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10-23-2013, 09:09 AM   #1
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DOF with FFs and APS-C

I've been playing around with this link:

Online Depth of Field Calculator

For example, when I calculate DOF with the Nikon D7000, I have less DOF than I do with a Nikon FF with the same settings. I thought I have been reading around here that FFs have a narrower DOF. Please explain.

10-23-2013, 09:15 AM   #2
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QuoteOriginally posted by traderdrew Quote
I've been playing around with this link:

Online Depth of Field Calculator

For example, when I calculate DOF with the Nikon D7000, I have less DOF than I do with a Nikon FF with the same settings. I thought I have been reading around here that FFs have a narrower DOF. Please explain.
APS-C is just a crop of the FF image circle. Say you put the same 50mm lens on a FF camera and an APS-C camera, and focus it on the same subject: the background blur will be identical.

Based on this logic, we can conclude that focal length is the key factor here. Because FF requires longer focal lengths to get the same field of view as APS-C, you get less DOF and therefore more background blur at the same field of view on FF compared to APS-C. Hope that makes sense!

Perhaps a practical example will help clarify: a 16mm lens on APS-C gives you the same field of view as a 24mm on FF. But 16mm lenses have more depth of field than 24mm lenses, so you can potentially get nicer bokeh with the 24mm. Conversely, if you want more of the frame to be in focus at all times, APS-C has an edge.

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10-23-2013, 09:17 AM   #3
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Are you using the same focal length for each sensor size? DOF should be pretty much constant for the same lens on different sensor sizes.

Try DOF with a 50mm on a FF and, say, a 35mm lens on a APS-C camera. Because these two combos have approximately the same field of view.
10-23-2013, 09:31 AM   #4
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QuoteOriginally posted by traderdrew Quote
I've been playing around with this link:

Online Depth of Field Calculator

For example, when I calculate DOF with the Nikon D7000, I have less DOF than I do with a Nikon FF with the same settings. I thought I have been reading around here that FFs have a narrower DOF. Please explain.
Yes, it is very confusing because all of the assumptions people are using when they make statements like that are rarely explicitly stated.

So people say things like "FF = less DOF" which makes one think that if you pop the same lens on an FF camera, suddenly the DOF will shrink. But in that case it is actually the smaller sensored APS-C image that will have less DOF, and even that is only because of another assumption -- that the resulting print sizes will be the same, which means you're enlarging the APS-C image more, so stuff needs to be sharper/more in-focus to gain the equivalent DOF.

But what people really mean about FF having less DOF is using completely different lenses on the different formats (but "equivalent") so you're taking the exact same picture framing-wise/field-of-view. Now, if you use the same aperture on both lenses, the FF will have thinner DOF. In order to take an exactly equivalent picture between the two formats, you stop down the FF more, so a picture taken with a 35mm lens at f/2.8 on APS-C will be roughly the same as a picture taken with a 50mm lens at f/4 on a FF. But while it is likely that your 35mm lens is already at the widest setting at f/2.8, surely the 50mm can be opened up another stop or two to get thinner DOF on the FF. And that's where that discussion often goes -- the lenses simply don't exist to get equivalently thin DOF images on an APS-C body with the same field of view as you can achieve with FF.

10-23-2013, 09:58 AM   #5
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This is a common problem with DOF calculators. The calculator is accurate, but not across formats. I made several tries at giving a concise explanation as to why this is so, but each time I found that the explanation grew too long. The short answer is that FOV should be the constant for comparison rather than distance to subject.

I know the text says that crop factor is corrected for, but if that were true DOF would be the same for a given shooting distance across all formats.

There is a more complicated calculator on the Web that factors in final image size and viewing distance and which allows for a proper comparison across formats, but I could not find the link. If don't mind doing the math, the Wikipedia article is helpful.

https://en.wikipedia.org/wiki/Depth_of_field

The specifics for comparison across formats are here:

https://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size


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10-23-2013, 10:03 AM   #6
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I'll try to explain it in my own way.

FF has more control over DOF because lenses that have the same FOV as APSC will have longer FL in FF, therefore producing less DOF. A 50mm lens in FF will have less dof than a 35mm lens in APSC, even when aperture and focusing distance is the same.

This is compounded with the fact that under 100mm, longer FL lenses are typically easier and cheaper to get in wide aperture flavor. A 24mm/1.4 doesnt exist, while 35mm/2 is cheap and plentiful; 50mm/1.2 is rarer and more expensive than 85/1.8, etc etc.

OTOH longer FL lenses typically have longer MFD, so if you like focusing close on small objects, maybe you'll lose something when using FF. For people shots I never seem to get anywhere close to MFD on any lens.

