KimP wrote: ↑Fri Feb 03, 2023 6:39 am
Hi Rik,
This is a very nice and relatively simple setup using the AF focus of the relay lens controlled by the Camranger
I have a few questions:
Kim,
I apologize for late reply -- this one slipped through a crack in my reminder system.
You asked:
1) Is the microscope objective a finite or an infinite objective (does it matter)?
The objective is infinite, but at this low magnification it does not matter much. At higher magnifications such as 10X-50X objectives, infinite becomes progressively more important.
2) Which focal length would be optimal for the relay lens?
For most purposes 200 mm is good. Some objectives and some relays work well with a shorter relay, down to around 100 mm. The sharpest 5X setup in my kit consists of a 10X objective on front of a Canon EF 100mm f/2.8L Macro IS USM focused from infinity going shorter. See
AF motor focusing with a microscope objective for some discussion of that setup.
Infinite objectives give their rated magnification at one specific focal length for the relay lens. That is 200 mm for Nikon objectives. Using longer or shorter relay lenses simply increases or decreases the magnification, in proportion to the focal length of the relay. This process is limited on the long end because you get empty magnification, and on the short end because you get vignetting and/or aberrations in the corners of the image. The extent of vignetting depends strongly on unpublished details of the relay lens, so as a matter of practice you have to just try it, or find a report by somebody else who has tested the exact model of lens that you have. Zooms are particularly prone to vignetting.
3) Have you refined/changed the method since the original post.
Not really. The big change since then is that many cameras now include an in-camera facility for shooting a focus stack, so the Camranger is not needed to automate that part of the process. Some people shoot a lot of stacks this way, especially at lower magnification in the field. Most of my own work is done in studio at higher magnifications, with stack lengths and optics for which the AF motor approach does not work well. For me the setup shown here was mostly useful for real-time demo at a conference.
4) I have the old Camranger. Do you know any advantages of the Camranger 2 in addition to the ability to set the start and end points and automatically calculate the step?
I do not have any experience with the Camranger 2, sorry.
5) What would the minimum step length be (probably depending on the actual lens)?
If the relay lens changes physical length with focus, so the objective moves with respect to the subject, then the minimum step size will depend on that, and yes it will vary a lot from one relay lens to another. The particular cheap zoom tele shown here works OK with a 4X objective, but if I recall correctly it gives focus banding with a 10X objective. On the other hand, if the relay lens does
not change physical length (internal focus), then the minimum step length is always short enough, even if with a high power objective "short enough" means just a couple of microns. This is because the objective turns a subject that is physically small and close, into one that is virtually large and far, and then the relay lens just steps focus through the large/far virtual subject. Any lens with AF must be capable of focusing well enough, at any position in its focus range, or its users would be unhappy and the lens would be hard to sell. Focus-stepping with the AF motor just takes advantage of this same capability.
Again, see
AF motor focusing with a microscope objective for a longer discussion.
--Rik