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JohnGC
22-Dec-2008, 13:30
I have a beautiful 9 1/2 Ronar that the f stop ring is out of adjustment. Can I reset it by checking the dia of the opening at a certain f stop? I'm thinking that the focal length 9.5 divided by the fstop should yield the needed diameter...is that right?

Alan Davenport
22-Dec-2008, 15:16
Someone will probably correct me, but I think you're on the right track...

For sure, yes the diameter of the aperture is given by the f/stop. Be sure you use the same units, of course, so you'd calculate 240mm / 8 = 30mm aperture at f/8.

It's been my understanding (tho I've not checked it) that you can measure the diameter of the aperture as seen through the front of the lens, and that will be reasonably accurate. So you should be able to adjust the aperture so it measures 30mm (as seen through the front element) and set the ring to read f/8.

As I said, someone might correct me on this...

22-Dec-2008, 15:35
The question is a bit complicated since the physical aperture may be magnified (or demagnified) by the front cell of the lens. The effective aperture diameter you need is the magnified one, not its actual physical diameter. Making the problem more complicated is that the degree of magnification will vary with the distance you observe from. To accurately observe the diameter, you need to view it from infinity.

One clever suggestion to resolve these problems was to place a point light source at the focal point of the lens and measure the diameter of the projected beam. The approach is discussed in some detail here:

David A. Goldfarb
22-Dec-2008, 16:07
Here's a thread on photo.net demonstrating the method of measuring the aperture with a calipers through the front cell of the lens--

http://photo.net/large-format-photography-forum/00EJyf

I did this recently, and I was a bit unsure as to how to measure the "diameter" of a pentagonal iris, but I think I'm pretty close based on exposures.

JohnGC
22-Dec-2008, 16:20
Thank you both. Yup, I was gonna remove the element...Glad I asked, before I opened that can of worms an got dust in it!

I think I'll try the caliper method, and take some test shots under controlled light conditions (studio) and see how it does. My test shots last week looked about 1 full f stop off.

Dan Fromm
22-Dec-2008, 17:46
John, re test shots, are your shutter's speeds correct?

JohnGC
22-Dec-2008, 23:07
Hi Dan,

Yes, I was using my Packard bulb, and they were both good clean the exposure "squeezes".

Thanks