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DPChallenge Forums >> Photography Discussion >> DOF different? 35mm SLR Lenses vs DSLR only lenses
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Showing posts 1 - 9 of 9, (reverse)
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 09/09/2009 06:26:56 PM · #1
Hi all. I feel (find, perceive) that having moved from SLR to DSLR I get much shallower DOF with the SAME lenses used on my DSLR's than I did with them on my SLR's, especially in close-up or Macro. Are my eyes deceiving me or is this somehow due to the 1.5x picture angle difference due to the small(er) CMOS size of DSLR (23,6mm x 15.8mm vs 36mm x 24mm)?
Technical (mathematical) help will be appreciated, as is real world experience with your own gear. I will try and do a "lab" test when I get a gap to satisfy my own curiosity. Are the "budget" style lenses i.e. DC-Sigma(those intended only for use on small format DSLR's) giving better DOF than their 35mm counterparts i.e. DG-Sigma, when used on a smaller CMOS? One of the reasons I'm not keen to use DSLR only (budget) lenses is that the outer border area (peripheral brightness and MTF ((Modulation Transfer Function) is one of the measurements that evaluates a lens' performance, and it's contrast sensitivity at different spatial frequencies), drops off way more on "budget" DSLR lenses than the equivalent traditional 35mm SLR lenses when used on a DSLR.

Message edited by author 2009-09-10 04:54:26.
 09/09/2009 06:33:34 PM · #2
There is nothing about the glass that makes it different. The smaller sensor does have an impact, because for a give print size, you have a greater magnification of the recorded image to get to the final print size.
The biggest factor, though, is that you *notice* the narrower DoF more because the digital sensor records more detail in a given area of sensor compared to the same are of film, so the real-life CoC is smaller. When you view a digitally recorded image at 100%, you will see blur that is not detectable on a sample of the same image recorded with the same lens but with film.
 09/10/2009 03:43:48 AM · #3
Hi kirbic,

Thanks for your (rapid) response. What you say makes absolute sense to me. Using the same glass on SLR and then on a DSLR does make a noticable difference because of the difference in picture angle but to my (analogue) mind the SLR 35mm glass vs a newer DSLR (say a 50mm) glass of the same brightness should thus have less DOF because it is pronounced MORE due to the magnification of 1.5? Not so if you read on:

Here's a great page I found this morning regarding Understanding DOF with a great Depth of Field Calculator that calculates DOF using 1.6x, 1.5x, 1.3x Crop Factor and other formats.

I used their DOF calculator to determine the difference of DOF on 35mm vs DSLR (1.5x)
"Depth of field calculations ordinarily assume that a feature size of 0.01 inches is required for acceptable sharpness (as discussed earlier), however people with 20-20 vision can see features 1/3 this size. If you use the 0.01 inch standard of eyesight, understand that the edge of the depth of field may not appear acceptably sharp.
The depth of field calculator assumes this standard of eyesight", however they also provide a more flexible depth of field calculator. that also calculates (factors in) Print Dimension (relative to kirbic's explanation) which I am going to play with later and give more feedback here later...

Camera Type:: 35mm
Selected Aperture F11
Actual lens focal length: 50mm
Focus distance (to subject): 1 meter
0.882 m Closest distance of acceptable sharpness
1.154 m Furthest distance of acceptable sharpness
0.272 m Total Depth of Field (MORE DOF than DSLR)

Camera Type: digital SLR with CF (Crop Factor) of 1.5x
Selected Aperture: F11
Actual lens focal length: 50mm
Focus distance (to subject): 1 meter
0.919 m Closest distance of acceptable sharpness
1.097 m Furthest distance of acceptable sharpness
0.178 m Total Depth of Field (LESS DOF than SLR!!)

Then I bumped focus distance to 10 metres to see what happens. Very interesting result indeed!

Camera Type:: 35mm
Selected Aperture F11
Actual lens focal length: 50mm
Focus distance (to subject): 10 meters
4.165 m Closest distance of acceptable sharpness
Infinity m Furthest distance of acceptable sharpness
Infinite m Total Depth of Field (MORE DOF than DSLR)

Camera Type: digital SLR with CF (Crop Factor) of 1.5x
Selected Aperture: F11
Actual lens focal length: 50mm
Focus distance (to subject): 10 meters
5.202 m Closest distance of acceptable sharpness
128.94 m Furthest distance of acceptable sharpness
123.738 m Total Depth of Field (LESS DOF than SLR!!)

