Field of view coverage, plan/non plan comparison

[Hobbyst46]

These title subjects have been covered a lot, but since questions about them often arise, I thought that posting a small demonstration might help folks who consider objective upgrades etc. It is a snapshot of my present setup, old and far from optimal. I just received, from a colleague in a research lab, a nice test sample - a professionally-made H&E stained mouse tissue slide.

On the Zeiss Standard GFL microscope, there are Olympus 10X, 20FN eyepieces and Zeiss objectives. Afocal setup. The camera lens is a Canon prime 50mm/1.8 EF (focused at infinity), connected to the EOS-M camera via a Chinese electronic EOSM-EF adapter. A 49mmx0.75 -> 1.25" telescope adapter connects the 49mm filter thread of the camera lens and extends over the phototube. A custom-made PVC spacer sleeve provides tight fitting between the adapter and the outer surface of the photo tube. The Zeiss KPL 8X, 18FN "photo" eyepiece rests normally in the photo tube without contact with the adapter or spacer. Parfocality between the camera and viewing eyepieces is verified. The photo tube is side-clamped to a shelf, to enhance mechanical stability.

Field of view coverage: estimated from the camera LCD screen and the FOV of the eyepieces ("bino"). The camera image size (length) is 64% of the bino. Since the latter is larger by 20/18 than the photo tube FOV, the FOV coverage by the camera is 71% of the theoretical, length-wise.
Previously, my camera lens was an EOS-M 15-45mm zoom, f3.5-5.6, locked at 45mm, directly connected to the camera. It yielded much higher coverage - 90+% of the FOV, yet I switched because prime lenses are said to be better suited to micrography than zoom lenses.

Planarity of the camera image: the uniformity of focus across the FOV clearly depends on the objective. However, I restrict my conclusion to the camera image, since it catches the central portion of the sample. The Planachromat 25X objective yields edge-to-edge uniform sharpness, whereas the non-Plan 25X phase-contrast yields uniform sharpness in barely 60% of the FOV. With the 40X, there is no such difference: both the Neofluar (NA 0.75) and the Planachromat (NA 0.65) yield uniform sharpness across the FOV. In principle, these results are expected from literature, yet are still nice to see, in my opinion.

All are single images, resized t0 1022 pixel width, not cropped, saturation tweaked a little in post processing.

[mrsonchus]

Looks pretty conclusive to me, the plans have it.

John B.

[apochronaut]

The term plan is entirely relative, though. There is no standardization and a plan objective used with a recommended eyepiece can become semi-plan with a similarly corrected eyepiece of a wider field.

Plan f's.o.v. have increased over time too. It used to be common to see plan objective/eyepiece combinations of 16 or 17mm, even as recently as the early 90's and now they are more often 20mm minimum. Earlier plan objectives pushed that far would have hopeless curvature of field.

[einman]

I agree with Apochronaut. The term "plan" is relative, both within a brand, across brands and across eras.

[MicroBob]

Hi Doron,

as ever a nice and conclusive comparison!

You could optimize you field of view by using a pancake 40mm lens like the old Pentax-M 40 1:2,8 from the late seventies.
This an be found on a Pentax ME camera that nobody pays much money for.

Bob