rjlittlefield wrote:And there it stalled.
Yeah, I've never been happy with the way that thread stalled out, either. As best I can recall, it ran afoul of the constraints of time and higher priority problems.
I think we've all been there. The term I usually use is "life got in the way".
rjlittlefield wrote:
Any contributions you can make toward fully explaining the experimental results will be most appreciated.
However, please note that "drive-by criticism" is generally not helpful or productive.
My apologies for that. I tend to come on a bit strong, sometimes.
rjlittlefield wrote:
You write that
Of course the 38mm lens "degrades" at f5.6.
and imply that I am surprised by that. In fact, I am well aware of effective aperture and diffraction, and why those phenomena imply that a macro lens on bellows "should" give best resolution when operated wide open. However, only the best lenses actually meet that expectation. Lesser lenses, typical of those used by many of this forum's members, often perform best at less than maximum apertures. In context, the point of the sentence that you complain about is only to emphasize that different lenses behave differently in practice -- which is why it's a Very Good Idea to test your own lenses to see how they actually perform.
Agreed, obviously. I was having a bit of trouble discerning your test methodology. I'll have to give it another read.
rjlittlefield wrote:
You also write that
Rik noted the problem of diffraction and effective aperture, but didn't actually do the calculations that would have shown him the light...
As far as I know, the only issue that I'm still in the dark about is why the microscope objectives exhibit so much larger cone angle than the macro lenses, despite calculations to the contrary based on NA and f-number (not all shown in the thread) .
I would be most grateful if you can provide those.
I cant, because there is no relationship between cone angle and NA (or f stop).
The cone angle is determined by "field stops", which are independent of whatever stop sets the system aperture. Basically, you just ray trace the lens design, and this will show you what stops the light.
The field stops usually have to be pretty far from the aperture, to reduce the field without stopping down the wanted ray bundles. The cones we're dealing with here are so narrow (minimum extension on the bellows setup I described earlier is 156mm. For a 43mm "full frame" sensor, that's a 2*arctan(43mm/2/156mm) = 15.7 degrees cone. A lens is typically field stopped by the sizes of one or more elements, but for macro lens (and objective) designs, that's typically a huge cone, at least 30 degrees.
Now, if we pick on the 16mm Luminar, it's field stop is the actual rear of the lens tube. The tube is pretty much as large in diameter as is possible for the RMS mount, so it's essentially "stopless". The rear element is 6mm in diameter, 26mm from the opening of the tube, which is 17mm in diameter.
(I'm using the plastic caliper for some quick measurements, instead of putting the plastic extensions on the good digital caliper, so the numbers are crude, and I'm using whole mm).
That means the angle where the tube first starts to occlude the rear element is arctan((17mm-6mm)/2/26mm) = 11.9 degrees. In other words, even at the shortest bellows extension, there should be no vignetting in a 76mm diameter circle.
The Nikon CF M Plan 20 ELWD, by comparison, has a very different construction, the rear element is recessed about 19mm into a hole the same diameter as the rear element, 9mm. You get some vignetting as soon as you move even slightly off center, but you need to go 0.3x the diameter of the circle (if memory serves) to be 1/2 stop down, and that's arctan(0.3*9/19) = 8 degrees. Add that 19mm to the 156mm for the bellows and adapters, and that 1/2 stop down circle is 48mm in diameter.
Vignetting should be a pretty rare phenomenon, and actual stopped fields, ever rarer.
)
So the "N" has uses in the database, but not for identifying tube length. Is the parfocal distance for the 160mm tube CFN still 45mm?
