Lenses and Micro objectives

Starting out in microscopy? Post images and ask questions relating to the microscope and get answers from our more advanced users on the subject.

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Lenses and Micro objectives

Post by hotrodder19 »

Am currently using a 200mm Nikkor lens with my 10x Infinite objective and only get a small part of an insect. Firstly why is a 200mm lens beneficial. Secondly what happens if I drop to a 100mm lens with the 10x micro? What is the result if I drop to a 4x micro with a 100 and 200mm lens? Just trying to get most of an insect in view but magnified. Options ?

Info and ideas please, just getting started.

Cheers hotrodder19

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Post by rjlittlefield »

With the objectives, you'll run into two issues: magnification and coverage. Objectives in the Nikon BE series are designed to give their rated magnification when used with a 200 mm rear lens. Assuming that the rear lens does not introduce vignetting, then on 200mm rear lens the objectives will cover a full frame sensor with decent quality image corner to corner. If you shorten the rear lens to 100 mm, then the magnification will drop to 1/2 of the objective's rating and its coverage will also drop by 1/2. So, a 10X objective on a 100 mm rear lens will actually deliver 5X, and on a full frame sensor it will vignette pretty badly. Likewise for a 4X objective on 100 mm rear lens -- it will give 2X and vignette.

If what you really need is 2X onto a fullframe sensor (you're using a D750, right?), then the classic solution is to use an old enlarger lens reversed on bellows. Probably the most frequently used enlarger lens for this purpose is the EL Nikkor 50mm f/2.8, set around f/4-f/5.6 so as to give effective f/12-f/16.8 back at the sensor. That is certainly not the world's sharpest lens, but they're cheap and conveniently available. The Schneider Kreuznach Componon-S 50mm f/2.8 is better, at least if you get one that's still in good condition. There are lots of other possibilities, but I'd be completely comfortable using either of those, even on my D800.

I'm not intending to derail this new thread, but I am mindful that when last we talked, you were struggling and failing to produce a sharp image using your 10X objective. Working at 2X is a lot simpler, since the same movement will produce only 1/5 the blur. But it's not enough simpler to overcome all the difficulties you had with the objective. So since you asked for ideas, I'll suggest making friends with your flash units for this sort of work.


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