it does, you could try pacing it out!
 
Ruby are you using an extenda view eyepiece,if so this magnifies the image by 2x and if setting the camera in manual focus adjust the focus up as far as it will go,then you can focus the scope on the bird,the image will seem to shimmer when its in focus,if you take several shot and slightly adjust the scopes focus now and again some should be sharp.If you use youre camera in manual focus mode you dont have to worry about it hunting in and out of focus or have to keep your finger on the shutter release.
 
Hi Graham,

I do have an EagleEye eyepiece thingy, but haven't found it very useful so far - it magnifies the image right enough, but the enlarged image is so pixellated (is that a word??) that it's no help for telling whether the thing is exactly in focus or not.

Your suggestions do sound sensible though, I'll give that a whirl... at the moment, I've been using macro focus...
 
Ruby i need glasses for reading and found it a pain having to put these on to focus ,so used the extenda view instead.Yes it does give a pixellated image but i found i soon got used to this and now cant take a picture with out it.If your having trouble focusing keep trying with the extenda view its very good.
 
Basically you can get extra depth of field digiscoping but you need to zoom way back on your camera for it to be effective. I remember doing some test stuff for birds-pix or something years ago, it was interesting if not particularly relevant for digiscopers, at least it proved the effective f# of the set-up was a lot less than people thought at the time.
If you really are struglling to get a good focus via the monitor, try the eagleeye 12x ds eyepiece, you barely need to focus with that as the d.o.f. is so deep...albeit you're not going to get huge magnifications.
 
In digiscoping the DOF is dictated by the aperture of the scope, as opposed to the aperture of the camera lens, unless you are stopping down beyond f8-11 ( the usual scope aperture ). However, due to the optical compromises that occur when digiscoping the smaller apertures can lead to softer images for other technical reasons than vibration etc.
 
Ruby said:
You see, I have a bit of a problem (failing eyesight!!) in telling when the image in the little LCD screen is absolutely in sharp focus

Does your camera have "focus confirmation"? If so, it can help determine when you are in focus. The contrast between edges of objects that are in focus will be highlighted with thin black lines, creating sort of an exaggerated effect like an over-sharpened picture. It is subtle but with practice, the lines are readily apparent.

The other thing you can do, of course, is to rely on your camera's autofocus. Then it doesn't really matter if you are in sharp focus or not before you take the shot. Glen
 
Adey Baker said:
<snip>
Stopping the lens down will only increase the amount of vignetting, i.e. you will get a smaller and smaller image circle as you stop the lens down.

Stopping the lens down does not generally increase vignetting (at least in a rig with proper eyepiece to camera spacing), and in fact, stopping down the camera lens while digiscoping has the same effect - in principle - as it does with conventional photography.

I say "in principle" because the extremes and special cases of digiscoping can make the practical effects much less noticeable and can render the camera aperture completely useless.

One of the special cases is when digiscoping at relatively high magnifications.
Suppose you are using the "classic" 80mm Swarovski with a CP4500 with the scope at 30X and the camera at 21mm focal length. In this case you have about a 3000mm equivalency at f7.9. Your camera can't stop down much more (f10.3 is the most it can stop down, but that is at 31mm focal length). So stopping down will probably have little or no effect on the image or exposure.

However, if you are shooting at lower magnifications, for instance the scope at 20x and the camera at 16mm, you get an equivalency of around 1500mm at f4. Now you can stop down by a couple of stops and increase your DOF a little bit.

I now stop my camera down to around f8 as a standard. In low light I'll open it up if I'm at low magnification and need a bit more light. But I stop down for better overall image quality, not DOF. I've found that stopping down generally improves image contrast as well as edge resolution. The improvement is not the same with all scopes and eyepieces, so it is best to test for yourself. But in general, you get a somewhat better image by stopping down to f8 or so. Stopping down further will probably start introducing softness due to diffraction issues. I suspect this is what Nigel was alluding to.

I'll post links to some test images showing this improvement later on this evening.
 
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The following images show the same test target taken from approximately 50 feet away with a moderate digiscoping magnification of around 1400mm. The scope was an ATS80HD with a 20-60X eyepiece zoomed to 20X. The camera was a CP5000 with its lens zoomed to a focal length of 17.5mm. The test target is on 8.5 x 11 paper. The lighting was indirect from the sky.

