How should I determine lens aperture corrections for tested shutter speed variances?

[JMO]

I have recently obtained a Phochron XA digital shutter speed tester and have made a little jig to mount my dozen or so lenses (all on Technika-style boards) so I can quickly test them through the various shutter speed settings. Luckily, only one of my lenses/shutters is off, and a bit too inconsistent, on certain speeds such that I will need to send it out for CLA or repair. All the other lenses seem to be quite consistent from one click to the next, and fairly close to the various speeds indicated on the shutter dials -- BUT OF COURSE THERE ARE SOME VARIANCES!

My test data are speeds expressed as fractions like 1/28.4 seconds (where the target was 1/30), and these data can be easily converted to milliseconds. My question is how do I determine the aperture exposure corrections to apply when taking photos with these lenses, in order to correct for the results of my shutter speed tests (whether the lens in question was found to be a bit fast, or slow)? My Compur or Copal lenses shutters allow me to add or subtract exposure in 1/3 stop increments, but how do I relate those increments to the shutter speed test data (which are on an arithmetic scale)?

Finally, if someone might have, and be willing to share, an Excel spreadsheet set-up for this purpose, it would be greatly appreciated.

[torashi]

I have recently obtained a Phochron XA digital shutter speed tester and have made a little jig to mount my dozen or so lenses (all on Technika-style boards) so I can quickly test them through the various shutter speed settings. Luckily, only one of my lenses/shutters is off, and a bit too inconsistent, on certain speeds such that I will need to send it out for CLA or repair. All the other lenses seem to be quite consistent from one click to the next, and fairly close to the various speeds indicated on the shutter dials -- BUT OF COURSE THERE ARE SOME VARIANCES!

My test data are speeds expressed as fractions like 1/28.4 seconds (where the target was 1/30), and these data can be easily converted to milliseconds. My question is how do I determine the aperture exposure corrections to apply when taking photos with these lenses, in order to correct for the results of my shutter speed tests (whether the lens in question was found to be a bit fast, or slow)? My Compur or Copal lenses shutters allow me to add or subtract exposure in 1/3 stop increments, but how do I relate those increments to the shutter speed test data (which are on an arithmetic scale)?

Finally, if someone might have, and be willing to share, an Excel spreadsheet set-up for this purpose, it would be greatly appreciated.

I wrote an Excel spreadsheet for you. I can email it to you, if you send me your address through a private message. For some reason I can't attach an .xlsx file here.
I'd suggest not to worry too much, if it's within the shutter's speed tolerance specs. I would suggest, if your work is very critical, that you could also test the shutters at different temperatures. For example by putting them in the freezer inside a sealed plastic bag (dew & moisture barrier, you know) or heating them a bit up with an incandescent bulb (and always with a thermometer) so as to mimic real environmental situations.
This should also let you know beforehand how they should behave in different weather conditions during summer/winter.

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[torashi]

Here's how the sheet looks like. It's really simple. First, the exposure time percent difference is calculated as the difference between the theoretical and the experimental values, divided by the theoretical. Then it's just converted to tenths of stop in the next row by simply applying a format.
A positive number indicates compensation by closing of the aperture (think plus, bigger number) and a negative the opening. If you prefer it the other way around, I can change that, too.

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[ic-racer]

No compensation is needed if one uses meter that displays shutter speeds at 1/3 stop intervals.

[Doremus Scudder]

I simply made a sticker for my lens board with the marked shutter speeds and an indication of compensation rounded to the nearest 1/3 stop. Example: say the 1/30 second speed is about 1/3 stop too slow. I'll mark this as 1/30- (minus to indicate slower). I have a few shutters marked in old speeds. With these I'll use modern standard speeds with an adjustment marking. E.g., 1/5 sec becomes 1/4+ (plus indicates faster).

Then, when exposing I simply make an appropriate adjustment with the aperture. 1/30- needs 1/3-stop less exposure from the aperture as 1/30.

FWIW, I figure everything from milliseconds and have made tables to show where the 1/3-stop speeds are. I fire my shutter at f/22 (where I work most) several times and take an average. Of course, if the shutter is erratic, it goes in for a CLA.

Hope that makes sense.

Best,

Doremus

[JMO]

Doremus, Thank you for your answer, but how do you decide which variances in measured shutter speeds require 1/3 stop correction, versus 2/3 stop, versus some other level of exposure correction? I don't want to make more of this than what it is, but at this stage in my relatively neophyte LF career I am not sure how to convert the shutter tester results into aperture adjustments. How do you make those adjustments? I have sent a PM to torahsi with my email address, so I can receive the Excel spreadsheet, but I am not clear on how I should make judgments on what aperture corrections to make in relation to the shutter speed test results. Thank you....

[Dan Fromm]

If you have a spreadsheet package, here's the magic formula:

exposure error in stops = log10(measured shutter speed in seconds/ideal shutter speed in seconds)/log10(2)

[jp]

My light meter has a dial... If my lens is setup for 1/250 and I know it's more like 1/200 because of cold, age, etc.. I use the aperture that corresponds with 1/200 on the light meter dial.

[JMO]

These last three answers are what I was looking for - THANK YOU ALL. I plan to use the Excel spreadsheet, with function from Dan, and then make little labels to put on each lens board with the aperture stop corrections. I have thought that another useful bit of info to put on these sticker labels will be the lens image circle, so I'll know in the field how much room I have for movements.

[Jerry Bodine]

...I am not clear on how I should make judgments on what aperture corrections to make in relation to the shutter speed test results...

One can never have enough data, I believe. If you're still interested in an Excel spreadsheet to answer you needs, I can provide you with one if I, too, have your email address via PM.