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Besides producing the legendary JML 21mm f/3.5, JML Optical have also given the world a nice 20x NA.30 objective.

20:1 objectives are hard to find, expensive and tricky to use: a leap up from 10:1 especially in terms of dust on the sensor.

JML 20x NA.30 objective, not in the same league as the Mitutoyo MPlan Apo 20x but at a fraction of the cost, the value for money isn't so awful.

A Curious Objective

The magnification of the JML 20X NA.30 is a bit of an ongoing mystery as it does not actually give you 20:1 on a 200mm tube lens when used as an infinite like you might expect.

When it is on a 200mm tube lens I get approx 17.5:1 out of it. To obtain 20:1 from it, it needs to be mounted on bellows at about 22cm extension.

Extreme Macro Performance

With a working distance of 12mm, there is enough room for the lighting to make its way down into the troughs and valleys of your mounted specimen. This corresponds roughly to a 20:1 LWD objective. The JML exhibits lateral chromatic aberration, visible as colour fringing, especially in corners.

corresponds roughly to a 20:1 LWD objective

In the centre, the JML 20x NA.30 has good resolution, but it does degrade quite quickly. Given this fact, it might be interesting to try this objective at 30:1.

This might not be a big problem for photographers doing portraits of insects which tend to be centred. The lens is not in the same league as the Mitutoyo Plan Apo 20x, which is remarkably free of CA and has about twice the working distance - but that is the class leader, an infinite objective which costs between 30 and 60 times as much (US$1000-$2000).

Lateral Aberration

Chromatic aberration lateral aberration is a lot easier to correct in post processing is a failure of a lens to focus all colours to the same convergence point and can be easily worked away by using a Photoshop filter with 'Fix Red/Cyan Fringe' put to the left on the control. Lateral aberration occurs when different wavelengths are focused at different positions in the focal plane (because the magnification and/or distortion of the lens also varies with wavelength). The main thing is though, lateral aberration is a lot easier to correct in post processing than axial aberration (the purple kind).

Price

The JML 20x NA.30 is hard to find and was available as a one time deal for obsolete equipment in 2012 for UK£30. This represents outstanding value for money for anyone working with bellows who does not have a way of reaching 20:1.

There are other 20:1 objectives that are more easily found such as the Nikon 20x/0.40 ELWD finite objective and the Mitutoyo 20x/0.42, but both of these are much more expensive, usually north of £1000.

Besides the JML, there are no good, inexpensive, and regularly available 20X infinite objectives with decent working distances. Counter findings very welcome!

More From JML Optical

JML Optical are better known in extreme macro circles for the iconic JML 21 f/3.5, a very hard to find lens. This company builds outstanding optics for industry and it would be worthwhile trying out more of these for extreme macro.

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