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Thread started 16 Aug 2015 (Sunday) 15:24
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Sensor size affect DOF? Methinks not.

 
DreDaze
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Aug 16, 2015 21:04 |  #31

This whole topic started on another thread where the OP mentioned FF, or crop cameras set to the same aperture, focal length, and distance would give the same DOF- I pointed out that online calculators say it would be different...changing any distance, or focal length to match the framing is not what the discussion is about. The only change is sensor size


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Aug 16, 2015 21:07 |  #32

davesrose wrote in post #17671547 (external link)
That correlates to perceptual resolution (and is important for enlargements that exceed CoC...if your perceptual resolution doesn't exceed the original image's captured resolution, you'll see no difference in sharpness). Guess there is some difference with nomenclature/terms (where you say DOF, I say perceptual resolution)...but I think we are agreeing with major concepts. I think the only main point I'm adding to is that with digital images: perception of sharpness first starts with perceptual resolution (the arguments with DOF are for larger prints where the CoC is exceeding perceptual resolution: and becomes more apparent with close viewing distance/etc). Aperture is a function of the lens, and does seem to correspond to sensor size the way FL does... it's all equivalences, and there are pros and cons of different sensor sizes/may be no difference depending on application.

DoF is exclusively about perception. As good as any out there, the wiki definition has the basic concept:
In optics, particularly as it relates to film and photography, depth of field (DOF), also called focus range or effective focus range, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.

Others use the term "appear as sharp as the sharpest parts" as a subsitute for the bold text. The key is "appear" - hence the perception requirement. And the viewing requirement. Once something "appears" to be softer than the sharpest parts, it is outside the DoF. The distance range changes depending on viewing distance and image size.


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Post edited over 3 years ago by AJSJones. (3 edits in all)
     
Aug 16, 2015 21:14 |  #33

DreDaze wrote in post #17671555 (external link)
This whole topic started on another thread where the OP mentioned FF, or crop cameras set to the same aperture, focal length, and distance would give the same DOF- I pointed out that online calculators say it would be different...changing any distance, or focal length to match the framing is not what the discussion is about. The only change is sensor size

That seems consistent with the OP's description. If I take a
1) fixed tripod location and
2) a given lens and aperture and focus distance and
3) put it on a APS-C crop and shoot and
4) then on a FF camera and shoot
5) take the central APS-C area of each capture in 3) and 4) and
6) make the same size print from each,
7) they'll have the same DoF.

But who would do that??? Note that in 5) the FF is only using an APS-C size sensor area too.


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davesrose
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Aug 16, 2015 22:09 |  #34

AJSJones wrote in post #17671557 (external link)
DoF is exclusively about perception..

Well that's the only main point I'm taking some exception (we're only debating a fine point, and agreeing overall). Could be getting back to the times we argued about HDR: I'm involved with absolute computer graphics;) Within the digital realm, resolution is the absolute measure of visual sharpness (disregarding OOF areas: a digital image will appear very soft if the perceptual resolution is very low: IE printing a large scale image at 72DPI when you're viewing within a foot away will appear soft). I appreciate that the CoC values/ analogue DOF theorems had to do with perceived sharpness (and conceptually, the science is all still equivalent now). Whether you want to follow CoC values and DOF calculators (that also correlate with crop factor proportions), it's all relative. Because we are working digitally, perceptual resolution (IE, printed size vs DPI) is the main factor for sharpness. Most all examples of judging DOF with your classic examples is viewing really large scale prints (and correlate to my premise of perceptual resolution exceeding CoC limit). Maybe it is because I do deal with other graphics standards...I would rather keep DOF the focus range of lens aperture (which has always been the definition of DOF anyway) :) IMO, it's best to think of DOF as the lens's focus depth, and perceived sharpness of print as your perceptual resolution. Again, I defy you to see a deference between any high MP image having a difference in sharpness (at any viewing distance) printed at 5x7 vs 8x10.


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Aug 17, 2015 00:36 |  #35

davesrose wrote in post #17671608 (external link)
Well that's the only main point I'm taking some exception (we're only debating a fine point, and agreeing overall). Could be getting back to the times we argued about HDR: I'm involved with absolute computer graphics;) Within the digital realm, resolution is the absolute measure of visual sharpness (disregarding OOF areas: a digital image will appear very soft if the perceptual resolution is very low: IE printing a large scale image at 72DPI when you're viewing within a foot away will appear soft). I appreciate that the CoC values/ analogue DOF theorems had to do with perceived sharpness (and conceptually, the science is all still equivalent now). Whether you want to follow CoC values and DOF calculators (that also correlate with crop factor proportions), it's all relative. Because we are working digitally, perceptual resolution (IE, printed size vs DPI) is the main factor for sharpness. Most all examples of judging DOF with your classic examples is viewing really large scale prints (and correlate to my premise of perceptual resolution exceeding CoC limit). Maybe it is because I do deal with other graphics standards...I would rather keep DOF the focus range of lens aperture (which has always been the definition of DOF anyway) :) IMO, it's best to think of DOF as the lens's focus depth, and perceived sharpness of print as your perceptual resolution. Again, I defy you to see a deference between any high MP image having a difference in sharpness (at any viewing distance) printed at 5x7 vs 8x10.

