FAQ: What is a JML 21mm f/3.5 lens?

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rjlittlefield
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FAQ: What is a JML 21mm f/3.5 lens?

Post by rjlittlefield »

Question: What is a JML 21mm f/3.5 lens?

Answer: It's an excellent lens with a wide field of view and almost no chromatic aberration that appeared on the surplus market in April 2010 selling for around $10. We have since figured out that it's probably a JML Model Number 61580, no longer being manufactured, with an original list price around $300.

Discussions at photomacrography.net include:

Specifications on the 61580.

Suggestions for mounting the JML 21mm.

Thread in which the JML 21mm was introduced to photomacrography.net.

Early discussion of trying to identify the lens.

Comparison with numerous other lenses at 4-5X.

--Rik

abpho
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Post by abpho »

Thanks Rik. Maybe now I can find that post where someone mentioned the ideal magnification for this lens.
I'm in Canada! Isn't that weird?

rjlittlefield
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Post by rjlittlefield »

abpho wrote:ideal magnification for this lens
Phrases like that have probably been used several places. Just remember that "ideal" is a moving target that depends on circumstances.

The design specs diagram mentions MAG= 0.0827, which for our purposes would correspond to 12.09X reversed. But an f/3.5 lens at 12X would give roughly f/45 effective. That would be deep in diffraction territory even on a 24mm x 36mm sensor.

With the DSLR sensors we use, the JML gives a sharper image at lower magnification on shorter extension. As the magnification drops, at some point the image goes soft starting in the corners. Exactly where that happens depends mostly on the size of sensor you're using. On my T1i APS-C sensor, at 2.5X on the shortest extension I can provide, my JML 21 delivers a high quality image circle that covers most but not quite all of the frame height. The edges and corners are not good. Adding extension to reach 5X, the image becomes good clear into the corners. For me, around 5X would be the "ideal" magnification. On a full frame sensor, the corresponding magnification would be somewhat larger although the field size on subject would be about the same.

--Rik

Ecooper
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Post by Ecooper »

rjlittlefield wrote:
abpho wrote:ideal magnification for this lens
Phrases like that have probably been used several places. Just remember that "ideal" is a moving target that depends on circumstances.

The design specs diagram mentions MAG= 0.0827, which for our purposes would correspond to 12.09X reversed. But an f/3.5 lens at 12X would give roughly f/45 effective. That would be deep in diffraction territory even on a 24mm x 36mm sensor.

With the DSLR sensors we use, the JML gives a sharper image at lower magnification on shorter extension. As the magnification drops, at some point the image goes soft starting in the corners. Exactly where that happens depends mostly on the size of sensor you're using. On my T1i APS-C sensor, at 2.5X on the shortest extension I can provide, my JML 21 delivers a high quality image circle that covers most but not quite all of the frame height. The edges and corners are not good. Adding extension to reach 5X, the image becomes good clear into the corners. For me, around 5X would be the "ideal" magnification. On a full frame sensor, the corresponding magnification would be somewhat larger although the field size on subject would be about the same.

--Rik
Thanks for setting up this FAQ Rik!

Any idea as to how this would translate to the best magification for using a jml 21mm with an Olympus 4/3 17.3×13.0 mm sensor?

Cheers.
EC

rjlittlefield
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Post by rjlittlefield »

Ecooper wrote:Any idea as to how this would translate to the best magification for using a jml 21mm with an Olympus 4/3 17.3×13.0 mm sensor?
I'd expect around 4X. That sensor has a 21.6 mm diagonal, versus the 26.8 mm that I see as OK around 5X.

--Rik

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