Checking calibration of aperture scales

[Leigh]

"Any lens set to f/11 puts exactly the same amount of light* on the image plane as any other lens set to f/11 when both are focused at infinity."
And that is your error. If you do it Andrew's way without taking in to consideration the light loss from the GG and Fresnel - and for this to work you need a Frenel to even the light across the gg) and bellows factor then you have not really found f11.

Bob,

You're intentionally twisting my words to cover your posterior. C'mon, Bob. You're above this tactic.

I said "the same amount of light on the IMAGE PLANE".
This has nothing to do with GG or fresnels or anything else.

In the comparison test that Andrew described, the same GG/fresnel is used for both lenses, so any issues
related to its transmissivity or dispersion are identical for both lenses, and thus irrelevant to the results.

I also said specifically "when focused at infinity", which eliminates bellows factor and other issues
related to the distance between the lens rear node and the image.

Let me paraphrase the test procedure in an attempt to clarify the technique...

Set up the test environment, and adjust the reference lens to f/11. Note the luminance reading*.
Replace the reference lens with the test lens. Adjust it until you get exactly the same reading.
The test lens is now set to f/11 within a very small margin of error.

- Leigh

*Note: Maximum accuracy is obtained by taking the readings directly on the optical axis for both lenses.

[Jim Jones]

Why not just measure the apparent diameter of the diaphragm as viewed from the front of the lens, and divide that into the focal length to get the optical f/number? Comparing image brightness introduces possible instrumental errors. True, such measurements do give something more like the old T/numbers, which may be desirable with some lenses and applications. However, with modern coated lenses, f/numbers seem useful enough and certainly more universal than our individual measurements.

[Leigh]

Comparing image brightness introduces possible instrumental errors.

Comparative tests do not introduce errors.

This is one of the fundamental concepts of metrology.

It's the basis for all calibration of all measurements in all fields in all countries.

- Leigh

[Bob Salomon]

"I said "the same amount of light on the IMAGE PLANE".
This has nothing to do with GG or fresnels or anything else."
But the meter is behind the image plane, not at it, unless the reading was done with the Sinar/Prontor/Gossen in the film plane probe. And it wasn't. So the gg/Fresnel factor has to be taken into consideration for the calibration to be correct on any camera. Not just the one it was done on.

[Leigh]

But the meter is behind the image plane, not at it, unless the reading was done with the Sinar/Prontor/Gossen in the film plane probe. And it wasn't. So the gg/Fresnel factor has to be taken into consideration for the calibration to be correct on any camera. Not just the one it was done on.

Yes, the meter is behind the GG/fresnel/whatever. But that fact is absolutely irrelevant to this test protocol.

We're doing a comparison, not making an exposure calculation.

When the measured values from the two lenses are equal, their apertures are equal.
It makes absolutely no difference how bright or dim the test image is at the point of evaluation.

The factor that you're talking about certainly is important when making actual exposure calculations.
That's not what we're doing in this test.

- Leigh

[Bob Salomon]

Yes, the meter is behind the GG/fresnel/whatever. But that fact is absolutely irrelevant to this test protocol.

We're doing a comparison, not making an exposure calculation.

When the measured values from the two lenses are equal, their apertures are equal.
It makes absolutely no difference how bright or dim the test image is at the point of evaluation.

The factor that you're talking about certainly is important when making actual exposure calculations.
That's not what we're doing in this test.

- Leigh

Forget it Leigh. The OP asked a question and I answered him. He will not correctly calibrate a lens with his technique. And if you are going to calibrate a lens then do it correctly so the calibration is correct for any camera. Not just the one you used to do the test.

[Leigh]

The calibration protocol described previously is correct for any lens, regardless of the camera on which it's used.

In fact you don't even need a camera.
You could do the comparison with the lenses on an oatmeal box and a piece of mylar at the back.

Too bad you don't understand metrology.

- Leigh

[Andrew]

The OP asked a question and I answered him. He will not correctly calibrate a lens with his technique. And if you are going to calibrate a lens then do it correctly so the calibration is correct for any camera.

I threw this open to constructive feedback and criticism....
if you think the idea is flawed that's fantastic but how do you suggest it should be done ??

I'm always trying to learn and I do want to sort out some equipment and not waste my time trying to do so

[Andrew]

addit

Bob, the initial experiment that I did to try validating the idea was to get a small number of lenses covering a reasonably wide range of focal lengths and go thru the same measurement procedure with each and the result was that the EV readings that were within small fractions of an f-stop for each lens [except one where I'd swapped cells between shutters!]

Can you suggest an experiment that would be more convincing OR something that would invalidate the initial observation ??

[Flauvius]

It might be helpful to view the original poster's statement in terms of: How are f stops defined?

If defined in terms of the traditional formula: [F]ocal length divided by aperture diameter, your know whether your aperture scale's numbers are correctly positioned if the variable diameters of your lens' front element - when divided by the lens' focal length - equal the nominal exposure values shown on your aperture scale.

As such it is irrefutable that Leigh's analysis is correct: Given that f stops are geometric functions that are totally independent of a lens' light transmission characteristics, Bob Solmon's claims are not applicable to the op's comments.

Flauvius