For the definition I quoted for DoF it does not matter (within reason) how sharp the sharpest parts of the image are (those on or near the actual focus plane), it's which parts are perceived as "not as sharp those sharpest areas". (If none of it is perceived as sharp, then you are simply too close for that image presentation - I'm not discussing that condition:D)

There is a sensor side counterpart to depth of field and that is depth of focus. You can draw ray diagrams back from the near and far points of the depth of field back to planes above and below the sensor/film to see the "depth of focus".

In computer graphics, unless you tell the renderer that (there is a z dimension and) it should do a computerized lens blur, it won't have OOF areas. If you do, then you will create an image that (if you did it well) will resemble an actual optical capture from a real lens at that aperture. From then on, the print size and viewing distance will determine the areas perceived by the viewer as "sharp" and "not as sharp as the rest" and the transitions between those two areas mark the edge of the DoF. How you tell the renderer to compute the blur is going to affect the rendered image in the same way as choosing the aperture for the real world situation - it will constrain the DoF for a given print size and viewing distance. I'll guess this will be different based on the anticipated viewing experience AoV - close-up to Imax vs on an iPhone screen, for example:D

Rather than your experiment, I'll propose a slightly different one with no need to print (better for internet discussions:)): a retina display set at a fixed distance behind eyepieces, so that viewing distance cannot be changed and so that the display has more resolution than the eye can perceive. On the screen is part of aa very high resolution image of (yes) a slanted ruler taken with a relatively wide aperture (where conventional DoF calculators say only a small part of the scale will actually be "sharp"). I set the onscreen magnification to value X and ask a range of viewers to tell me the where the distances (the numbers on the scale) start getting soft. I then change it to Y and repeat, and Z and repeat etc. I then plot the distances against magnification and find there is a good correlation. As I zoom in on the image (cf. increasing print size for fixed viewing distance) the range of distances on the ruler people report as sharp will go down. (Easy to find any image and do an approximation of this experiment with a good imaging program that can scale an image down from 1:1 image pixel:display pixel). One of these onscreen magnifications will approximate the 5x7 and one will approximate the 8x10 and with enough observations, the graph will show the DoFs are different.

On the other hand, if you didn't get the renderer to do the lens blur, then the whole scene will be rendered as sharp (at your desired resolution) and the whole ruler will be legible and there will be no transitions from "sharp" to "not so sharp" and so the whole ruler will be within the DoF. We can do that in the real world by stopping down (and/or using lens movements) - landscapers strive for this condition - to where the CoC and pixel size are close, so all appears sharp, all the way to 100% on screen. Then the scene is all within the DoF and we take a 50MP image. Now, I'll agree that whether this is printed at 5x7 or 8x10 I won't be able to distinguish which is sharper because my visual acuity is the limiting factor. Go up to 20x30 and 40x60 and things might change in perceived image sharpness but not in DoF:D


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Nalauk
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Aug 17, 2015 03:55 |  #36

To the OP.

It's not the sensor size that affects DOF - it's the lens used to get the same field of view (FOV) between sensor sizes, so you are correct in some ways and articles are wrong in others.

Take a 70D (1.6 crop) and a 6D (FF) - both are 20MP so the image is going to come out the same size, no resizing to mess things up.

Put a 50mm on the 6D and to get the same field of view on the 70D you'd need a 32mm (Mythical lens, so use a 35mm). Shoot a scene on both at f5.6 and the 50mm as it's longer is going to have less DOF than the 32mm (35mm) at f5.6.

Swap the 50mm onto the 70D and shoot again and the DOF will be the same (as the 6d and 50mm) but the FOV will be narrower.

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Aug 17, 2015 04:37 |  #37

Nalauk wrote in post #17671861 (external link)
To the OP.

It's not the sensor size that affects DOF - it's the lens used to get the same field of view (FOV) between sensor sizes, so you are correct in some ways and articles are wrong in others.

Take a 70D (1.6 crop) and a 6D (FF) - both are 20MP so the image is going to come out the same size, no resizing to mess things up.

Put a 50mm on the 6D and to get the same field of view on the 70D you'd need a 32mm (Mythical lens, so use a 35mm). Shoot a scene on both at f5.6 and the 50mm as it's longer is going to have less DOF than the 32mm (35mm) at f5.6.

