20X Mitty

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

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leonardturner
Posts: 713
Joined: Thu Mar 14, 2013 11:40 am
Location: Atlanta, GA, USA

20X Mitty

Post by leonardturner »

Will the 20X Mitty NA .42 cover a 24 X 36mm (full frame) sensor using an approximately 200 mm intermediate lens? What about with different focal length tube lenses?

Thanks!

Leonard

Greenfields
Posts: 116
Joined: Thu Nov 25, 2010 10:54 am
Location: Nottinghamshire, England

Post by Greenfields »

Yes. I would recommend this objective, especially for its working distance.

All microscope objectives produce images which have lower contrast and resolution away from the centre of the frame because of aberrations at higher field angles.

With the 20x 0.42 objective the n.a. no longer increases in proportion to the magnification in the way it does with the well-regarded 5x, 7.5x and 10x objectives in the same series, so - if you look for it you may see evidence of degradation closer to the centre of the frame than with the others but I would say that this is rarely pictorially significant and you do not have to correct for corner shading.

For example:

With a 5D MarkIII sensor and the lower magnification objectives [5x, 7,5x and 20x] the objectives will easily out-resolve the sensor in the centre of the frame. Moving away from the centre the gradual drop in resolution and increase in aberrations is not detectable until the resolution of the sensor takes over, and then there is a small drop in image quality into the corners. Its clear on a flat subject full of high frequency detail [like a silicon wafer of dies] but almost never pictorially significant.

The 20x approximately matches or slightly exceeds the resolution of the same sensor in the centre of the field, so you will notice signs of deterioration closer to the centre - but only if you look for it and with a sensitive subject. Inevitable that means that the corners will appear to be "worse" than those of the lower magnification lenses - but that will rarely matter.

Whether its worth using a tube lens with a longer focal length depends on the resolution of your sensor and judgement. In my judgement the 20x 0.42 only slightly out-resolves the 5D2 sensor in the centre of the frame so a longer focal length tube lens will produce mostly "empty magnification" but at least you will capture every bit of detail the lens can record.
Feel free to edit my images.

leonardturner
Posts: 713
Joined: Thu Mar 14, 2013 11:40 am
Location: Atlanta, GA, USA

Post by leonardturner »

Thanks, Greenfields, I very much appreciate your input. This helps considerably.

Leonard

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