Checking calibration of aperture scales

[Bill Burk]

Let's take a step back for Andrew's sake...

At stake here is a set of lens cells in a foreign shutter without suitable markings.

Seems the original plan is fine... compare to some known lens/shutters and take it from there.

[Andrew]

Bill, thanks but don't worry about me too much... I like to DIY but if it comes to it, I can access a technician who can make new scales for me [I'd just prefer not to]

the comment to my email enquiry was: "Best I can do is to calibrate the lens & diaphragm. Diaphragm scales are made especially for each focal length & each lens configuration. We rely on calculations then test transmission before completion of the scale" and "Can make up a correct one but will need the lens & shutter to be able to calculate & measure the transmission in order top make up an accurate scale".

he keeps coming back to measurement of light transmission being critical

To a large extent I'm trying to think thru how he could be measuring light transmission directly and rework it around the gear I already own.

from the comments so far, it sounds like most people agree the idea has validity.

worst that can happen if I'm wrong is that I flaff up a few sheets of test exposures.... but if it is as simple as I'm thinking, the method would be an easy fix for a other people who are in similar situation too.

[Bill Burk]

Transmission stops, as used in cinema, are critical... But I worry if I calibrated to transmission, would it throw off my normal calibration ... For example, I expose TMY2 at EI 250 based on experience and sensitometry. If calibration to T-stops gives me a more accurate measure of my aperture... does that mean I can shoot at EI 400 now?

[Leigh]

For example, I expose TMY2 at EI 250 based on experience and sensitometry. If calibration to T-stops gives me a more accurate measure of my aperture... does that mean I can shoot at EI 400 now?

For still photography, T-stops are not necessary.

They're designed to produce extremely accurate levels of illumination at the film plane.
The are used so movie scenes shot with multiple cameras and lenses will appear uniform when combined.

The light lost through a lens of the type used on view cameras should be a very few percent.
This will not introduce a noticeable error in exposure. The error in true film speed will exceed the aperture error.

Movie camera lenses have many more elements, and thus transmission loss is increased.

- Leigh

[IanG]

Let's take a step back for Andrew's sake...

At stake here is a set of lens cells in a foreign shutter without suitable markings.

Seems the original plan is fine... compare to some known lens/shutters and take it from there.

I'm actually in this position, I bought an f4.5 14" telephoto lens at a flea market in July it's marked f4.5 but has no scale (or shutter) - it appears to have been a military lens. In fact I'll draw up a scale by measuring the diameter of the aperture and then double check it comparatively before testing. I've a front mounted Thornton Pickard shutter that'll fit it unfortunately my Speed Graphic was modified for Wide angle use or I'd use the lens with it.

Ian

[Leigh]

Hi Ian,

In this case I suggest reversing the "roles" of the two lenses as follows:
Mount your f/4.5 lens and take a luminance reading. Record the value.
Mount the reference lens and adjust its aperture to achieve the same reading.

The reading shown on the reference lens scale should equal the true aperture of the f/4.5 lens.

- Leigh

[IanG]

Thanks Leigh, I'll try that - I figured on using the closest lens I have with similar bellows draw. I was also going to measure the aperture diameter at f4.5 and work from that to find the diameters needed for other f-stops. My Minolta spotmeter is fine for this.

Ian

[ridax]

I agree this is a useful test. In fact, I've gone a similar way more then once. BTW, focusing may be done not only at infinity; any distance is OK if only its the same for all the lenses. And one can also make this a projection test putting the lens on an enlarger and the meter on the enlarger base/table (but in this case the lens is to be mounted backwards as a lens's entrance and exit pupils are most often not identical). BUT...

May we assume the difference between T-stops (which are actually compared in the test discussed) and f-stops to be negligible for a modern LF lens? Well, that certainly depends in the accuracy we want to imply... Heaving measured lens transmission myself, I'm pretty sure we can't speak of 0.1 of a stop (that our meter yields) at all.

No, LF lenses don't usually have many elements but f/1.7 to f/2 50mm small format Planar derivatives don't either, and transmission in those differs quite a lot. 0.1 EV difference is even seen in comparing Zeiss T* to Pentax SMC - two really top-quality brands of coating that in fact were developed in collaboration from the start. And the pretty excellent in contrast control CZJ GDR coating lacks exactly 1/3 of a stop in transmission compared to SMC. And there are lots of brands pretty inferior to CZJ in their coating quality.

