How to calibrate a shutters apeture

[stompyq]

I have a seiko #0 shutter that i have mounted my 150mm f/9 lens to. I was wondering if anybody knew how i could calibrate the shutters apeture stops for the lens? I have to add that i don't have the shutters manual.

 

[Donald Miller]

The aperture at any given setting will be the apparent aperture diameter divided into the focal length of the lens. It should be recognized that your 150 mm F9 lens will only be F9 at an apparent aperture diameter of 16.6mm.

 

[stompyq]

Thanks for the reply. How do you mearure the apperent apeture diameter? just measure it off a ruler?

 

[Donald Miller]

Yes, by viewing it through the front lens element..

 

[Tony Egan]

Hi, I got some help on a similar topic recently on this thread and there are some photos I posted of how I checked and then created a new scale for my shutter. Some good advice was, as Don indicates, to leave the elements on the lens as they can act as a magnifier of the "physical" aperture diameter.
(there was a url link here which no longer exists)

Your approximate aperture diameters will be:
f9 = 16.7mm (150/9= 16.667)
f11= 13.6mm (150/11= 13.64 etc)
f16= 9.4mm
f22= 6.8mm
f32= 4.7mm
f45 = 3.3mm

 

[Neanderman]

I don't have the specs for the Seiko #0 (or, at least, I can't find them...), but it shouldn't be appreciably different than the Copal #0, which has a maximum iris opening of 24mm and a minimum size of 1.5mm.

 

[Larry H-L]

Schneider will sell you a new aperture scale for $ 79.00. Pricey, but it looks good!
Just tell them the lens and shutter sizes.
818-766-3715

 

[phfitz]

Hi there,

Tony Egan, how did your f/stop scale work out for exposure? Rereading the thread got me thinking so I had to check. On some lenses it measures correctly viewing the aperture from front-to-back, on others it only read correctly viewing from back-to-front thru the lens.

Curiouser and curiouser.

 

[Tony Egan]

The new scale is very accurate based on my testing and comparison with other lens and shutters given all the possible variables which can affect a final developed negative. My knowledge of optics is laughable but intuitively the amount of light passing through the front element and the "pupil" is the critical measure of effective aperture. Once the "volume" of light gets "through the hole" the rear element's job is to complete the bending/magnifying. I would suspect looking through the back element and trying to measure aperture in the opposite direction is not advisable. Interesting that you would say this appears to correspond more accurately with the scale in some cases? Others much more expert will have to help here.

I have checked all my other lens/shutters and scales are within 5% accuracy at each stop measuring roughly with a ruler and naked eye. This would confirm to me the scales are all correct. With a non-click stop aperture there will always be a bit of play so I don't intend to do any more testing unless results are way off. Previously I was overexposing by up to two stops with the old scale which is something you notice! I don't think I'm good enough to control exposure and development finely enough to know if I was up to a quarter of a stop out.

 

[Jim Jones]

Like Don Miller and Tony say, measure from the front with the front cell in place. With many lenses, front or back will give similar results. With telephoto or retrofocus lenses, it can make a lot of difference. I just checked two f/4 Nikkor lenses. A 20mm retrofocus measured f/4 from the front and measured f1.8 from the back. A 200mm telephoto measured f/4 from the front, and f/12 from the back. A Rodenstock Rotelar 400mm showed the same effect.