200 mm as tube lens

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BugEZ
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200 mm as tube lens

Post by BugEZ »

It was suggested (wisely I think) that my 10X Olympus SPLAN 0.30/170 lens would benefit from a tube lens. After I figured out what a tube lens is, I purchased a 200 mm telephoto to place between the objective and my camera. As I have a Pentax DLSR body, I chose an inexpensive fixed focal length lens. I am quite pleased with the result, recognizing that I expect things to get better when I shift to my other actuator that moves the subject with smaller step size. The step size should be reduced by 4X if I have interpreted the formulas on this website correctly.

Pictured is the Pentax 200mm f4.0 lens with the adapted microscope lens on the end.

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As was suggested by others, I used a "hollow" filter as a stand-off and a drilled out lens cap to attach the lens. After I convince myself that the rig is sound, I will blacken the threads to reduce glare inside the lens.

The subject is a rather tiny ant, the head being .5mm from eye to eye. The image is from 56 exposures, with .001" (.025mm) steps. I processed using CombineZM and CombineZP. I like the results best with CombineZM and my customized macros. After I get a good stack with reduced step size I may try another software package.


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Aloha

SONYNUT
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Post by SONYNUT »

they sell a din to 52mm adapter on ebay
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Just shoot it......

rjlittlefield
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Re: 200 mm as tube lens

Post by rjlittlefield »

BugEZ wrote:It was suggested (wisely I think) that my 10X Olympus SPLAN 0.30/170 lens would benefit from a tube lens.
Hmm... If you are referring to postings made in the thread HERE, then I am concerned that you misunderstood what was being said.

I see two comments in there about additional glass:
NikonUser wrote:The Oly 10x SPLAN objective, and all the Oly SPLANs and SPLAN Apos. need a corrective lens (usually an eyepiece or a NFK relay lens) to function at 100%.
rjlittlefield wrote:It may be counterintuitive, but all of the newer excellent objectives require additional glass between the objective and the sensor. This is because they are "infinity" designs that require a "tube lens" to complete the image formation.
The comment by NikonUser refers specifically to the lens that you have. It relates to the fact that Oly SPLANs are designed to have a certain amount of chromatic aberration by themselves, which is removed by a compensating opposite amount of chromatic aberration built into specific corrective lenses such as an Olympus eyepiece or relay lens. When the objective is used by itself or in conjunction with another lens that does not have the proper amount of CA built into it, the image will not be as good as it could be.

The comment by rjlittlefield about tube lenses was not referring to your lens in particular, but rather to your general concern that "The need for additional glass between the bug and the sensor is a bit discouraging". The "newer excellent objectives" that it speaks about are things like the Nikon CFI and Mitutoyo objectives, which are specifically designed to work with tube lenses and are not so good without them.

In contrast, the lens that you have is a finite design, not infinity, and the telephoto that you're using with it is not serving the role of removing the CA that's designed into the Oly objective. I'll trust your judgment that it's better with the telephoto than without, but neither configuration is using the Oly to best advantage.

When you're looking at objectives, take note of designations like 160, 170, and ∞ (the infinity symbol, like an "8" lying on its side). 160 and 170 indicate a finite objective, intended to be used without a tube lens. ∞ indicates an infinity design, intended to be used with a tube lens. Other designations are described HERE.

Just thought I should clarify...

--Rik

BugEZ
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Tube Lens concerns...

Post by BugEZ »

Hmmmmmm.... I will have to digest these comments. I'll shoot a comparison with and without the 200mm.

If nothing else I have a compact telephoto lens that is sharper than my 140-300mm zoom for taking photos of the wrens in my wren house.

:D

ChrisR
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Post by ChrisR »

I would have thought the CA would show more than it does here. I have a couple of older Olympus and other objectives which "need" correction, which make it pretty obvious.
BugEZ, try a pic of some black dots/lines printed on white paper?

Cyclops
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Post by Cyclops »

Hmm another good reason for going with Pentax! Wish I had to be honest!
Canon 5D and 30D | Canon IXUS 265HS | Cosina 100mm f3.5 macro | EF 75-300 f4.5-5.6 USM III | EF 50 f1.8 II | Slik 88 tripod | Apex Practicioner monocular microscope

BugEZ
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Post by BugEZ »

Many thanks for comments. The attached show the results of a "shoot off" this afternoon.

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I will look over the telephoto lens (recently purchased on E-bay) and make sure there are not any obvious issues. The exposed front lens and back lens looked pretty clean but I'll give it a second look with stronger light. Aside from that, the shoot off suggests I am more pleased with less glass in the setup.

Thanks again for the consultation RiK and ChrisR...

Keith

ChrisR
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Post by ChrisR »

On the ant pic the CA only really seems to show in the corners.
Just use the middle 3/4 of the frame, then :wink:
You probably know that
>> CA can be improved somewhat in Photoshop and
>> this distance should be 10mm less than the tube length marked on the objective:
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- yours look a bit long,,
>> Your objective is probably not designed to cover something as big as your sensor anyway, but
>> going a bit long on the extension usually helps a bit with that.

rjlittlefield
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Post by rjlittlefield »

Nicely documented!

I agree about the CA -- definitely present with the objective alone, and definitely much reduced when used with the tube lens. I would not expect the telephoto to have much CA of its own to cancel against the objective's, so I'm thinking the reduction in CA must be a side effect of dragging the finite objective far away from its designed focus arrangement by using it like an infinite.

That dragging away from designed focus is probably responsible for a lot of the loss of overall clarity in the tube lens shot. What that does is to introduce a lot of spherical aberration in the objective. The center and edge of the objective no longer focus at the same depth, and as a result the image loses contrast for fine detail.

With some careful reading, you can see HERE how that works with a Nikon lens of similar specs to your Olympus. The illustration shows a degraded image paired with a good one. The exact images shown are from a finite CF on telephoto (the degraded image) versus an infinite on telephoto (the good one). But it's shown earlier in that thread that the CF on bellows (focusing as designed) is almost identical to the infinite on telephoto. So for practical purposes you can think of the comparison as illustrating finite CF on telephoto (degraded) versus finite CF on bellows (good).

--Rik

BugEZ
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One more test...

Post by BugEZ »

I'll add a test with the objective distance ~ 160mm on the extension tubes. Thanks for the sketch. The distance (measured with a meter stick) of the testing that I described in my previous post is about 215mm.

The addtional test will have to wait as I have work commitments that will keep my nose to the grindstone for a week or two...

Another "fun" test would be to focus the 200mm lens to something other than infinity... Say 170 mm... :wink:
BugEZ

rjlittlefield
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Re: One more test...

Post by rjlittlefield »

BugEZ wrote:Another "fun" test would be to focus the 200mm lens to something other than infinity... Say 170 mm... :wink:
I'm sensing some possible confusion, so let me offer a "pre-emptive explanation" that may save you some work.

Your objective is designed to produce light rays that converge as they come out the back of the lens, coming to a focus around 170 mm away. With the telephoto focused at infinity, you're forcing the objective to produce light rays that are parallel as they come out the back of the lens. If you focus the 200mm lens closer than infinity, then you'll be forcing the objective to produce light rays that diverge as they come out the back of the lens. In other words, focusing the telephoto lens closer than infinity pushes the objective even farther away from its design point. I grant that the test would be amusing and educational, but I wouldn't expect the image quality to improve.

--Rik

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