This shows that the same lens from a 35mm gives MORE DOF on a 35mm camera than if used on a (Small Format) DSLR with 1.5x picture angle, so my old eyes weren't deceiving me! This off course is good news if you are buying 2nd hand 35mm glass for your DSLR. Cheaper and better!

Use it, don't use it :)

PS: Would this thread be worth adding to "Tutorials" so that it doesn't get lost in time?
PPS: I am NO expert, if I have made an error and you can explain it better than I, please do so.

Message edited by author 2009-09-10 05:15:39.
 09/10/2009 06:26:11 AM · #4
You need to be *very* careful about the assumptions you make when calculating DoF, specifically how the CoC is determined. I'll just say this, the CoC used by many DoF calculators is less than optimal. I won't call it wrong, but I will say that if you use it for calculating hyperfocal distance and think you'll get sharp images of objects at infinity (which the calculation suggests you should), you'd be wrong.
Depending on the assumptions you make, you will find that the results can indicate either more, or less DoF with a digital crop body than 35mm film. In each case, the calculation is correct, if the assumptions are valid.
I'll leave you with this thought exercise:
Assume you take an adequately sharp lens and mount it to a tripod. You set the focus and aperture manually. Now you mount first a film body, and then a digital body to the back of the lens. The image projected by the lens on the film, then the digital sensor, is precisely the same (assuming, of course, that the register distances are accurate in both cases). It's only how the image is captured that differs. The lens makes *no* difference in DoF, it's how the image is recorded, and what you do with it afterward.
09/10/2009 10:06:54 AM · #5
It would make a lot more sense, and probably help to clear things up for you to beg, borrow, buy, or steal a full frame DSLR for the comparison using the same lenses.

Though I don't recommend stealing one......8>)
 09/10/2009 02:42:26 PM · #6
The way I understand it is that a fixed focus P&S works because a very wide angle lens can be used, and shorter focal lengths have greater DOF. The sensor on these types of camera is small and you get a small angle of view.

I've heard that MF and larger formats have even smaller DOFs than a FF (D)SLR

One example is that a 50mm on a 5D has a smaller DOF than the same lens on a 1.6 crop (at the same aperture).
 09/10/2009 05:10:10 PM · #7
FF and APS-c have the same DAMN DoF. The old saying, "APS-C is like getting a free 1.5 or 1.6 teleconverter" is a shameful marketing ploy for saying you get 50% - 60% less FoV.
The reason you see less Dof with FF is due to more FoV, you can get physically closer or zoom in to your subject to fill the frame. This causes your DoF to much more shallow than APS-C or 4/3 system.

I am going to get some coffee now.
 09/10/2009 06:24:17 PM · #8
Originally posted by hankk:
The way I understand it is that a fixed focus P&S works because a very wide angle lens can be used, and shorter focal lengths have greater DOF. The sensor on these types of camera is small and you get a small angle of view.


Yes, in general the smaller the focal length, the greater the DoF, given that the Av and angle of view are constant. Example: for a typical P&S, the focal length equivalent for a "50mm-like" angle of view (about 45 degrees) might be about 7 or 8mm. That's tiny. If you calculate the DoF for an 8mm lens at f/4, it's big, even considering the very small CoC for the sensor required.

Originally posted by hankk:
I've heard that MF and larger formats have even smaller DOFs than a FF (D)SLR


Again, typically yes, because the focal lengths are longer for similar angles of view

Originally posted by hankk:
One example is that a 50mm on a 5D has a smaller DOF than the same lens on a 1.6 crop (at the same aperture).


Careful here, it really depends on what constraints you put on the comparison. If the AV, CoC and subject distance are the same, then the DoF is the same, period. For other sets of assumptions, e.g. equal framing instead of equal subject distance) the results will be different. The results also depend entirely on your assumptions on how to calculate CoC. Don't get me started there.
 09/10/2009 06:26:18 PM · #9
Originally posted by ben4345:
FF and APS-c have the same DAMN DoF. ...


From a practical perspective, usually not, no, even though the image projected by the lens is exactly the same!
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