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View Full Version : Two lenses remounted, which aper. scale is right?



Michael_527
24-Jan-2001, 13:47
Hello all,

I have recently purchased two 150mm copy lenses that have been remounted in shut ters. The aperture scales were done by two different people (before I purchased them). Now, I was curious if the aperture scales would agree, so that I could match the better lens with the better shutter, and then sell the other.

Well, they don't agree, there is a visibly different aperture at the same settin g on both lenses. I don't have the original barrels, so I can't determine the c orrect aperture off of them.

Any recommendations on how to see which one is correct? I thought I might set u p a shot with a third lens, meter a spot on the gg, then change to the lenses in question and see which one agrees (all at the same aperture, of course). I don 't have a third 150, but I do have a 180 than I can use, and I'll try to meter a subject that is very uniform so that there is little chance for metering deviat ion.

Does this work? I think it should, as long as I work at the center of the I.C., and don't have any extraneous light from the back.

As a side note, I assume that when you have a lens mounted in a shutter (by Sk G rimes, etc) they don't apply a formula to determine the aperture. They measure off the iris in the barrel, right? How do they do it if they don't have the ori ginal iris, or if the existing iris is suspect (as in my case)?

Any thoughts would be appreciated.

---Michael

Michael_527
24-Jan-2001, 13:50
One additional note I neglected to mention:

They are identical lenses, so there is no reason the apertures should be different.

Thanks,

---Michael

Steve Grimes.
24-Jan-2001, 16:40
Surprisingly there are some variations between "identical" lens iris scales as supplied by lens manufacturers. This may be because of manufacturing conveniences or "rounding off". I really don't know why. I always prefer to have on hand the original factory iris scale from the lens in question. Second best thing is that I consult my records and/or look to see if I have another factory original shutter or lensbarrel for that same lens. The last resort if no original and no sample or record is on hand is to install the lens into the shutter and open the iris until the iris just stops being visible as it is opened. This, in ordinary lenses, corresponds to the maximum aperture marked on the lens. Such as, in the case of a f-5.6 150mm lens where it says 5.6 around the name ring. With that set, I measure the diameter and apply the formula fd=fl to determine a "trial" focal length, which should be approximatly 150mm. I use that trial focal length to calclulate the diameter of the other remaining stops.

Steve Grimes.

Guillermo Penate
24-Jan-2001, 20:40
One way to check verify which shutter has the correct aperture scale is by comparing it to other lens that has a known correct aperture scale. For that you would need a spot meter. This is what you do: 1- mount the known good lens on your camera and focuse it to infinity 2- aim the camera to a wall having even color and even- not changing illumination 3- set the aperture to -let's say- f/8 and take a spot meter reading of the ground glass "in axis" with the lens. 4- mount the lens you want to check and focuse it to infinity 5- aim the camera to the same wall and same spot of the wall as in #2 6- set the aperture to f/8 and take a spot meter reading as in #3

If the 2 readings #3 and #6 are similar, the aperture for f/8 is correct.

If your 2 lenses are incorrect, the same procedure can be used to make a new scale.

Guillermo