View Full Version : measuring aperture opening

Roger Vadim
22-May-2011, 04:19
Hello all,

so finaly I mounted the 150mm Sironar lens cells I bought for cheap in a Compur shutter - but I am a bit puzzled how to measure the new f-stop scale needed. so the question is:

the diameter of the f-stop aperture should be:

focal length / f-stop = d
(i.e. 150mm / f 8 = 18,75mm)

ist that right?

but when comparing the calculations I did for 127 mm with the diameter of the aperture my 127mm ektar the numbers doesnt seem right, they sort of work when I measure from the front through the lens (which is more a "sort of measure")

can anybody explain the proper way to measure the f-stop aperture? directly at the aperure - or through the lens - and if how?

thank you.

Helen Bach
22-May-2011, 05:17
Roger, it's correct to measure 'through the front of the lens' - you should be measuring the entrance pupil, not the physical diameter. The entrance pupil is the virtual image of the physical iris when seen from the front of the lens. If there are no cells in front of the iris, then the iris and the entrance pupil are the same.

Look at the lens from arm's length and hold a scale or other measuring device close to the front of the lens, and view with one eye. That's usually good enough.


Roger Vadim
22-May-2011, 06:50
Helen, that is very helpfull!
I spend a whole afternoon trying to get my head around this issue - and suspected this conclusion. just wasn't sure.

So of to start shooting with the new lens - I am curious about the difference to my 127mm Ektar, which is sharp but lacks massively in contrast...

22-May-2011, 08:16

An alternative method is to focus the lens on a very distant object, then take the camera into the darkroom (a dark closet will do). Mask the GG completely, except for a pinhole in the center. Attach a piece of photo paper to the front of the lens, then shine a small flashlight through the pinhole. Developing the paper will reveal a projection of the entrance pupil onto the paper. It's easy to measure then.


26-May-2011, 19:55
Lenses with a central diaphragm compress the diameter of the beam of light flowing through the lens. Thus the physical aperture in the lens is smaller in diameter than the simple f/d formula suggests. Typically this means the actual aperture is 80% to 95% of the one the formula suggests.
I haven't measured a 127mm Ektar, so I can't help you on that one. The reduction for
a Symmar is 0.85 and I'd expect a Sironar, a similar design, to be very close to that.
Bill Peters

Roger Vadim
29-May-2011, 05:06
Valuabale Information everybody! Thanks.
I now made a new scale doing the eyeballing evaluation from the front and will shoot a couple of sheets to see if it is precise enough. But, well, the shutter is old and not tested, the lens has some separation issues and even a tiny amount of fungus, so we'll see if it performs niecely anyway.

Maybe a affordable Apo Symmar crosses my path soon in a good shutter ;) so I could get rid of DIY solutions... (but then this is part of the fun).

Bill: that mathematical solution is interesting: is the 0.85 is fixed value for all Symmar lenses? This would make things a lot easier. Never heard of that before - is it the same for all focal lengths? I have a 210 Sironar which would fit in the #1 shutter of the Ektar - it needs a new scale as well...

great forum btw.

29-May-2011, 05:40
Use a metric measuring device (easier math). Set the aperture. Measure the hole as above. Then measure your bellows extension. Divide. This is your f stop. No further bellows calculations are needed. (ie 10mm hole. 100mm of extension you are at f10)

Measure the different hole sizes use multiples of 5 for easy math. Mark the shutter. Then follow the above paragraph.