LOMO 3,7x parameters

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JKT
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LOMO 3,7x parameters

Post by JKT »

I tried to measure lens parameters from my LOMO 3,7.

With extension lengths of 50 mm and 100 mm I measured the magnification to be 2.47 and 3.99 respectively. That was with T2 fixing, so there is additional 55 mm to sensor.

From those numbers I calculated that the nominal lens is 9.15 mm in front of the RMS flange and the focal length of the objective is 32.9 mm. The interesting part is that with those values the nominal magnification 3.7 requires 91 mm of extension. That would make the tube length for nominal magnification 156 mm instead of 160. I'll have to see if I can define the tolerance, but 160 mm seems a bit far. I'd also appreciate if someone checked my numbers...

I intended to calculate the FOV too, but that didn't work out - APSC sensor was perfectly covered with the lower magnification as well. I'll have to try even lower extension to see where the limit actually is.

It would be nice if those with other LOMO:s would do similar tests as the information about their parameters seems to be somewhat scarce.

The pictures are unsharpened 1:1 crops of the stacked output. The center and corner of 3.99x and corner of 2.47x. The subject was Mitutoyo caliber. With a bit of sharpening those should look quite reasonable.

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Pau
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Re: LOMO 3,7x parameters

Post by Pau »

JKT wrote:...That would make the tube length for nominal magnification 156 mm instead of 160. I'll have to see if I can define the tolerance, but 160 mm seems a bit far. I'd also appreciate if someone checked my numbers
The 160mm specification refers to the distance of the objective shoulder (not the thread) to the end of the mechanical microscope tube where the eyepiece sits. The actual primary image is placed 10mm below it (at least in DIN standard), so the optical tube length is 150mm, closer to your calculation. Also be aware that the nominal magnification of objectives often is not exact.
Pau

JKT
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Re: LOMO 3,7x parameters

Post by JKT »

Pau wrote:The 160mm specification refers to the distance of the objective shoulder (not the thread)
Pau wrote:The actual primary image is placed 10mm below it (at least in DIN standard)
Both were covered. 91mm extension + 55 mm (T2) + 10 mm = 156 mm
Pau wrote:Also be aware that the nominal magnification of objectives often is not exact.
Apparently not. :D It would have been pretty accurate for 155 mm tube. With 160 mm it should be 3.84.

rjlittlefield
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Re: LOMO 3,7x parameters

Post by rjlittlefield »

JKT wrote:I'd also appreciate if someone checked my numbers...
I get the same numbers within measurement error. Calculated focal length 32.895 mm (=(100-50)/(3.99-2.47)), extension for 3.7X is 90.46 mm (=50+(3.7-2.47)*32.895)). Then distance to sensor at 3.7X would be 145.46 mm, versus magnification of 3.84 at 150 mm. This discrepancy from nominal is not big enough to make me think there's anything wrong.

--Rik

JKT
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Post by JKT »

OK - thanks!

Have you by any chance tested the FoV for this LOMO?

rjlittlefield
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Post by rjlittlefield »

Possible confusion... I get the same numbers by computing from your observations, to check your computations. I do not have a copy of this lens to test personally.

--Rik

JKT
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Post by JKT »

No harm done. I'm pretty sure of the measurements. I was more suspicious about the formulae. 8)

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