Ugly Hedgehog® - Photography Forum
Exit and Entrance Pupil and Depth of Field
Sep 16, 2020 18:09:51   #
pquiggle Loc: Monterey Bay California
 
I would like to be able to calculate the depth of field with my Canon 100/2.8L macro lens. The formulas are very sensitive to the ration of the exit pupil diameter to the entrance pupil diameter. Has anyone else tried to measure these? The values change with magnification starting near 1 at infinity and going down to 0.41 at 1:1. I placed the lens on a light table and set the focus at the marked magnifications on the lens barrel and measured both the exit and entrance pupil. The values I got were:

Pin Pout M P
∞ 25 25 0 1
1:5 28 24 0.2 0.857142857
1:3 26 22 0.33 0.846153846
1:2 26 18 0.5 0.692307692
1:1.5 27 15 0.67 0.555555556
1:1 32 13 1 0.40625

Where the first column contains the markings on the lens, Pin is the diameter of the entrance pupil, Pout is the diameter of the exit pupil, M is the magnification, and P is the ratio Pout/Pin.

The formula I found for depth of field is: D = 2*CoC*N*(M/P+1)/M^2

Where D is the depth of field, CoC is the diameter of the circle of confusion, N is the nominal aperture, M is magnification, and P is the pupil ratio defined above.

Has anyone else measured the exit and entrance pupils, it was not very easy to do so I am not to sure of the values I got.

Thanks.

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Sep 16, 2020 18:19:00   #
CHG_CANON Loc: the Windy City
 
Why not use an online DOF calculator? No need to re-invent the wheel: https://www.photopills.com/calculators/dof

You can download similar apps to your phone too.

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Sep 16, 2020 18:32:58   #
pquiggle Loc: Monterey Bay California
 
CHG_CANON wrote:
Why not use an online DOF calculator? No need to re-invent the wheel: https://www.photopills.com/calculators/dof

You can download similar apps to your phone too.


Yes, but they require the user to input the pupil magnification ratio, that is what I am trying to find.

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Sep 16, 2020 18:35:46   #
CHG_CANON Loc: the Windy City
 
pquiggle wrote:
Yes, but they require the user to input the pupil magnification ratio, that is what I am trying to find.


You don't seem to have actually followed the link ....

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Sep 16, 2020 18:55:57   #
pquiggle Loc: Monterey Bay California
 
CHG_CANON wrote:
You don't seem to have actually followed the link ....


That calculator is not for macro work. The link that is:

https://www.photopills.com/calculators/dof-macro

asks for the pupil magnification.

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Sep 17, 2020 08:44:34   #
bleirer
 
I just gave it a try with my Canon non L 100mm 2.8 macro which is also 1:1. I held dof preview at f4 as I removed the lens so I could see the aperture, so my numbers are different from yours but the ratio should be consistent. It is not easy to measure. I used calipers and held the lens against a bright background. Even so measuring twice yielded slightly different values, so I kept it to the nearest .5. All results in mm. For infinity entrance was 25, exit 23. For 1:5 entrance 24 exit 20, for 1:2 it was 26.5 and 15.5, for 1:1 it was 30 and 11. So my ratios were .92 for infinity, .83 for 1:5, .58 for 1:2, .49 for 1:1.5, and .36 for 1:1.

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Sep 17, 2020 09:16:55   #
bleirer
 
Also interesting impact on diffraction. https://www.photopills.com/calculators/diffraction-macro

For my camera at 1:1 at 100% viewing diffraction begins to be noticable at f 3.2, for print viewing distances at f 6.7 because set at f4 the effective aperture works out to be f 13.9.

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Sep 17, 2020 12:32:37   #
pquiggle Loc: Monterey Bay California
 
bleirer wrote:
I just gave it a try with my Canon non L 100mm 2.8 macro ...


Thank-you. I surprised how close the two lenses are. I tried averaging your values with mine and when I graphed it the best fit line was a near perfect line. Thank-you for helping me to verify my results.

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Sep 17, 2020 12:38:52   #
pquiggle Loc: Monterey Bay California
 
bleirer wrote:
Also interesting impact on diffraction. https://www.photopills.com/calculators/diffraction-macro


That is my next project. I saw how to calculate it in a book by Enrico Savazzi, "Digital Photography for Science." The results from his formula are very different from the Photopills calculator. Looks like I am going to make a few prints. Thanks for you data as well. Which camera are you using it with, sensor dimensions and resolution as a role to play as well.

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Sep 17, 2020 13:25:33   #
bleirer
 
pquiggle wrote:
That is my next project. I saw how to calculate it in a book by Enrico Savazzi, "Digital Photography for Science." The results from his formula are very different from the Photopills calculator. Looks like I am going to make a few prints. Thanks for you data as well. Which camera are you using it with, sensor dimensions and resolution as a role to play as well.


A Canon RP, 26.2 mpx 4160x6240 full frame. Cambridge in color also has a diffraction calculator, scroll way down here https://www.cambridgeincolour.com/tutorials/macro-lenses.htm

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