# Thread: Calculating Difference in DOF at Different Focal Lengths / Same Framing

1. ## Calculating Difference in DOF at Different Focal Lengths / Same Framing

Quick question: How do I calculate the difference in DOF using two lenses at different focal lengths, same aperture, same framing?

The problem is I cannot figure out how to change the distance to compensate for the different focal lengths. Specifically...
• What is the difference in DOF at f/2 between the 135L and 85L when the framing is identical?

2. Can you just multiply the focus distance (ie: distance to subject) by long mm/short mm and plug the number into DOF Master?

I tried doing that on one set of values, and found that whether I zoomed with my feet or the lens, the DOF ended up identical. I don't know if that's a hard rule though.

edit: I tried this a week or two ago, so not with your example lenses.

3. They are going to be close to the same.

If you have the same framing, for instance you use your 5D II at 100' with the 135mm on it. Taking 85mm and divide that by 135mm and it gives you a ratio of .6296. So for 100 feet X .6296 gives you 62.96 feet for the exact same framing. At that distance the DOF calculator shows 135mm at 100' to be 20.2' and the 85mm shows to be 20.5'

I think the math is right, any one feel free to correct it.

4. DoF is determined by four factors: aperture, focal length, subject distance, and circle of confusion (linked to sensor format). With the same camera and two different lenses, if the framing is the same, the focal length and distance factors cancel each other out, and DoF is determined only by aperture (caveat - this doesn't apply for macro distances). Doesn't matter if the two lenses are the 85L and 135L or the 14L and 800L.

So, no need to bust out the calculator, if the framing is the same for the 135L and 85L, and both are at f/2, the DoF is the same. But note...the perspective is not the same.

5. So, how would this not apply to macro distances?

6. Originally Posted by HDNitehawk
No, at 135mm at 15.88 ' it would be the same as the 85mm at 10', .49'.
Both would have same framing
I deleted my calculations to avoid confusion. :-)

7. Originally Posted by Sean Setters
I deleted my calculations to avoid confusion. :-)
Well I will delete my response then

8. Originally Posted by HDNitehawk
So, how would this not apply to macro distances?
The equations on which the DoF calculators are based have an underlying assumption that the lens is 'thin' (a single element with no thickness). At 'normal' subject distances, the error introduced by the difference in entrance pupil position between the assumed thin lens and the real lens is insignificant, but at macro distances that error has a real impact on the DoF.

9. Thanks Neuro. I appreciate the explanations.

10. Thanks, that makes sense. Theory vs Actual. I suppose if you wanted the actual DOF of the 100mm L IS and the 180mm L I would have to find someone who has actually plotted it out.

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