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Depth of Field Muddle, muddle, muddle.......
Posted By: PenPusher, 11-26-2015, 09:32 AM

I'm looking at the Depth of Field Tables for Takumar screw mount lenses in Keppler's "The Pentax Way" from around 1970.

I am assuming that the same figures apply approximately to K, M, A, F and FA lenses as they are all 35mm full frame lenses, spot on accuracy does not matter so as long as they are in the same field it's OK.

The muddle I'm getting into starts when I'm trying to translate these figures for these lenses when used on APS-C cameras.

I initially assumed that say, the figures for 35mm lenses could be swapped for those for 50mm as the crop factor being 1.5x makes a 35mm lens 52mm which is near enough.

Then I started thinking that can't be correct and I've been going round in ever decreasing circles ever since.

Any help would be appreciated.

CD
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11-26-2015, 10:35 AM   #2
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The depth of field is unchanged at any given distance for any given aperture with any given lens. This is an optical property of the lens.

What is changing is the field of view. To keep the figures round, a 100mm lens on crop has the FIELD OF VIEW of a 150mm lens on full frame. The depth of field only changes when you alter (i.e. increase) your distance to the subject in such a way as to reproduce the field of view you would get if viewing through a full-frame system. Then the distance is greater so the depth of field increases (provided you keep the same aperture setting).
11-26-2015, 10:49 AM   #3
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It is not straight forward and the values you see in tables depend on which standard for determining the DoF (i.e. diameter of the "circle of confusion") was used.

With the value that Pentax used way back when (may differ from what Keppler used) I can calculate the following example:

Given:
Distance to subject: 5 m

I then get:

35mm lens on APS-C, F4: Hyper focal distance: 14.2m, DoF from 3.7 to 7.7 meters

52mm lens on 35mm FF format, F5.6: Hyper focal distance: 14.6 m, DoF from 3.7 to 7.6 meters

So that's how you can use the tables: Find the equivalent focal length, then compensate by one F-stop.

Hope this helps a little.
11-26-2015, 10:53 AM - 1 Like   #4
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THis might be of some use: Online Depth of Field Calculator

11-26-2015, 02:20 PM   #5
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QuoteOriginally posted by jatrax Quote
THis might be of some use: Online Depth of Field Calculator
Or from the same site, one can use:
Depth of Field Table
11-27-2015, 01:13 AM   #6
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QuoteOriginally posted by Alex645 Quote
Or from the same site, one can use:
Depth of Field Table
Thank you all, very much appreciated.

CD
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