If you want thin DOF, FF is the way to go.

Last edited by Andi Lo; 10-23-2013 at 10:10 AM.
10-23-2013, 10:05 AM   #7
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QuoteOriginally posted by tuco Quote
Try DOF with a 50mm on a FF and, say, a 35mm lens on a APS-C camera. Because these two combos have approximately the same field of view.
Thanks for this suggestion! It provides the best answer.

Alternatively, I guess a person could simply spend some time with a larger format camera. The difference is immediately apparent when you move up to medium or large format and noticeable when using 35mm FF.


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10-23-2013, 10:13 AM   #8
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To boil it all down (from the Wikipedia article):

"DOF is determined by subject magnification at the film / sensor plane and the selected lens aperture..."

FF requires more magnification for a given FOV, hence delivers less DOF.

To add to that, DOF as perceived is influenced by final magnification and viewing distance. For example, consider my avatar image. Now consider the larger version of the same from my Flickr account. Both had the same DOF on the sensor.




Now back up a few feet from the monitor...


Steve

10-23-2013, 10:24 AM   #9
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To make it a bit more complicated.... you have to actually take the picture to see difference in DOF as today's viewfinder is unable to show the effects of aperture wider than 2.8.

If you dont have access to an FF digital, just bring your SD card to a store and test a 50mm lens at various apertures at a moderate focusing distance, then shoot the same with an APSC and 35mm.

You can test it just with apsc through cropping as well, since really, that's all APSC does., but I think the effect is more appreciable when you have the full picture.
10-23-2013, 10:34 AM   #10
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What Adam said.

Personally I don't believe much of what I read. SO I once went out and from the same distance and f stop. I shot one 50 mm shot to represent full frame, and one 35mm image to represent APS-c with a ruler in the frame. The difference was about twice as much DoF for the 35mm lens as the 50. APS-c image 12mm DoF, representative FF image 6 mm. Even after correcting the magnification so both images were the same size. To me, that's all you need to know. You get more DoF with APS-c. Others may quote all kinds of things to show you other takes on it, but the bottom line for me is what I see with my eyes and camera, not what some guy thinks based on various theories and tables.

Not only that , the original image I shot with the 35, was too close for the 50 to focus. You may be able to achieve focus using APS-c you couldn't even achieve with FF. If you are into hyper-focal images as opposed to narrow DoF images, APS-c is defintiely the way to go.
10-23-2013, 10:45 AM   #11
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People might be making this to complicated.

A 50mm lens is always going to be a 50mm lens no matter what sensor you have behind it. DoF will be the same if you maintain the same working distance and aperture. Of course if you maintain the same working distance you have a much smaller FoV. If you back away from the subject to maintain the same FoV then you are increasing the DoF.

The reason the DoF calculator is showing a narrower DoF for APS-C is because of the compression that occurs when cropping/magnification occur to equalize for final output.

The reverse way to look at it is that with a FF 50mm lens I have to get closer to my subject to get the same FoV. As I get closer the DoF will get narrower so I have to stop down to maintain the same DoF as i get closer.
10-23-2013, 11:25 AM   #12
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There is one stop less depth of field with full frame than with APS-C. Basically, if you are shooting with a 55mm at f2.8 you will get an equivalent photo with an 85mm on full frame shot at f4. This assumes that you want to keep your framing the same between the formats. If, however, you shoot a 55 mm at f2.8 on APS-C and on full frame and keep your shooting distance the same, your photo will look very different and while the APS-C image may have narrower depth of field, it probably won't be useful.
10-23-2013, 11:48 AM   #13
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QuoteOriginally posted by Rondec Quote
There is one stop less depth of field with full frame than with APS-C. Basically, if you are shooting with a 55mm at f2.8 you will get an equivalent photo with an 85mm on full frame shot at f4. This assumes that you want to keep your framing the same between the formats. If, however, you shoot a 55 mm at f2.8 on APS-C and on full frame and keep your shooting distance the same, your photo will look very different and while the APS-C image may have narrower depth of field, it probably won't be useful.
This is where folks lose me... first thing is, very few of my 2.8 APS-c images are useful, so I'm guessing very few of my ƒ4 images would be useful on FF. I shot 100 images today, at ƒ2.8, ƒ5/6, ƒ11 and ƒ22. All the keepers were either ƒ5.6 or ƒ11. SO really, FF or APS-c for everyone of those shots doesn't matter. You have the leeway to get the DoF you want in either system.