Swap the 50mm onto the 70D and shoot again and the DOF will be the same (as the 6d and 50mm) but the FOV will be narrower.

:)

Resolution (either digital pixel count or film graininess) has absolutely NOTHING to do with depth of field calculations.


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Aug 17, 2015 06:18 as a reply to  @ SkipD's post |  #38

http://www.cambridgein​colour.com/tutorials/d​of-calculator.htm (external link)

In the advanced options, print size is the first factor in DOF calculation. Resolution is the primary factor for print size ( and perceptual resolution is the DPI of the final output). In a print, if you're able to print at an image size that's 300 DPI, it will appear sharp at any viewing distance (and is the exact definition of perceptual resolution). If you have to print larger, so that the image's native resolution goes to say 72 DPI, then the whole image will appear softer when you view it close up. Graphic artists will still print large banners at low DPI if the intended application is say a billboard. No one is going to get in close to visually see a blurry image.


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Aug 17, 2015 06:21 |  #39

GeoKras1989 wrote in post #17671392 (external link)
I am still struggling to understand how anyone can look at two photos from vastly different size sensors, taken at the same aperture, distance and focal length, that display identical DOF, can continue to insist that what they are staring at is not possible. Those photos are a demonstration that sensor size does not affect DOF. I sincerely regret opening this can of worms. Apologies all around.

You're cropping one photo. That invalidates your point. Try to reproduce the results by obtaining the same framing WITHOUT cropping either photo, only by changing lens/moving forward or backwards, and see what happens.


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Aug 17, 2015 06:33 |  #40

DOF depends only of three things:
- distance to subject;
- focal length of the lens;
- Aperture.

That's it.


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Aug 17, 2015 06:39 |  #41

I'm glad I subscribed to this thread. I have learned so much my head hurts! -?


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Aug 17, 2015 06:44 |  #42

davesrose wrote in post #17671922 (external link)
Resolution is the primary factor for print size ( and perceptual resolution is the DPI of the final output). In a print, if you're able to print at an image size that's 300 DPI, it will appear sharp at any viewing distance (and is the exact definition of perceptual resolution). If you have to print larger, so that the image's native resolution goes to say 72 DPI, then the whole image will appear softer when you view it close up. Graphic artists will still print large banners at low DPI if the intended application is say a billboard. No one is going to get in close to visually see a blurry image.

I feel that's slightly orthogonal to the main discussion though; that DOF is the range of focus distances where the resulting print goes from perceptually sharp to perceptually soft.

It's certainly true though that insufficient pixel resolution for the print size and viewing distance may render a print too soft, regardless of what was in focus, and that a large print viewed close will have shallower perceived DOF.

My main point is that you could do all the DOF calculations based on camera settings, print size, viewing distance etc, and then confirm that the final print would have sufficient pixel resolution (if the image source was a digital camera). I guess there'd be comparable metrics for film grain/resolution; but I'm not familiar with that.

Azathoth wrote in post #17671929 (external link)
DOF depends only of three things:
- distance to subject;
- focal length of the lens;
- Aperture.

That's it.

...and all the other things that have been discussed on the thread, such as print size, viewing distance, visual acuity etc. Take a look at Circle of Confusion for more details.


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Aug 17, 2015 06:44 |  #43

Sirrith wrote in post #17671924 (external link)
You're cropping one photo. That invalidates your point. Try to reproduce the results by obtaining the same framing WITHOUT cropping either photo, only by changing lens/moving forward or backwards, and see what happens.

I think for most people this is all that really matters. If I want the subject to fill a certain area of the frame, and that is almost always my primary factor, I want it to fill that area at capture, not after cropping.

If I want the subject to fill the frame at 50 on a crop, but can use 85 on the FF, the 85 will show a different DOF with both at 1.8.


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Aug 17, 2015 06:48 |  #44

Left Handed Brisket wrote in post #17671945 (external link)
If I want the subject to fill the frame at 50 on a crop, but can use 85 on the FF, the 85 will show a different DOF with both at 1.8.

Very roughly; if you used 50mm on the APS-C and 85mm on the FF (for the same framing, at the same subject distance), f/1.8 on the FF would require approx f/1.1 on the APS-C to match the DOF. Just about doable with a 50mm f/1.2L I guess.


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Aug 17, 2015 06:50 as a reply to  @ SkipD's post |  #45

SkipD

Even in the most simplistic terms someone has to not see why keeping all variables the same in an example is needed.

True, if you only print small at 7x5in then resolution will make no difference.

However, if you view at 100% on a computer screen it will.

Lots of Nikon users suddenly found that when they moved from D700s to D800s they started to get 'less' DOF at the same aperture as before - simply because they had a larger size (12MP to 36MP) to view and the lack of focus was now noticeable where as it wasn't before.

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