typestar wrote:... I assumed -- according to all the testing with NON official "tube lenses" like even simple zoom lenses - and (sometimes far) outside of the range of 200 mm --that the distance from tubelens to microscopelens is not "super critical".
This is a case of "it depends". Changing the separation can change several different aspects of off-center behavior -- notably CA, astigmatism, and flatness of field. The best separation for one aspect is not necessarily the best for another. For example some lenses I've tested (I don't offhand recall which) show best corner sharpness when there is significant curvature of field. Changing the separation can flatten the field but degrade best sharpness. That's why I specifically said "for focus-stacking applications", because focus stacking removes curvature of field as an important factor.
Lenses that are designed for use by themselves, such as all standard telephotos, have internal apertures that are placed for good performance. There are solid theoretical reasons to think that corner performance will be degraded by stopping the lens at a much different place, such as by sticking an objective in front of the telephoto's front element. As a matter of practice, this does not seem to be a big problem on APS-C. But full-frame is harder. Quoting John Hallmén's comment at
http://www.photomacrography.net/forum/v ... 6273#76273:
From an image quality stand point I wouldn't really recommend the morfanon
to anyone not using a FF sensor.
I tried at least 10-15 different primes and prime+telephoto combinations before I got tired of it. With the Mitutoyo 10x/0.28 I could never get good edge/corner definition on FF. I imagine many of these solutions would have been great on an APS-C sensor though since contrast, center sharpness etc often impressed me and the "good" field often extended at least beyond the height of the frame (24mm). What I liked with the morfanon was that with some objectives (Mitutoyo M Plan Apo 10/0.28, Nikon CF Plan 50/0.55 EPI ELWD) it gave a very consistent performance across the frame on FF – even at lower magnifications.
What is special about the official tube lenses is that they are specifically designed for good optical performance with significant separation between the tube lens and the objective.
It turns out that some other relatively thin lenses like the Raynox 150 and some simple achromats also hold up well to significant separation. In some cases the image quality gets better with say 25-50 mm added separation than it is with a minimum adapter.
rjlittlefield wrote: ThorlabsPlus3LongFocusedAtFFCorner: Thorlabs ITL200, ~90 mm separation to aperture ...
So, (sorry for my silly question, again)-- the setup means: for optimal results, mount your microscope lens at least 90 mm from the frontside of the tubelens = distance from front of tube-lens to shoulder of microscope lens should be ~90 mm (?)
And, what is the
optical length needed from Camera-bajonett up to the tubelens-shoulder with the 200mm tubelenses -- as this seems to result in a rather long setup, then...-
thorlabs specs are: 148 mm (image plane to back of tubelens + 28 mm length of tubelens + 90 mm to shoulder of microscope lens = a total of 266 mm up to the microscope lens -- is this correct? So rather a long tube then.
Yes, that's correct. It's quite a long setup.
rjlittlefield wrote: However, for focus-stacking applications the differences are small when each lens is used in its best configuration
As the appreciated & passionated users of this fine forum spend so much time with their equipment -- for their pictures - it is good to know, which brings up the best results in their setup... -- not to "waste" further time...
You know, I really wish I could tell other people how to get the "best results in their setup". But I can't.
In addition to the complexity of what happens with different lenses, there is the huge problem that different people have different criteria. Personally I tend to weight sharpness over color, so if you give me a subject that is 4 mm long I will reach for a Nikon CFI Plan Achromat 10X NA 0.25 (MRL00102) and push it down with a short tube lens so that I have say 5X NA 0.25 with significant axial CA. In contrast, one of my friends is very sensitive to color -- he will reach instead for a Mitutoyo Plan Apo 5X NA 0.14. Even though we agree completely on what the lens characteristics
are, we disagree emphatically about which lens is "
better".
So really, I can only answer a few questions:
1.
Do the official tube lenses cover full frame? Yes, optically. But beware: the long distance from sensor makes them vulnerable to clipping by other components. Having a thick-walled extension tube mounted on the camera is practically guaranteed to cause hard vignetting with a full-frame sensor.
2.
Does separation between tube lens and objective matter? Yes. Follow manufacturers' recommendations if available. Otherwise, test to optimize against your own criteria.
3.
What tube lens would I recommend? Thorlabs ITL200; unless cost matters, in which case Nikon MXA20696 (Edmund 58-520); unless convenience matters, in which case Raynox 150 or a telephoto because all the required adapters are cheap and easy to get. In other words, "it depends".
4.
How does an "ABC" tube lens compare to some particular "XYZ" telephoto/enlarging/process/viewcamera lens? I have no idea. If you have one, test it and see if you like the results. I try to organize my testing to get information that can be reused as widely as possible. As a matter of practice that means restricting my efforts to lenses that are routinely available, typically still being manufactured.
So, what kind of misuse happens to the optical power of the whole in comparison, if we use the non official "tubelenses" so far outside the specifications as 135 mm or even down to 100 mm, as you showed it so nicely with the Nikon 10x/0.25 lens?
I don't know what sort of information you're looking for. Can you specify more precisely what you want to know?
--Rik
