Matching depth of field between formats

ParkerSmithPhoto · May 6, 2013

Here's a puzzler for you, or maybe just for me:

I shoot almost all medium format, but due to a need to a) keep expenses down; b) shoot more frames; and, c) get a bit more tooth and grain in the prints, I wanted to shoot 35mm, but match the shallow depth of field I get with my RZ. Now, assuming that you had normal lenses on both cameras, is there a scientific way (other than trial and error) that one would go about matching DOF?

For example, if I am focused on a subject five feet away with the RZ with a 90mm lens, and I switch to my Canon with a 50mm lens and maintain the same distance, is there an equation of some sort that would convert f5.6 on the RZ to f-whatever on the Canon?

Thanks in advance to youse!

revdocjim · May 7, 2013

Not sure if this is scientific enough but it works!
https://play.google.com/store/apps/details?id=com.aimenrg.dof&hl=en

piu58 · May 7, 2013

My simple rule: A lens with the same image angle should be opened two stops using 35 mm.
I got this by comparing DOF markings at the lenses.

Bill Burk · May 7, 2013

I don't have a scientific basis for it... I shot a couple comparison negs to compare 4x5 to 35mm, but I haven't printed them yet. So I have not come to a conclusion yet. My gut tells me piu58 is probably right... open two stops to f/2.8 and you will likely get similar DOF.

Tom1956 · May 7, 2013

I bet Ralph Lambrecht already did a table on that.

polyglot · May 7, 2013

It's very simple: you compare the physical size of the aperture. If they match, the DOF will match for the same framing.

So 90mm/5.6 = 16mm, which would be the same as 50mm f/3.1 (because 50/3.1=16).

For conversion between 6x7 and 35mm, a factor of two is a good approximation; the only issue is that the aspect ratios differ. So 90mm f/5.6 is equivalent to 45mm f/2.8, 110/2.8 = 55/1.4, etc.

dorff · May 7, 2013

polyglot said:
It's very simple: you compare the physical size of the aperture. If they match, the DOF will match for the same framing.

So 90mm/5.6 = 16mm, which would be the same as 50mm f/3.1 (because 50/3.1=16).

For conversion between 6x7 and 35mm, a factor of two is a good approximation; the only issue is that the aspect ratios differ. So 90mm f/5.6 is equivalent to 45mm f/2.8, 110/2.8 = 55/1.4, etc.

+1. To make it clear, this refers to the DOF you will see in the OUTPUT, i.e. if you print to the same size paper from different negative sizes. On the film, the DOF is purely dependent on the f-stop and nothing else. One thing to also remember, though, is that lenses are not symmetrical in DOF either side of the focus plane, unless they are ideal lenses (with a P-value of 1). Some lenses are designed to give more DOF behind the focus plane than in front, and vice versa. So you have to use similar lenses for the different formats if you truly want matched DOF. My own take is that I prefer lenses with good bokeh, and do not obsess over DOF, unless it is for instance landscape photography. I just shoot my lenses where I know they perform well, and where I get the shutter speed that works for the situation.

I can't explain it better than Paul van Walree: http://toothwalker.org/optics.html. I suggest you visit his site and have a good read. The other characteristics of optics are also explained. Interesting stuff.

pdeeh · May 7, 2013

http://dofmaster.com/ is a useful resource

polyglot · May 7, 2013

dorff said:
I can't explain it better than Paul van Walree: http://toothwalker.org/optics.html. I suggest you visit his site and have a good read. The other characteristics of optics are also explained. Interesting stuff.

doublepluss on Van Walree. It's an excellent reference that explains all the major issues with lenses nicely.

Tom Richard · May 7, 2013

Thank you so much for the Paul Van Walree link. Brilliant!

ParkerSmithPhoto · May 7, 2013

Thanks everyone. Very much appreciated!

Alan Gales · May 7, 2013

polyglot said:
doublepluss on Van Walree. It's an excellent reference that explains all the major issues with lenses nicely.

Very interesting! I have not seen this before.

Thanks for sharing, Polyglot.

polyglot · May 7, 2013

'twasn't me that posted the link here! I'm merely agreeing with it. It's one of the references listed in my FAQ.

narcosynthesis · May 10, 2013

A bit of playing around on www.dofmaster.com should tell you what you need - note the depth of field of your medium format at the settings you use, then input your 35mm setup and adjust the options until you get a similar dof

wiltw · May 10, 2013

First one needs to start off with a common basis for FOV, negating the differences in aspect ratio...

135 format using 48mm as lens on 135 format and
RZ67 using 112mm on RZ67
4x5 format using 192mm on 4x5 format

(all three equate to 2 * short dimension of frame)...

Using the DOF calculator available on Cambridge Color web site, and using 20/20 visual acuity (rather than the much more tolerant 'manufacturer standard')

48mm f/5.6 focused at 100' yields DOF zone 67'-197'
112mm f/8 focused at 100' yields DOF zone of 69'-179'; f/11 DOF zone 62'-252'
192mm f/16 focused at 100' yields DOF zone 66'-211'

Matching depth of field between formats

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