Depth of field for various formats and lenses

Started May 30, 2012 | Discussions thread
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amangupta
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Depth of field for various formats and lenses
May 30, 2012

It is common to discuss DOF for various formats and lenses. There are two statements commonly made:

1) Small format means more DOF
2) Longer lens means shallower DOF

I wanted to verify this by doing actual DOF calculations. I used http://www.dofmaster.com/dofjs.html to do these calculations.

Methodology:

I set the standard to compare with as using a full frame camera with an 85mm f/1.4 lens, standing 2.5 metres from the subject - an acceptable distance for head and shoulder portrait I guess. Then, for a given format and lens spec, I calculated the 35mm equivalent focal length and adjusted the subject distance. For e.g., if I am using a 50mm f/1.4 lens on Nikon D7000, the subject distance would be 75/85 * 2.5m = 2.2m.

The overall assumption is that the framing of the subject is the single most important factor and has to remain constant.

Following are the results for some common format/portrait lens combos.

(focal length, aperture setting, subject distance - DOF in metres).

Full frame:
85mm, f/1.2, 2.5m - 0.06m
85mm, f/1.4, 2.5m - 0.07m
50mm, f/1.4, 1.47m - 0.07m
50mm, f/1.0, 1.47m - 0.05m
100mm, f/2, 2.94m - 0.1m

APS-C (Nikon size):
50mm f/1.4, 2.2m - 0.11m
60mm f/2.2, 2.65m - 0.17m
85mm f/1.4, 3.75m - 0.11m
100m f/2, 4.4m - 0.15m

MFT:
45mm, f/1.8, 2.65m - 0.18m
75mm, f/1.8, 4.4m - 0.18m
25mm, f/1.4, 1.47m - 0.14m

(I chose 60mm f/2.2 with APS-C because that is the combo I have right now using the Tamron 60mm f/2 SP Macro lens with Nikon D40)

What I found:

1) Small format means more DOF - TRUE. Mainly because for the same lens focal length, subject distance increases, increasing the DOF.

2) Longer lens means shallower DOF - FALSE. As the focal length increases, so does the subject distance and the eventual DOF remains constant.

I hope I've not made a mistake above. What do you think?

 amangupta's gear list:amangupta's gear list
Nikon D40
Nikon D40 Nikon D7000
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