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FORUMS General Gear Talk Camera Vs. Camera 
Thread started 05 Nov 2015 (Thursday) 22:06
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APS-C vs FF Depth of Field - Solved!

 
n1as
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Nov 06, 2015 08:56 |  #16

Archibald wrote in post #17773999 (external link)
If your objective is to have more background blur, then FF is your friend. FF has shallower inherent DOF, plus there are more wide aperture EF lenses available than EF-S lenses.

Which makes it sound like FF (i.e. the larger sensor) somehow impacts DOF all by itself.

Archibald wrote in post #17773999 (external link)
There is a useful rule of thumb that is worth noting, but which boggles the minds of many. The rule is that for equivalent framing, the DOF depends only on format (MF or FF or crop, etc) and aperture. Check it out. (Don't forget, though - equivalent framing!!)

Now there ya go! That "equivalent framing" part says that you can't just change sensor size without changing the lens. So my quick test was to figure out which of those 2 changes actually affects DOF.

Archibald wrote in post #17773999 (external link)
The other thing useful to note is that longer lenses, even though they give the same DOF as wider lenses (with equivalent framing), give a more pleasing bokeh because of the perspective change.

I think I disagree here on a somewhat minor but important part. Longer lenses give shallower DOF than wide lenses "at the same perspective" regardless of the sensor size. Stand in one place, shoot the same scene with 2 cameras, 1 lens and you get the same DOF. It isn't the sensor, it is the lens and the perspective.

JeffreyG wrote in post #17774093 (external link)
Once you crop the FF image to use a sensor that is exactly the same size as the 1.6X format you are comparing it to, the camera isn't (functionally) a FF camera any more. Put another way....a FF camera doesn't make a shallower DOF image just because there is a FF sensor in there someplace.

Thanks JeffreyG, this is my point.

JeffreyG wrote in post #17774093 (external link)
You just compared a 1.6X format camera to a 1.6X format camera and discovered that they make the same image. Don't slap yourself in the forehead too hard with this eureka moment.

Cropped FF vs APS-C was the purpose of the test. I was wanting to confirm what happens to DOF in the focal-length limited scenario which is where I often find myself while shooting sports.

And the eureka moment was mostly a tongue in cheek statement. I forgot the smily face in the thread title! :oops:


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n1as
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Nov 06, 2015 09:04 |  #17

BigAl007 wrote in post #17774096 (external link)
Depth of Field under the SPECIFIED VIEWING CONDITIONS of any photographic image is actually only dependent on TWO factors. The first is the Absolute Reproduction Ratio, which is the ratio of the size of the subject (at the focused distance) in real life, to the size of the subject in the final image (usually specified as a print). The second dependency is to the absolute effective diameter of the aperture.

Alan

Unfortunately both of those main drivers are obscure and not something we directly control. We have focal lengths, f-stops, shooting distances and fields of view to play with.

Although it does bring an interesting point to the surface. When I used the 85mm lens on both cameras, the FF image was a wider field of view. That meant the absolute reproduction ratio (ARR) was smaller and the DOF would be larger. When I cropped the FF image to match the field of view from the APS-C image, I brought both images to the same ARR, thus ensuring the same DOF since they were shot with the same lens / same effective aperture diameter.

Thanks for the info!


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Wilt
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Post edited over 4 years ago by Wilt. (8 edits in all)
     
Nov 06, 2015 09:20 |  #18

n1as wrote in post #17773980 (external link)
Well the root question I was trying to answer is how does crop factor affect DoF in the focal length limited scenario. Stand in the same place, shoot with the same lens, crop the FF image because the lens is too short. Does FF still give less DoF. The answer is "no".

I am thinking the DoF advantage of FF diminishes as you crop and is gone once you crop the FF image by a factor of 1.6 as I did in my 2nd example.

Yes you are correct in a principle that is NOT understood by folks...that DOF calculators assume the printing of the FULL AREA of the frame of the taking camera. If you took an APS film camera with 50mm lens, and you took a FF film camera with 50mm lens, shot from the same spot and THEN CROPPED the 36x24mm film frame down to 25.1 × 16.7 mm and printed a 16x20" print from both, the resultant DOF on the final print is indeed identical from the two cameras...because the so-called FF shot is actually an APS format shot when cropped in the printing of the 16x20" !

So many folks 'crop off the annoying/distracting parts of the frame' when post processing, and do not consider that the DOF of the final 8"x12" print from that crop is NOT WHAT THE DOF CALCULATOR said it would be, because the additional magnification to make the print from a smaller starting area CHANGES the DOF of the shot!

Archibald wrote:
Certainly cropping a FF image is equivalent to using a crop frame. That's why they are called crop cameras.

^
My previous statements earlier is this post reflect Archibald's statement.

n1as wrote:
Now there ya go! That "equivalent framing" part says that you can't just change sensor size without changing the lens. So my quick test was to figure out which of those 2 changes actually affects DOF.

And I said that if you


  1. use a lens FL which is proportional the the frame size difference, AND
  2. use a shooting aperture with that (above) lens which is proportionally (inversely) smaller

your DOF and your degree of background blur will be identical, regardless of the format size

n1as wrote:
I think I disagree here on a somewhat minor but important part. Longer lenses give shallower DOF than wide lenses "at the same perspective" regardless of the sensor size. Stand in one place, shoot the same scene with 2 cameras, 1 lens and you get the same DOF. It isn't the sensor, it is the lens and the perspective.

Not quite right. If 1) the framing of the subject is identical it is the SIZE of the subject within the framed area which is the ultimate determinant of DOF...for a given format. When format size differs, the 3) aperture size needs to be proportionally different, as well as 4) the FL of the taking lens, too, though!


