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05-22-2011, 07:13 AM   #1
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Variation in true focal length between copies of the same lens

How much variation in true focal length (at a given focus distance) might one expect between two "identical" lenses due to sample variation? I'm only asking only out of curiosity and not out of concern

I happen to have both a newer FA43 and and older FA43 that I was testing to determine if I could see any difference in their output. Specifically, I was trying to see if there was any truth to the notion that the older 43 is actually superior to the newer 43, possibly due to different (leaded?) glass.

Anyway, the first thing I noticed when I took some test shots was that there was a visible difference in the magnification of the shots between the two lenses. These were shots taken on a tripod of a static object and care was taken not to disturb the tripod while changing lenses. The end result was that there was 1.6% more magnification when using the older 43. This translates to about 0.7mm of focal length, so certainly nothing major, but I was just wondering if one would expect this kind of variation between sample of the same lens. I suspect the answer is that this is normal variation, but I was curious if anyone else had any insight.

05-22-2011, 07:36 AM   #2
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QuoteOriginally posted by dgaies Quote
How much variation in true focal length (at a given focus distance) might one expect between two "identical" lenses due to sample variation? I'm only asking only out of curiosity and not out of concern

I happen to have both a newer FA43 and and older FA43 that I was testing to determine if I could see any difference in their output. Specifically, I was trying to see if there was any truth to the notion that the older 43 is actually superior to the newer 43, possibly due to different (leaded?) glass.

Anyway, the first thing I noticed when I took some test shots was that there was a visible difference in the magnification of the shots between the two lenses. These were shots taken on a tripod of a static object and care was taken not to disturb the tripod while changing lenses. The end result was that there was 1.6% more magnification when using the older 43. This translates to about 0.7mm of focal length, so certainly nothing major, but I was just wondering if one would expect this kind of variation between sample of the same lens. I suspect the answer is that this is normal variation, but I was curious if anyone else had any insight.
A deviation of 1.6% is neglible. The fl printed or engraved on the lens is in most cases much more off, as most manufactureres stick to well-known and well-used lens types in their denomination. So you may find a 21mm lens, with 20mm effective fl or 22mm. I once compared that for a couple of makes and lenses and especially on the shorter end of the focal range, the roundion up or down to the next standard fl is often much more than thos 1.6%.

And yes, a slight difference between lenses from different production runs is not unreasonable.

Ben
05-22-2011, 10:46 AM   #3
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QuoteOriginally posted by Ben_Edict Quote
A deviation of 1.6% is neglible. The fl printed or engraved on the lens is in most cases much more off, as most manufactureres stick to well-known and well-used lens types in their denomination. So you may find a 21mm lens, with 20mm effective fl or 22mm. I once compared that for a couple of makes and lenses and especially on the shorter end of the focal range, the roundion up or down to the next standard fl is often much more than thos 1.6%.

And yes, a slight difference between lenses from different production runs is not unreasonable.

Ben
Thanks for the reply. I agree the difference is negligible. Like I said, I certainly wasn't concerned in anyway, mostly just curious
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