I have found 1891 B&J lens for 8x10 in a Unicum shutter that I want to use for portraiture but the iris blades have been destroyed (were made out of some sort of paper) I was wondering if I could just insert a piece of paper of a fixed apeture ala "waterhouse" type stop so that I can use this lens on my beat up old 8x10 , the Unicum shutter actually still works on T & B..so my shutter is working But how do I calculate the size of the hole to make my stop? It was set up for stops from F6 to F128 but I was thinking a stop around F11 or F16 would be about right or would it? Any suggestions? Thanks.

-- bill zelinski (willy226@yahoo.com), November 22, 2000

Answers

Divide the focal length by 4, 5.6, 8, 11, 16, etc.

F'rnstance 300/5.6 = 53.571428571428 ad infinitum. So drill a hole as close to 53.5714 mm as possible and that is your f 5.6. THe same would apply for f/64 etc. Bear in mind though that those values are for focus at infinity only.

-- Sean yates (yatescats@yahoo.com), November 22, 2000.

Bill, there is actually another step to be added to Sean's procedure. If you were sticking the aperture on the front of the lens you could follow his instructions. However, since the aperture is (I presume) physically within the lens, more than likely it will seem larger viewed through the front of the lens. The view through the front of the lens is where the aperture needs to be the size Sean described.

I don't think there is any reasonable way you can accurately predict the effective change in size of the aperture, so I would suggest trial and error. Simply make an aperture of known size, install it, then measure the apparent size from the front. To measure from the front, a crude method would be to lay a scale across the front of the lens and read it from as far away as you reasonably can; ie, from arm's length, etc. Once you find the size relationship between actual drilled hole and apparent size through front of lens, this should stay constant. Good luck!

-- Bill C (bcarriel@cpicorp.com), November 22, 2000.

I did this once by metering off the ground glass with a spot meter. You can compare the light readings to a known lens. You cannot have ANY stray light on the glass. I put the meter right against the glass and used it sort of like a general meter on a large area. It probably gives the most realistic idea of how the apertures compare, rather than trying to measure little holes. Besides, you probably don't really need to have perfect normal numbers like a modern lens. You just need to know what it is for the exposure. So what if a certain hole works out as f11 and a half? I would try to make the stops out of thin metal, however, for a cleaner edge. You could determine hole sizes in paper first, though. The lens I have actually is a Waterhouse stop lens, but is missing the metal slides. At least it has the slot in the barrel.

-- E.L. (elperdido65@hotmail.com), November 22, 2000.

thanks, its trial and error time.:)

-- photo123 (willy226@yahoo.com), November 22, 2000.

I just read about a really neat way to measure the effective size of the stop. You take the lens cap and (in the darkroom) put in a piece of enlarging paper. With the camera set up for infinity focus, you replace the ground glass with an opaque card having a small hole in the middle. Illuminate the hole briefly then develop the enlarging paper in the cap. There should be a dark circle that represents the effective aperture size. I haven't tried this and it's probably a pain in the rear, but thought it was kind of a clever idea.

-- Conrad Hoffman (choffman@rpa.net), November 22, 2000.

I've used a slightly more sophisticated version of Conrad's 'exit pupil' method to measure the actual aperture diameter of a range of 35mm lenses. This involved using a diffused laser spot in the film plane, and measuring the diameter of the exit beam using engineering calipers.
It's a very accurate method, but I found the measured and stated apertures to be extremely close, usually within the limits of measurement (~0.1mm).
In other words, you can be pretty confident that if you simply divide the focal length by the desired aperture number, then the resultant diameter will be close enough for all practical purposes.

-- Pete Andrews (p.l.andrews@bham.ac.uk), November 23, 2000.