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Nov 06, 2015 11:23 |  #19

BigAl007 wrote in post #17774096 (external link)
Depth of Field under the SPECIFIED VIEWING CONDITIONS of any photographic image is actually only dependent on TWO factors. The first is the Absolute Reproduction Ratio, which is the ratio of the size of the subject (at the focused distance) in real life, to the size of the subject in the final image (usually specified as a print). The second dependency is to the absolute effective diameter of the aperture. The effective aperture diameter is the size the aperture appears to be from the sensor side of the lens. The important thing to remember is that we are using the actual physical diameter, not the f/number. In both cases, reproduction ratio, and aperture they are actually inversely proportional. So as both the reproduction ratio, and the aperture diameter get smaller, the DoF will get larger.

It is also important to remember that the specified viewing conditions are very important, probably more so than either of the two variable proportionalities. So changing the viewing conditions is likely to have a far more profound effect on DoF than changing either the reproduction ratio, or the aperture diameter. Also of note is the fact that all of the things that are usually considered when calculating the DoF, such as Sensor size, focal length, focal distance f/number etc, are just convenient ways of pre-defining the reproduction ratio, and aperture diameter, based on the equipment that is in current use.

I think this is a pretty good summary of the factors that influence DOF.

DOF is just the result of geometry. A bundle of light emanating from the lens and focusing on the sensor is in the shape of a cone. The base of the cone is at the lens - more specifically at the exit pupil of the lens. The apex of the cone is on the sensor. Now if the distance between lens and sensor is changed, then the sensor intersects the cone at a place other than the apex. This forms a small circle, and is the blurriness we observe. So DOF and blurriness is just the result of this geometry.

Thin cones give greater DOF than fat cones, as a consequence of geometry. Fat cones come from having large exit pupils relatively close to the sensor. *

I realize this is all a bit geeky, but for some of us, it is interesting to see how DOF works. This little analysis also says that DOF depends on the relative size of the exit pupil, and not on the absolute effective diameter of the aperture. The exit pupil, as far as I understand it, is the apparent, not actual diameter of the aperture.

* (Fat cones mean large apertures and short focal lengths. So large apertures give less DOF, as we know. And wide angle lenses have less DOF :-) at equal lens extensions, and is another example of how one can baffle people with DOF.)


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John ­ from ­ PA
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Nov 06, 2015 14:30 |  #20

kawi_200 wrote in post #17773945 (external link)
This perfectly proves the point that the 1.6x crop factor "gives you more reach" is complete BS. Like saying that 250mm on crop is the same as 400mm on FF. It may frame the same, but it is not the same. Just like in this example 85mm = 85mm = 85mm, it doesn't matter the camera it is on.

Define what you think is "reach". If I have a cropped body with a 250mm on my left shoulder and a 400mm on FF on my right, and look at the same subject they essentially will appear the same in the viewfinder. But if I have both cameras with the same focal length lens, say the 250mm, is it wrong to say under those circumstances that the cropped body "has more reach"? What would be your choice of words, besides of course "complete BS."

For those that want to play with the numbers, there is a good online depth of field calculator at http://www.dofmaster.c​om/dofjs.html (external link).




  
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Nov 06, 2015 15:22 |  #21

John from PA wrote in post #17774553 (external link)
For those that want to play with the numbers, there is a good online depth of field calculator at http://www.dofmaster.c​om/dofjs.html (external link).

Unfortunately this calculator and most of the others I know about won't work at macro distances, where DOF is very shallow and where DOF numbers would be very useful.

It would be nice to have a macro DOF calculator for my phone. Such a calculator would need to have specific parameters for the different macro lens models, because their focal lengths and exit pupils changes with magnification, affecting the DOF.


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Nov 06, 2015 22:23 |  #22

EverydayGetaway wrote in post #17774001 (external link)
It still gives you more pixels on target with most cameras. Now that camera's like the 5DS and a7Rii are around it's less of an advantage, but those cameras also don't give you the same type of performance that a pro grade APS-C DSLR like the 7Dii would give you, thus the APS-C camera giving you "more reach" is still true and relevant imo.

Sensor size and "reach" is a complete red herring. It's the sensel, and thus pixel, density that matters. In the Canon DSLR ranges, the 20D/30D bodies have around the same "reach" as the 1DsIII/5DII/5DIII bodies for example, or even the 5DS and the 7DII. For the 5DS/7DII other than file size, buffer depth and FPS there is not a lot to separate the two cameras features wise. If you do not really need more than 5FPS and twenty odd RAW shots in a burst and cost is not an object, then the 5DS could indeed be a better choice camera for many.

If the 5DS and the 7DII both had the same high ISO options, I would expect that they would both have about the same pixel level noise characteristics, although the 5DS would still have the 2.56× sensor size advantage in total image noise.

Personally for my needs, 5FPS and the buffer size of the 5DS would be ample, so even though I am often very FL limited shooting at 600mm I would pick the 5DS over the 7DII. I would have (just about) the same pixel density to crop into when FL limited, ans two and a half times more sensor area to fill with the lens when it is not FL limited. Whats not to like in that situation?

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Nov 07, 2015 06:51 as a reply to  @ BigAl007's post |  #23

I agree, it's the continual dealing with huge files, limited AF, 3fps, and lag of viewing images that holds me back from the latest offerings. Once a few of these are mitigated and i get a new laptop, I will then pick one up myself. With the latest stm lenses though, good IQ at a very reasonable price is one very good advantage of crops today.


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APS-C vs FF Depth of Field - Solved!
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