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04-23-2014, 07:23 AM   #1
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Depth of field difference between FF & APSC

Hi this is my first post here in PF. I was just wondering why the DoF of a 50mm lens, lets assume set at f1.8 and focused at 10ft, is different between a FF camera and an APSC camera. Thanks in advance.

04-23-2014, 07:43 AM   #2
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QuoteOriginally posted by AtitG Quote
Hi this is my first post here in PF. I was just wondering why the DoF of a 50mm lens, lets assume set at f1.8 and focused at 10ft, is different between a FF camera and an APSC camera. Thanks in advance.
Digital Camera Sensor Sizes: How it Influences Your Photography
04-23-2014, 07:48 AM   #3
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QuoteOriginally posted by AtitG Quote
Hi this is my first post here in PF. I was just wondering why the DoF of a 50mm lens, lets assume set at f1.8 and focused at 10ft, is different between a FF camera and an APSC camera. Thanks in advance.
There are a lot of issues here... and many ways to approach this, all of which need some thought.

- You have to consider that you would have to back up with the APS-c to make the subject in the image the same size. As you back up DoF increases. (The closer you are to your subject, the less DoF you have.) To me, that's the easiest way to look at it. If both cameras have lenses at 10 feet and use the same lens, you get different Fields of View... so it's not really an equivalent image.

-The crop area within the 36x24 image will have exactly the same DoF as the uncropped APS-c image.. but you have to not need the rest of the image to make use of that. SO If you were actually going to crop your 36x24 image to the so you used only a 23x17 area of the sensor, then there would be no difference in DoF

- If you use a 35 mm on your APS-c and 50 on your 36x24, so you get the same FoV from 10 feet, it gets more complicated. You'll have more DoF with a 35 mm lens on APS-c ( a rough equivalent) than with a 50 (on a 35x24 sensor), but you have to enlarge the the APS-c image twice as much. In the enlarging, some areas which are in acceptable focus will become unacceptable, so you're going to lose some of the DoF you've achieved by using a wider lens.

As a general rule... the larger the sensor, the more choices you have... that being said, I've talked to wedding photographers who are shooting their outdoor and good light images with 4/3.. these days smaller sensors can be very capable... you just have to decide how much capability you need.

I
04-23-2014, 08:02 AM   #4
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Welcome to the PentaxForums! The difference is due to the size of the sensor. In simplest terms, there are a number of variables in the standard calculation for DOF. If all are kept constant except for media (frame) size, the DOF will vary according to that. In less simple terms...I don't think I really want to go there.


Steve


Last edited by stevebrot; 04-23-2014 at 08:08 AM.
04-23-2014, 08:16 AM   #5
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The simplest answer (for a Pentax w/ 1.5 cropped sensor) is to use the depth of field (DOF) for a film camera (FF sensor) at 1 stop more open (for the same focal length) --so it would be that of f/1.3 about. Better to use your using f/2.as the taking aperture. In this case the DOF would be that listed/shown on your lens (assuming it is an older FF lens) for f/1.4. (The correspondence is exact for a square root of 2 [1.414 cropped sensor]--which is essentially 1.5.)

Last edited by dms; 04-23-2014 at 08:24 AM.
04-23-2014, 08:22 AM   #6
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Just use a DoF calculator:

Online Depth of Field Calculator

A Flexible Depth of Field Calculator

There are also excellent apps for the same purpose.
04-23-2014, 09:14 AM   #7
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QuoteOriginally posted by normhead Quote

-The crop area within the 36x24 image will have exactly the same DoF as the uncropped APS-c image.. but you have to not need the rest of the image to make use of that. SO If you were actually going to crop your 36x24 image to the so you used only a 23x17 area of the sensor, then there would be no difference in DoF

I
This is what i thought also, but DoF calculators shows otherwise. Apparently as image is blown up by apsc sensors, sharpness decreases and DoF decreases as well.

Thank you very much everyone for the prompt replies.
04-23-2014, 09:24 AM   #8
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QuoteOriginally posted by AtitG Quote
This is what i thought also, but DoF calculators shows otherwise.
That is because DOF is based on the size of the final image (print or screen) as viewed at a standard distance by a person with average visual acuity. As I mentioned above, the non-simple answer is not something you want to try and read on a forum thread. The better DOF calculators are essentially correct.

It is enough to say that for a given field of view and final image size, a larger sensor (capture frame) will provide less DOF at a given aperture* than a smaller sensor. This is immediately obvious when moving from a miniature format (35mm and smaller) to something like 6x7cm medium format. With those larger formats adequate DOF becomes a serious issue.


Steve

* There are really only two variables, absolute aperture (not f-number) and magnification to the final image. Factor in the notion of acceptable blur and you have DOF. Of course, it does not really exist. The closer you pixel peep, the smaller DOF becomes to the point where you realize that there is only one true point of focus regardless of lens aperture.

04-23-2014, 09:26 AM   #9
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QuoteOriginally posted by AtitG Quote
This is what i thought also, but DoF calculators shows otherwise. Apparently as image is blown up by apsc sensors, sharpness decreases and DoF decreases as well.

