Effect on image quality of tube lens focal length?

[Josh]

Hi Guys,

I do most of my macro photograph with a macro lens and a Raynox, but I thought I'd try using some microscope objectives. I tried an RMS objective, and it worked great, but I also got an infinity corrected objective and that was pretty disappointing. The objective is an unbranded 10x 0.25NA lens which claims to be a plan achromat, and it seems to be. But it has pretty awful l longitudinal chromatic aberration and it is also quite soft overall.

The objective is designed to be compatible with the Olympus microscope systems, and so it expects a 180mm focal length tube lens. I was using a 100mm macro lens. It's my sharpest lens and the closest I have to 180mm. Obviously, the magnification was different to what would be expected, but I don't know if it would have any other effects. Since almost all of the converging is done in the objective I don't think the tube lens will have very much effect on the image quality, but I'm not super confident about that. I also have two different arguments, that 1) the best quality tube lens will yield the best results, vs 2) the objective is designed for a 180mm fl tube lens, so tube lens should be as close as possible to that fl.

Option 2 doesn't really make much sense to me, but optics is complicated so who knows? This person https://www.closeuphotography.com/tube-lenses suggests not to use zoom lenses as tube lenses, which makes sense. But doesn't compare prime lenses to macro filters he tests. On top of that, I'm a phd student in an optics lab and we just use simple achromatic doublets as tube lenses, and really use anything to set the magnification. In my current setup, I have a random 60mm lens I found in a box when the objective is specified for 200mm, although the system is pretty rough at the moment and we are not optimising for image quality at the moment. And I can correct for the aberrations in software easily enough.

Now, the objective was quite inexpensive, so I didn't expect it to be great, but I wanted to get the optimal image quality from it before I buy something else. Do you guys think using the right focal length lens would yield better image quality or would using one of the macro filters/dedicated tube lenses give better results? I also have an ancient zeiss 180mm f/2.8 lens for a medium format camera which I have, but the adapters to get it to fit on my digital camera cost more than I spent on the objective.


I had a quick search back on here, and couldn't find this same kind of question before. Well, not one in such a general setting. I'd greatly appreciate your thoughts and ideas, thanks.

[Lou Jost]

You've left out a really important factor: the size of an objective's good-quality image circle. There is a lot of discussion of this on the forum; if your tube lens is too short in focal length, you'll be seeing the bad-quality edges of the image circle. The difference between 100mm and 180mm is almost a factor of two; if you looked at the center half of the image, is that part decent? If so, using a longer tube lens will solve your problem. No need to use exactly 180mm.

[Josh]

I thought that also, the image circle fills the sensor (35mm) albeit with quite a lot of vignetting. But the image quality isn't great even in the dead centre.

[Lou Jost]

If it is bad in the center, not much can be done...shorter tube lenses should make the center sharper than normal.

[rjlittlefield]

Josh, welcome aboard!

Assuming the tube lens is focused at infinity, it would take an atrocious one to have much effect on image quality at center. Anything away from center can go bad quickly, especially with cheap objectives and short tube lenses.

The problem with zoom lenses is vignetting, not image quality. Zooms usually have their entrance pupil far back even at their longest setting, shrinking and moving even farther back when zoomed shorter. This results in blocking light rays headed to the corners of the sensor.

Raynox DCR-150 makes a good tube lens with nominal FL=208mm. But again the adapters add up in cost.

Longitudinal CA is common in objectives, even some nice modern objectives like the Nikon CFI's. See http://www.photomacrography.net/forum/v ... 672#147672 for example.

I hope this helps!

--Rik

[Josh]

Thanks very much, rjlittlefield. That does make sense. I think I've got a few things to try and experiment with. I was mainly surprised at the image quality as I also got a cheap 4x 0.10 plan achromat somewhere else, and it's pretty great. Since they get about the same magnification it was disappointing to find the higher NA one performing worse.

It's not the end of the world if I end up not using the 10x much. It's all part of the learning process. Maybe I'll even review it so that others will know what to expect if I find the time. That Mitutoyo, looks like an extremely well put together lens.