Pau wrote:
A 40/0.65 is an achromat or plan achromat, while the 20/0.75 is a modern plan apo and we are getting only the best part of the image circle. It will outperform the 40X for sure
Not sure about this. I didn't mention the structure of it, perhaps it was unclear. Let's control everything and make it an infinity apo objective. The mit 50x has an NA of 0.55 or something, not even 0.65. Welp, trade-off for the WD.
Pau wrote:
I don't understand it. Resolution (measured at the subject side as it should be) is dependent of NA not magnification. With infinite corrected objectives magnification can be changed with the tube lens FL as others have pointed. No problem here except about coverage when you go downwards.
I was conflating sharpness and resolution, so I was wrong here.
Pau wrote:
I don't understand the role of thermal expansion here.
Yeah that was brought out of nowhere. It was probably 4am when I posted the rather incoherent response. I was thinking about the thermal problems of specimens when one reaches very high magnifications. I've seen the problem demonstrated, hence the "could". I've never tried myself however.
Lou Jost wrote:Macro_cosmos, most of the things you've said are the very common misconceptions I tried to dispel in the first entry of this post. Most of your comments are mostly correct for finite objectives, but not for infinity-corrected objectives on tube lenses. As Pau has noted, and as Rik has noted elsewhere, the rated magnification is not the "native magnification", it's just the rated magnification when you use a 200mm tube lens (or 180mm for Olympus). You can use shorter or longer lenses without penalty, as long as the FOV and NA allow it.
Yeah I was conflating the concept of sharpness with resolution here. Of course how much "resolution (defined in my perspective)" one can get will depend on technique and lots of other issues, but that's just me confusing myself. I like to think of resolution in the sense of electronics (ADCs and etc), a rather silly false equivalence to get myself trapped in since optics is entirely different.
The unrelated problem here would be the availability of such tube lenses. Plenty 200mm tube lenses can be found, but I've never came across dedicated 400mm tube lenses. I'm sure they do exist somewhere. Meanwhile, one would have to use lenses designed for cameras. The 400mm f/5.6 nikkor you metioned is kind of a rarity. It's not expensive but kind of difficult to come by. Nikon also has an f/3.5 version which is rather expensive (and a great lens optically).