Checking calibration of aperture scales

[Andrew]

there seems to be a lot of people, myself included, swapping lens cells between shutters and then worrying about how accurate the aperture scales are. Or not worrying and perhaps getting caught out with bad exposures? Or having no scales at all and wanting to construct new ones.

I think I've got an easy method for checking the scale calibration [and perhaps constructing new scales] that just uses the gear most of us have to hand... but can those who know more please tell me if there's a basic flaw in what I've come up with ???

here goes:

Start with any lens that's know to be stock standard and assume that the aperture scale is correct because it's come from the factory. This becomes the reference lens.

Mount the reference lens on the camera, set it to any chosen f-stop and focus infinity. Then point the lens at a bright uniform light source like an area of clear sky and get under a darkcloth to reduce extraneous light. Get a light meter with the dome withdrawn, hold the light meter directly on the ground glass and measure the brightness in EV.

Then get the suspect lens and do exactly the same thing ie focus it to infinity, set to the same f-stop, point it at the same light source/ same area of clear sky and measure the EV of exactly the same "object" while under the darkcloth to reduce extraneous light

The EV should be exactly the same.... if there's a difference it could be the light has actually changed between measurements [!], you aren't measuring the same point or the aperture scales are inaccurate. If you're happy the light hasn't changed and the difference is due to the scales, the difference in EV gives you the magnitude of the error.

I've tried this with a few unmolested lenses from 125mm to 305mm and the EV readings are pretty consistent so I think it works. There's been some variations in EV readings that could be explained by measuring different areas of sky and the light changing between measurements but the variation was small.

If that's all valid and you find an error in the aperture scale you're testing, you could just open/close the aperture till the reference EV reading is reproduced and that'll give you the point which corresponds to the selected f-stop you started with on the reference lens.

feedback/ comments please!
.

[Bill Burk]

Seems like you should determine one aperture by actual exposure tests to get the transmission value... Then use your technique to find all the other stops.

[ic-racer]

Or just use a film-plane meter all the time and ignore lens aperture scales (Horseman meter for example).

[Leigh]

Hi Andrew,

Sounds like that method should work well.

Remember that shutter aperture assemblies have tolerances, just like every other manufactured item.
I'm sure they're not worried about a 1/10th stop error, so your measurements may vary a bit.
Modern meters will easily catch changes of 1/10th stop.

- Leigh

[BetterSense]

Why focus on infinity? If you are just comparing brightnesses, you could place the lens radically OOF, and it would help to blend together any variations in scene brightness.

[Leigh]

Why focus on infinity?

You focus at infinity because that's the position at which the aperture scales are correctly calibrated.
The scales are in error at any closer focus.
The error varies with the shift from infinity focus, increasing as the focus distance decreases.

As you focus closer, the lens moves farther from the film, and the area of the circle of illumination increases.

- Leigh

[Andrew]

Leigh, thanks for commenting.... I was hoping you would.

frankly, I'm not even too worried by 1/3 f-stop errors and if I can be that accurate reliably then I'm happy

[Bob Salomon]

The ground glass absorbs some light. So a direct reading through a ground glass will not be correct unless you factor in the absorbtion due to the gg. That means that you have to first measure a grey card with the light source you want to use through the gg. Then measure the same gg directly. The difference is the factor and you apply it to your meter accordingly. Otherwise your scales that you make will be incorrect by the amount of the factor.
This technique also works when there is any bellows factor in play due to distances less then infinity.

[Leigh]

The ground glass absorbs some light.

Hi Bob,

That's true, but irrelevant.

He's comparing the readings of a known lens with that of an unknown lens at the same aperture.

The known and unknown readings should be identical regardless of the characteristics of the GG.

The absolute magnitude of the reading is irrelevant to the comparison.

- Leigh

[Bob Salomon]

Until he puts it on another camera. Or changes the gg/Fresnel. Or go to sell it or loan it to someone else.