Entrance Pupil and how to properly measure Lens Speed

[goamules]

This may help people get more accurate maximum F-stop, lens speed measurements. The actual formula is often misinterpreted, and people mistakenly measure the physical iris inside the lens. Actually, the formula is Focal Length divided by Entrance Pupil. There is a difference, because the EP will take any magnification of the front glass into account.

Wrong method - measuring the physical iris itself will yield F4.4, (16 inch focal length divided by 3.6 inches).



Correct method - measuring the apparent aperture looking through the front glass gives the Entrance Pupil. This yields F4.2, somewhat faster. This method is difficult because parallax makes it hard to see both ends of your ruler and make sure you are just measuring the circle of light you can see. I.E. you try to measure the internal iris seen through the glass. Do not simply measure the glass.



Also wrong method - measuring the physical glass does not account for vignetting caused by the internal barrel and the iris. This yields F3.5.



The measurement that is technically correct for this lens is F4.2. What you will find is no lens ever (well, seldom) matches it's max speed rating exactly. Nor do the measurements for each aperture setting match exactly. Perhaps someone with more optical engineering than I can explain why. But this is the way I've been measuring old lenses (especially old Petzvals) for years.

[Emmanuel BIGLER]

Hi !

There is little to add to the excellent summary posted above.

Without any other nistruments than a ruler, a simple method to measure the diameter of the entrance pupil is explained as follows:
Attach a small flash-light at the focal point at the rear of the lens and illuminate the lens backward. Pay attention that all glasses are actually illuminated.
Place a piece of translucent paper as close as possible to the entrance of the lens, the diameter of the illuminated circle as seen on the translucent glass is the diameter of the entrance pupil.

[Jim C.]

Great post goamules, I had to sift thru a lot of posts to figure out my Petzval lens speed, having pictures
helps enormously.

Emmanuel, wouldn't just aiming the ground glass out the window do the same thing ?
Much easier than trying to jig up a flashlight.

[rosshj]

Thank you for this post goamules! I was measuring the internal iris and though my aperture was a lot smaller than what it turned out to be.

[Leigh]

wouldn't just aiming the ground glass out the window do the same thing ?

The image circle of a lens is usually larger than the format it covers.

For the measurement to be accurate, the light source must illuminate the entire image circle, and preferably more.

- Leigh

[Bernard_L]

All clear and correct. With one small proviso: in the "correct method" configuration as shown in second picture, the ruler is not in the same plane as the entrance pupil (image of the diaphragm through optics). So, reading the pupil diameter as shown on the picture is affected by parallax error. Can be avoided by moving one's eye so that the starting and ending measurements are taken along parallel directions.

[tih]

wouldn't just aiming the ground glass out the window do the same thing ?

No, you need collimated light out of the front element (all rays parallel to each other), so the light source should really be a point in the infinity focus plane.