Each image was resized to 640 x 480. No other processing was done. I then overlayed a full resolution detail circle from two spots on the image. One is very close to center and the other is in the upper left.

The first image was taken with a camera setting of f4.2 which works out to be an effective f4.38.
http://www.jayandwanda.com/digiscope/stpdwn/open.jpg
You can see that the upper left circle shows a pretty mediocre image. There is some green color fringing that tends to smear sideways. The center circle looks a bit soft, but is clearly much sharper than the area in the upper left.

The next shot was taken with a camera setting of f7.5 which yields an effective f7.5.
http://www.jayandwanda.com/digiscope/stpdwn/stopped.jpg
You can immediately see that the field is more evenly lit and the overall image contrast seems better. The full resolution spot near center is a bit sharper than the previous image, but not by a huge margin. The edge image, however, is clearly superior. The sideways color fringing is almost totally gone and the text looks much clearer. Not quite as clear as the text in the center, but still pretty respectable. The shutter speed has dropped from 1/358 of a second to 1/136 as we would expect.

So the general results are pretty much what we have come to expect from stopping down conventional lenses on a 35mm camera. A more evenly lit field, better sharpness near the edges of the image and moderate to nearly negligible improvements in the center. Other shots also show small improvements in DOF. There are other images that show similar results with different scopes and different eyepieces. Some combinations show very little or no significant difference when the camera lens is stopped down. But most show moderate to very significant improvements in edge sharpness and some improvement in image contrast.

These are relatively low magnification shots. When you start getting past 2500mm focal length equivalents with 80mm scopes, the likelihood of any visual improvement begins to diminish because such magnifications have the automatic effect of yielding larger f-numbers. This leave very little room for further stopping down.

The results with your rig may be a bit different. It probably depends a lot on how well corrected your scope optics are near the edges. But it may be worth taking a few test shots in the backyard to see if perhaps stopping down a bit can give you a a little more sharpness and contrast.
 
Ragna said:
Ruby i need glasses for reading and found it a pain having to put these on to focus ,so used the extenda view instead.Yes it does give a pixellated image but i found i soon got used to this and now cant take a picture with out it.If your having trouble focusing keep trying with the extenda view its very good.

Hi Graham,

Yep - I'm in the same boat.... long distance eyesight OK, but blind as a bat for close up work.

Interesting what you say, as I would have thought that if everything was in focus for dodgy eyesight (ie us without our specs on!) it would be out of focus for everything else (ie the camera, for instance) This is clearly not the case, but I don't really understand why....

I'll keep trying with the extenda-view pro-mini, as I suspect it will be a requirement when out in the field on sunny days....

Jay - Thanks very much for your time and trouble, I found all of that very interesting. I have a 20-60 zoom eyepiece, but tend to use it only in 20X for digi-scoping, so it may be worth trying with some smaller apertures on the camera if the opportunity presents itself - for image quality if nothing else. In grim old England though, we spend most of our time with the camera lens wide-open and struggling for enough light, so opportunities may be few and far between.

Glen - I do have the image confirmation feature, but turned it off, as I couldn't see the 'twinkling' that has been referred to.... I'll have another go.

I am very new to digi-scoping, but at the moment I am definitely in the 'soft-focus' camp and am just scratching around for ways of improving matters. I suspect a bit of decent light and some intensive practice would go a long way!!
 
Having just looked at this calculator I think that many of the calcs are incorrect. I checked the dof settings with 600mm and 400mm on Canon 1DS, the hyperfocal distance given was miles out, and some of the limits of acceptable sharpness did not tally with my bench tests by a wide margin.
 
Ruby said:
Glen - I do have the image confirmation feature, but turned it off, as I couldn't see the 'twinkling' that has been referred to.... I'll have another go.

Try digiscoping a stationary target with lots of contrast, like a newspaper taped to the wall. Roll you focus knob or ring slowly back and forth as you watch the monitor. As the page comes into focus, you should see the edges between the black letters and the white page sort of light up -- the contrast between the black and white looks exaggerated. Glen
 
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