How to test lens transmission

[kanishka]

I was wondering what is the best way to test the transmission of a lens.
I will receive in a few days an old lens and I want to know how to expose correctly with it.
Can you share with me what you think would be the best method?

Thanks

[Nodda Duma]

Assuming the glass is clean..

The best method is relative measurement on a spectrophotometer, but I'm guessing you don't have access to one.

The next best is to calculate based on number of air-glass interfaces using the formula 0.96^x, where X is the number of air-glass interfaces. That calculation is for uncoated optics. For coated, use 0.99^x. So an uncoated singlet with two air-glass interfaces has a transmission of 0.96^2 = 92%. An old uncoated Rapid Rectilinear or something like a Dagor has 2 cemented groups, or 4 air-glass interfaces, so 0.96^4 is ~85% Transmission. An uncoated Cooke triplet has 0.96^6 = 78.3%. Incidentally you can see why older designs minimized the number of air-glass interfaces as much as possible and also why coatings are so important. Any scientific calculator will do this calculation.

The next best method is use a 35mm camera's exposure meter to read the change in readings with a known lens vs. with the unknown lens in front of it. This is trickier to set up, since you'll need to minimize differences between the two measurements: block stray light with a fabricated tube, maintain a similar FOV, and also factor out the f/#. Once all is said and done, the results should be close to the calculated values above. If not, something went wrong with your setup.

As a lens designer, my recommendation is to calculate the transmission using the formula above. Assuming you count the number of surfaces correctly, the result will be very close to truth.

Keep in mind that if your lens has aperture markings, this transmission loss may very well have been factored in when the markings were scribed on the lens barrel.

[Tin Can]

All good until the last caveat.

However, big thanks OP and Nodda for addressing an obvious question considering recent threads.

[Nodda Duma]

All good until the last caveat.

My attempt to be generous to the original designers.

[Jac@stafford.net]

Any scientific calculator will do this calculation.

And if you are online, just type .96^2 in the search window. Very handy! The iPhone will do the same without its browser in the search window. (And U*X wizards know how to do it at the command line.)

[ic-racer]

The most practicle is a FP meter. I use the Horseman focal plane meter.

[Tin Can]

I use the Horseman focal plane meter.

I thought most of those failed long ago.

No matter, basicly too rare and old for most of us to find and use.

The question could be, do you find variation when changing glass and apertures? Do you read significant differences in lens transmission?

[ic-racer]

Perhaps even more practicle would be a Zone I test. That would give the correct exposure for the lens/shutter system.

[ic-racer]

Randy Moe posted the words "too rare" ...this guy collects and uses the rarest of rare equipment like a madman...

[ic-racer]

BTW that Horseman meter is a great tool, especially if you ever need io expose sheetfilm under an enlarger.