Besides, even all the rest thoroughly taken into account, our 0.1EV-accurate meter is as useless for evaluating anything close to real f-stops as a micrometer is for measuring a soft rubber band's width - unless every glass surface in each lens is perfectly clean. Think your second-hand lens that needs a shutter replacement is clean? Well, I've no doubt it is - for taking pictures. But not enough for tests. Apply a little ether with an eye-grade cotton swab and see the ugly shapes of dirt when the ether dries. No that dirt was not introduced by your swab. That's the dirt you had on your lens. Ether just made it visible to you by rearranging the dirt structure. And no the fact that you were unable to notice it does not mean it didn't influence lens performance. It did. To make a lens clean enough for tests, one needs dozens to hundreds of passes with those swabs with ether, each pass with a fresh swab, on each of the glass surfaces. And if not done in a proper laboratory-grade clean room, with any amount of efforts the lens still would not be as clean as needed for tests... not within 0.1EV of transmittance surely.

All the above is about just one problem: the light loss due to reflection. Alas, that problem is far from being the only one. As LF lenses tend to be bigger then those for other formats, for LF, light absorption in the glass itself becomes pretty prominent. Transmission of a 100mm Symmar is way greater then of a 300mm Symmar of the same formula. And an Apo-Ronar, regardless of the coating type, is transmitting noticeably less then a Sironar of the same focal length due to the much much yellower glass in the Ronar. The type of light and the meter spectral sensitivity do matter quite a bit, too (ever tried comparing an Aero-Ektar's T-stops to a Dagor's ones for a blue-sensitive film - or wetplate?).

So... as the repairman quoted above puts it, this type of test is useful as an additional verification to avoid an occasional blunder in calibration - but not as a method to do the calibration itself. And as far as I understand, the additional verification of the ready to use aperture scale was the very purpose the OP offered this method for. And as mentioned above, if in any doubt, I do occasionally perform similar tests, too.

But for actually making a new aperture scale, I'm pretty sure finding an original one to copy is the most sane way to go. Call a friend or post a question here, etc, to let someone owning the same lens measure at least one of the original manufacturer's iris diameters for you, and duplicate those. That the smartest thing one can ever do....

Except that for some old lenses, factors to multiply the iris diameters to get the entrance pupil diameters were actually published by the manufacturers. And with the factor known, there is no problem to make any new scale for any lens of the type. For example, below is the C.P. Goerz, Berlin data of 1908. For lenses specified, to get the actual entrance pupil diameter, multiply the aperture opening by:

Dagor III: (1+1/6)
Dagor IV: (1+1/7)
Pantar: (1+1/7)
Celor: (1+1/11)
Sintor: (1+1/9)
Alethar: (1+1/7)
Hypergon: (1+1/6)
Lynkeioskop C: (1+1/9)
Lynkeioskop D: (1+1/9)
Lynkeioskop E: (1+1/10)
Lynkeioskop F: (1+1/11)

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?

Because (as I've mentioned in this thread: http://www.largeformatphotography.in...ers-determined) you need a collimator, an optical bench, and a set of optical measuring instruments to get the real (not nominal) focal length and the entrance pupil diameter measured - at least if one insists on 0.1 of a stop accuracy... which I personally do not ever need in my own lenses BTW. :)

[jb7]

To Andrew- I use a very similar method to yours, except I use a DSLR mounted in place of the back. Set to manual, the shutter speed is adjusted until it produces a histogram to match the known value. Difference in shutter speeds will indicate difference in stops...

This does produce effective T stop differences, but I wouldn't complain about that-

For the purpose of testing transmission of lenses, perhaps it might be possible to cobble something together, perhaps even hand hold the camera under an effective dark cloth. Maybe a dedicated DSLR mount might even be overkill-

[Jim Jones]

. . . Because (as I've mentioned in this thread: http://www.largeformatphotography.in...ers-determined) you need a collimator, an optical bench, and a set of optical measuring instruments to get the real (not nominal) focal length and the entrance pupil diameter measured - at least if one insists on 0.1 of a stop accuracy... which I personally do not ever need in my own lenses BTW.

Quite true. Sometimes I get involved in the pursuit of perfection rather than in the practical production of photographs. Lens and shutter manufacturers have to do this. However, the functions of an optical bench and measuring instruments can be simulated well enough for most of us.