Thank you very much everyone for the prompt replies.
They do that by blowing up the APS-c image to the same size as the 36x24 image, if you left it at half the size of the 36x24 image (the same magnification) , the DoF would be the same..... but that is irrelevant to your image. A comparison from one format to another is in no way informative. All that matters is that you get the image you want at the size you want. SO when I blow up my 16Mp k-5 image to 20x30 and I'm satisfied with that image, it makes no difference that I could blow a D800 image up to 35x50 at the same resolution. That is irrelevant information. And that is the weakness of the DoF calculator. It tells you what you an achieve, but it doesn't tell you if you can achieve what you need or want. And that changes from picture to picture.

And I can show you example images I took, where I used a 35mm and a 50mm from the same distance, and the 35 on APS-5 had twice the DoF for the same FoV. It's not that the DoF thing is wrong, it's that it's often inappropriately applied.

If your image looks good at 2700 lw/ph as achieved with a K-3 it matters not at all that you could have achieved 3700 lw/ph on a D800. While it's good to illustrate a concept, I wouldn't be trusting the DoF calculator to predict accurately what might be achieved with any set of real lenses. It's more a simplification to illustrate a concept. But it takes itself way too seriously, as do most of the people who mess with it.

Last edited by normhead; 04-23-2014 at 09:39 AM.
04-23-2014, 09:34 AM   #10
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Did a quick check (DOFmaster or something like that) and FF gave hyperfocal distance of 27 ft at f/16, vs Pentax cropped dslr of 24 ft at f/11. For practical results 27 and 24 feet are essentially the same. Anyway the dependence I mentioned comes straight out of the equations for hyperfocal distance (same dependence as DOF)--which I programmed--but don't have them at the moment.

---------- Post added 04-23-14 at 09:35 AM ----------

BTW you should realize I mean for same focal length lens--not same field of view on each camera.
04-23-2014, 09:43 AM   #11
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The interesting thing to me is that if you look at the available f-stops etc. 80-90% of what you can achieve on an 36x24 sensor you can achieve on an APS-c.(IN terms of DoF, not resolution or low noise capability.) You have to want that last 10-20% awful bad to pay the big bucks, unless you have unlimited resources, or collecting cameras is one of your hobbies.
04-23-2014, 09:54 AM   #12
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QuoteOriginally posted by normhead Quote

...In the enlarging, some areas which are in acceptable focus will become unacceptable, so you're going to lose some of the DoF...

I
This statement is the clincher for me. Now the DoF calculators make sense.

FF 33mm f1.8 @ 10ft = 3.0 ft
Apsc 33mm f1.8 @ 10ft = 1.9 ft
FF 50mm f2.8 @ 10ft = 2.0 ft
Apsc. 50mm f2.8 @ 10ft = 1.3 ft

Thanks again.
04-23-2014, 10:03 AM   #13
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QuoteOriginally posted by normhead Quote
look at the available f-stops etc.
This part always gets me. DOF calculations assume no restrictions on photographer to subject distance and don't factor in the physical characteristics of different lenses. Depth of field (which is a fuzzy concept, as pointed out by stevebrot) is a factor to be worked around in order to get acceptable sharpness in the subject and suitable contrast with the background. Optimize for everything else before optimizing for DOF.

Last edited by RGlasel; 04-23-2014 at 03:38 PM.
04-23-2014, 07:32 PM   #14
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Using DOF tables/etc. as a starting point "model/method"

I think some of the comments about DOF results (tables/programs based on a consistent set of assumptions) are "throwing the baby out w/ the bath water."

The basic idea is to have a "method" ("a model" or "set of assumptions") that attempts to account for all the important parameters--and if you find it is not adequate--you adjust the model. e.g., you use the DOF for 1 stop more open or 1 stop more closed.

The alternative is to not have a method and consider all the variables each time--or just give up and use nothing. The former is impossible for ordinary mortals and the latter means no control.
04-24-2014, 03:28 AM   #15
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QuoteOriginally posted by dms Quote
I think some of the comments about DOF results (tables/programs based on a consistent set of assumptions) are "throwing the baby out w/ the bath water."

The basic idea is to have a "method" ("a model" or "set of assumptions") that attempts to account for all the important parameters--and if you find it is not adequate--you adjust the model. e.g., you use the DOF for 1 stop more open or 1 stop more closed.

The alternative is to not have a method and consider all the variables each time--or just give up and use nothing. The former is impossible for ordinary mortals and the latter means no control.
+1
This is the best response to the whole depth of field discussion.

Fundamentally, the real problem with all DOF calculators is that they they all tie back to the concept that the image being viewed is based upon blowing the full frame up to an 8x10 inch print, viewed from reading distance of about 1 foot and having a point if light represented by a circle of 1/100 of an inch.

That translates back to the "circle of confusion" being 20 microns on an APS-C sensor or 30 microns on full frame.

But as soon as we crop, pixel peep or what ever, that concept is lost, so depth of field calculations don't really matter all that much, with a little work as you suggest, you establish your own rules, based upon the size you enlarge to, the amount of cropping etc.

What people often forget, with respect to the whole argument about depth of infield and it's relationship to hokeh, is the relative magnification of the background, when you change the shooting distance with a shorter lens on APS-C format. This changes the relative magnification of foreground to background, so the out of focus elements are smaller on APS-C than with full frame, making the hokeh less pleasing because you see what appear to be more defined and smaller noobs as opposed to the gradual merging of larger less apparent shapes into each other.

The larger the format the smaller the difference in magnification of subject and background and therefor the more pleasing hokeh, and more apparent separation if foreground to background.

It is all about understanding your format, not playing with little calculators
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