# Matching depth of field between formats

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• 05-06-2013, 11:38 PM
ParkerSmithPhoto
Matching depth of field between formats
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?

• 05-07-2013, 12:04 AM
revdocjim
Not sure if this is scientific enough but it works!
• 05-07-2013, 12:21 AM
piu58
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.
• 05-07-2013, 12:33 AM
Bill Burk
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.
• 05-07-2013, 12:39 AM
Tom1956
I bet Ralph Lambrecht already did a table on that.
• 05-07-2013, 01:03 AM
polyglot
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.
• 05-07-2013, 03:01 AM
dorff
Quote:

Originally Posted by polyglot
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.
• 05-07-2013, 04:32 AM
pdeeh
http://dofmaster.com/ is a useful resource
• 05-07-2013, 05:29 AM
polyglot
Quote:

Originally Posted by dorff
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.
• 05-07-2013, 07:49 AM
Tom Richard
Thank you so much for the Paul Van Walree link. Brilliant!
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