# Thread: Pupil Factor, exit and entrance pupil diameters - how to measure

1. ## Pupil Factor, exit and entrance pupil diameters - how to measure

Code:
```               exit pupil diameter
Pupil Factor = -----------------------
entrance pupil diameter```
This factor affects strongly the depth of field:
http://toothwalker.org/optics/dofderivation.html
and maybe Airy disk diameter. Most of all in macrophotography.
When a lens is reverted the exit pupil becomes probably the entry pupil, what changes the f-number and i.e. even the hyperfocal distance.
Is it possible to measure it somehow ?
Actually the f-number definition gives the entrance pupuil. But what about the exit pupil ?
Perhaps very approximately
Code:
```    last rear lens diameter
P = ------------------------------------------------
focal length / lens brightness (for example 2.8)```
????

2. ## Re: Pupil Factor, exit and entrance pupil diameters - how to measure

Measuring the pupil magnification can be done, but it's not easy. In short, this requires estimating the ratio between the entrance and exit pupil images, as seen through the lens from the outside. You can try and guess this visually, but that will only give crude results. The more accurate way would be to set the lens in question on a table and photograph it from both the front and rear (using a different lens). This way, you can measure the size of each pupil precisely and calculate the ratio. The key is to make sure that you accurately represent scale in the two photos:

1. Place the unmounted lens in question on a table, and have a camera ready.
2. Use the widest aperture possible, then take a photo by focusing on the pupil as seen through the front of the lens
3. Keeping the focusing distance constant, turn the unmounted lens around and photograph the pupil from the rear. You will need to move the lens closer and further to get the pupil to align with the focusing distance.
4. Calculate the ratio between the diameters of these two pupils by measuring the ratio between their pixel width in each photo. That will be your pupil magnification factor.

Hope this is helpful. I personally wouldn't measure this unless I really had to though. There's a lot more on this topic in an older thread here: Pupil magnification and depth of field

3. ## Re: Pupil Factor, exit and entrance pupil diameters - how to measure

Thank you, I will try to do it tomorrow in the daylight.

4. ## Re: Pupil Factor, exit and entrance pupil diameters - how to measure

For 10-15 years old Canon Macro Lens EF 100 mm 1:2.8, if these holes are proportional to pupil diameters:

Infinity
P = 510px / 682px = 0.75
1:1
P = 417px / 666px = 0.63

Some of pictures are slighty unsharp, what affects accuracy.

The rear lens diameter is 35 mm. 35 / (100 / 2.8) = 0.98. Then it equals the entrance pupil diameter.

And depth of field.

f = 100 mm (focal length)
N = 16 (F - number)
β = 1 (magnification)
c = 0.029 mm (circle of confusion for 35 mm film)

DOF = 2·N·c·(β/P+1) / [β2 – (N·c/f)2]

Without pupil ratio (P = 1) DOF = 1.86 mm
For P = 0.63 DOF = 2.40 mm (29% more)

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