And... you can't anticipate which system and image will favor. With existing light, you may come to the conclusion that your ƒ11 APS-c image is the best. Going to 16 MP FF is going to introduce diffraction. But ƒ11 FF may not give you enough DoF. For any given image, you may have a situation that favours one system or the other. But this is made on a case by case basis, based not on what size the sensor is per se, but what the unique characteristics of that format is.

Until you realize, different doesn't mean better.... If you are a hyper-focal, macro, or telephoto, APS-c is your friend. If you are a narrow DoF shooter, not so much.

QuoteQuote:
your photo will look very different and while the APS-C image may have narrower depth of field, it probably won't be useful.
I'd say, much of the time you don't know what's going to be useful and what isn't in advance. You just go with what you have and make do. And you certainly don't know what system is going to have the advantage except in extreme cases. But as in my first example. most of the time it's not going to make much difference. Certainly not as much as some folks around here let on.

Last edited by normhead; 10-23-2013 at 11:55 AM.
10-23-2013, 12:00 PM   #14
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QuoteOriginally posted by normhead Quote
What Adam said.

Personally I don't believe much of what I read. SO I once went out and from the same distance and f stop. I shot one 50 mm shot to represent full frame, and one 35mm image to represent APS-c with a ruler in the frame. The difference was about twice as much DoF for the 35mm lens as the 50. APS-c image 12mm DoF, representative FF image 6 mm. Even after correcting the magnification so both images were the same size. To me, that's all you need to know. You get more DoF with APS-c. Others may quote all kinds of things to show you other takes on it, but the bottom line for me is what I see with my eyes and camera, not what some guy thinks based on various theories and tables.

Not only that , the original image I shot with the 35, was too close for the 50 to focus. You may be able to achieve focus using APS-c you couldn't even achieve with FF. If you are into hyper-focal images as opposed to narrow DoF images, APS-c is defintiely the way to go.
Frankly, I'm appalled at your methods. Taking pictures and relying on your own judgement to decide what works for you? Shocking!

Other than that, my own observations coincide with yours. APS-C offers some advantages in terms of depth of field for a given field of view, if you want lots of DOF- which is important to me as a landscape photographer. On the other hand, at the moment FF offers higher resolution which is important to me as a landscape photographer who makes large prints.

I'm not going to get too complicated about the tradeoffs involved in increased DOF and apparent sharpness vs diffraction effects and viewing distance. I do know that by closing down a stop or two on full frame I can come close to the DOF of APS-C, and have a bigger file to boot. From experience, I know that with 3-dimensional subjects gains in apparent sharpness with increased DOF generally outweigh diffraction effects. I usually work on a tripod, so shutter speeds don't matter much.

I did a lot of FF DSLR work before retiring from my old job, so I'm quite confident about what tradeoffs I want to make for my own purposes.

I work quite happily with APS-C, but ultimately will move to full frame for larger prints. I'm sure you are able to make very good large prints from APS-C files. I know I can. However, I would like to take quality up another notch.

What it really comes down to is different horses for different courses.
10-23-2013, 12:03 PM   #15
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QuoteOriginally posted by normhead Quote
This is where folks lose me... first thing is, very few of my 2.8 APS-c images are useful, so I'm guessing very few of my 4 images would be useful on FF. I shot 100 images today, at 2.8, 5/6, 11 and 22. All the keepers were either 5.6 or 11. SO really, FF or APS-c for everyone of those shots doesn't matter. You have the leeway to get the DoF you want in either system.

And... you can't anticipate which system and image will favor. With existing light, you may come to the conclusion that your 11 APS-c image is the best. Going to 16 MP FF is going to introduce diffraction. But 11 FF may not give you enough DoF. For any given image, you may have a situation that favours one system or the other. But this is made on a case by case basis, based not on what size the sensor is per se, but what the unique characteristics of that format is.

Until you realize, different doesn't mean better.... If you are a hyper-focal, macro, or telephoto, APS-c is your friend. If you are a narrow DoF shooter, not so much.



I'd say, much of the time you don't know what's going to be useful and what isn't in advance. You just go with what you have and make do. And you certainly don't know what system is going to have the advantage except in extreme cases. But as in my first example. most of the time it's not going to make much difference. Certainly not as much as some folks around here let on.
My point was not about which system is preferrable, it has to do with the difference in what you see in practice. I agree. I shoot a lot of landscapes and if I am at f8 with my DA 15, I know that on full frame I would need to be at f11 on a 22mm lens to get the same photo. Not a big deal in either system.

I do think the pursuit of narrow depth of field has gotten a little silly and I am bothered, frankly, by portraits in which half of a person's features are out of focus or soft. But each to their own...
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