View Full Version : Aperture scale for 90mm f8 Super-Angulon

Patrik Roseen
13-Oct-2006, 23:47
Hello, I hope you can help me with the issue of creating an aperture-scale for my 90mm f8 Super-Angulon which I have mounted into a Prontor-S shutter (copal-0).

I think I know the math for calculating the aperture size, but I am puzzled as to where to do this measurement. As a comparison I am looking at my 90mm f6.8 Angulon.

At first I thought I would just make the aperture holes within the shutters the same size using the Angulon as a reference, easily done.

Some people on this forum claim that the aperture (aparent?) is determined by looking through the front cell of the lens since the lens has a magnification factor.
So when I then mount the front cells into the respective shutters to view the 'aparent aperture' they differ quite a bit...the SA opening seems much smaller.

(It's also interesting that the 'endpoints' (rear lens element) of the front cells of the two lenses differ so much too, with the f6.8 being much smaller than the f8 SA being almost the size of an f5.6 aperture??

Please, what's the trick here?
(Or even better perhaps...does anyone know the shutter aperture hole sizes for f8-f32 for the 90mm f8 Super-Angulon?)

Desperate, Patrik

Patrik Roseen
16-Oct-2006, 06:29

Jim Jones
16-Oct-2006, 11:01
Measure the apparent diameter of the aperture when looking into the front of the lens and use the (focal length/aperture diameter) formula to determine the aperture. This can be trickier than it seems. You should be looking directly into the front of the lens. Parallax can cause an incorrect measurement. This can be eliminated by placing a ruler close to the front of the lens, and observing ruler and aperture from a considerable distance. I usually observe from a close enough distance to read the ruler scale, but move my eye sidewise by the apparent aperture diameter when measuring from aperture edge to aperture edge. Others have measured the projected entrance pupil of the lens by projecting a point source of light at the infinity focus film plane. Sound confusing? Maybe others can phrase it better.

Ed Richards
16-Oct-2006, 12:00
How about taking the elements out and using a caliper to measure the iris directly? That should be the most accurate way to do it, just be careful to not screw up the iris with a finger or poke from the caliper.

Peter Lewin
16-Oct-2006, 13:55
Perhaps someone else can help flesh this out. In an article on mounting old lenses in new shutters, I saw an approach to determining the f-stops based on light meter readings on the ground glass. You would establish your f8 starting point by getting a reading from your other 90mm lens set at f8. (Or, I imagine, you could simply take a meter reading with your "to be calibrated lens" wide open, which should be f8.) With the lens you are calibrating, when you get the same meter reading, that's f8. When you shut it down to the point where your meter reads one stop less, that's f11, and so on. Admittedly this is going by memory from an old View Camera article, and I've never tried it, but conceptually it should work.

Patrik Roseen
16-Oct-2006, 15:53
Thanks for the replies so far...

Ed Richards, your idea is what I started out from and even used my 90mm Angulon shutter as a reference, but this seems not to work since the 'magnification' of the lenses are different (according to what others say). Unless someone with a 90mm Super Angulon can provide me with their shutter data as a reference.

Jim Jones, Ok so I need to measure the size of the magnified aperture when viewed from the front through the lens. Well this is the tricky part as you say since the aperture is so far into the glass of the Super-Angulon. I could ofcourse use my eyes to try to compare the size to the 90mm Angulon as a reference...

Peter Lewin, I think I have seen this before too. One thing which is funny with the Super Angulon, which I have also noticed with my 75mm f7 SA is that the f8 is not with the aperture fully open...I can not really figure out why. I will try to do this light measurement and see where it ends up.

So a combination of Jim Jones's and Peter Lewin's suggestion should probably do the trick.

Hmmm, Could I simply use my 75mm SA as a reference instead and measure its physical aperture and then recalculate into 90 with a 90/75 -factor?

Ernest Purdum
16-Oct-2006, 16:14
The usual way of measuring the entrance pupil is by using a "traveling micrometer". Okay, you haven't got one, but you may be able to come up with a reasonable facsimile. What you need is a ruler, preferably a good steel one reading in millimeters, and a sighting device of some kind. The idea is that you line up a couple of vertical hairlines, threads will do, with one side of the opening, take a reading from the ruler, then slide them over to the other side and subtract the first reading from the second. Divide that figure into your focal length to make sure you have arrived at f8. With reasonable care in making your contraption you should obtain a good dimension for maximum aperture. From there on it's mathematics, using your gadget to establish all the rest of the apertures.

Peter Levin has given you the basis for another method. I would just add a few procedural suggestions. Aim at a blank wall which doesn't have to be in focus. Using a comparison lens would be a good idea. You might find out that your diaphragm needs to be closed down a ways to put you at f8. The sensor of your meter should be shielded from extraneous light which could make reading very awkward with some meters. Watch out, of course, for any light changes while you are taking readings. You might have to wait for a cloud to go by. You could, of course, set up something indoors by artificial light, but the illuminated area should be large and uniformly lit. A helper would be very desirable.

Ernest Purdum
16-Oct-2006, 16:24
Patrick, I started the above before your latest post came up. I see that you are already aware that the full opening is not necessarily f8.

Jim Jones
16-Oct-2006, 16:46
Comparing light meter readings on the ground glass introduce variables such as consistancy in reading the meter, meter inconsistancies, and differences in light transmissions of the lenses. It should be reasonably accurate when modern lenses are compared, using a reliable meter.

Since one size of shutter is intended to accomodate a variety of lenses, the maximum aperture of the lens often requires less than the maximum shutter aperture.