Depth of Field question...

[vodid]

I'm trying to do a little project, adding a closeup diopter to a Horizon 202 panorama camera. It's a fixed focus 28mm f2.8 lens, and using small apertures is the only way one can focus anywhere near close...so this project necessarily involves depth of field. I figured I'd just do the tests (in order to determine the focusing range with the added closeup diopter) by using a 28mm Nikon lens on one of my old Nikon SLR's, and placing the diopter in front of the Nikon 28mm lens in order to check the distances that were in focus, figuring it would be the same.

Out of curiousity, however, I compared the depth of field tables for both lenses, and they are wildly different. Now why would this be? Shouldn't all 28mm lenses have the same depth of field at a given aperture? For example, the depth of field (in meters) for the 28mm Nikon at f2.8 is 8.77 meters to infinity, whereas the same setting using the 28mm lens on the Horizon is listed as 5.5 meters to infinity. At f8, the Nikon is 3.16 to infinity, and the Horizon is 2 to infinity. Can any of you knowledgeable types out there explain this?

 

[Philly]

Your Horizon, though using 35mm film, is really classed as a medium format, where larger circles of confusion are applied when calculating depth of field. Thus the apparent larger dof of the Horizon.

 

[vodid]

So what you're telling me, Philly, is that the depth of field of the two lenses is actually the same, if the same size circle of confusion criteria is applied to both lenses? In which case, I'd be getting better accuracy by going with the figures in the depth of field table for the Nikon, correct?

 

[Philly]

I'd be getting better accuracy by going with the figures in the depth of field table for the Nikon, correct?

Yes, in a way, though it's more a question of neg/print size relating to viewing distance.

That's why for medium or large format we can accept the greater circle of confusion criteria because the neg is not enlarged as much (as 35mm) and even if we do print large, we view the print from a greater distance.

Download this free dof calculator and play around with it. You will see that by choosing different film formats (the buttons on the left) the dof scale on the round chart will change.

DOFMaster Depth Of Field Calculator.