Microphotography with microscopes that have a finite mechanical tube length (usually 160 or 170 mm) can sometimes be challenging because the photographic adaptation was optimized for old analog camera systems. Photographic projectives or projection eyepieces from the analog past are often difficult to use in a satisfying way with today's digital cameras with their relatively small sensors. When using those old projectives, mostly only a small portion of the field of view will be captured. A possible solution for this is afocal photography, in which the virtual image from a normal eyepiece is turned into a real image with a camera lens and projected onto the sensor. This setup usually works well, but it also can have some problems like hotspots, reflections and optical aberrations that are introduced by the camera objective. And in general the mechanical connection to the microscope is more complex. If you want to do focal photography with a projective, then hybrid eyepieces are a good solution, see also: 'Microphotography with 'hybrid' eyepieces'.
There is also another way to make a good projective from a normal eyepiece by increasing the distance between field lens and eye lens with an intermediate ring. There is some resemblance with a hybrid eyepiece, the difference however is that an existing eyepiece is modified by placing a ring between the barrel and the eye lens, so the eyepiece is being extended. The necessary spacers may come from other eyepieces. Carl Zeiss and Zeiss-Winkel eyepieces, for example, have a black ring on which the eyepiece rests in the tube. These rings can be removed from certain eyepieces. The rings from some Zeiss-Winkel eyepieces contain screw thread which makes it possible to insert them easily. For spacers that don't have screw thread I currently I don't have a good solution for attaching them other than using tape. But maybe it's possible to make spacers with screw thread using a 3D printer.
I modified a Carl Zeiss Kpl8x eyepiece with different rings that came from a Zeiss Kpl10x eyepiece (10.5mm), Kpl12.5x eyepiece (17mm) and Zeiss-Winkel 5x eyepiece (25mm). Only the ring of the Zeiss-Winkel eyepiece contains screw thread, which makes it possible to firmly screw the 3 parts together.
Projectives based on a Kpl8x eyepiece. The eye lens is unscrewed and an intermediate ring is fitted between the barrel and the eye lens. A: Intermediate ring of Kpl10x eyepiece, 10.5 mm. B: spacer ring of Kpl12.5x eyepiece, 17 mm. C: Intermediate ring from Zeiss-Winkel 5x eyepiece with screw thread, 25 mm. D: final projective with intermediate ring of Zeiss-Winkel 5x eyepiece.
Using a slide of the diatom Arachnoidiscus and a stage micrometer, I tested the various Kpl8x projectives with the Canon 600D camera. With decreasing length of the intermediate ring, a smaller part of the field of view will be captured and at a certain point there is no longer any vignetting.
From left to right (1-4): stage micrometer and Arachnoidiscus photographed with Kpl8x projective using different spacers. 1: intermediate ring Zeiss-Winkel 5x, 25 mm. 2: intermediate ring of Kpl12.5x, 17 mm. 3: intermediate ring of Kpl10x, 10.5 mm, but with an additional rubber ring extended to 12.5 mm. 4: intermediate ring of Kpl10x, 10.5 mm. Camera: Canon 600D. Objective: Carl Zeiss Plan 25/0.45.
With the Canon 600D there was still vignetting when the Kpl8x eyepiece was extended with a 17 mm intermediate ring, but for a micro 4/3 camera like the Olympus PEN E-Pl1, this configuration turned out to be very suitable.
Stage micrometer and Arachnoidiscus photographed with the Olympus PEN E-Pl1 camera using the Kpl8x eyepiece extended with a 17 mm intermediate ring. Objective: Carl Zeiss Plan 25/0.45.
For cameras with a smaller sensor such as the 1" Nikon1 J1 an intermediate ring of approximately 19 mm turned out to be optimal. For this I used the ring of a Kpl12.5x eyepiece and extended it with an additional 2 mm thick ring.
From left to right (1-3): Stage micrometer and Arachnoidiscus photographed with Kpl8x projective with different spacers. 1: intermediate ring Zeiss-Winkel 5x, 25 mm. 2: intermediate ring of Kpl12.5x, 17 mm, but with additional ring extended to 19 mm. 3: intermediate ring of Kpl12.5x, 17 mm. Camera: Nikon1 J1. Objective: Carl Zeiss Plan 25/0.45.
The simplest and cheapest compensating eyepieces built by Zeiss were the C eyepieces. As a basis for a projective I used a C8x eyepiece and I extended this with the ring of a Kpl10x eyepiece. C8x eyepieces as such can hardly be used as projectives. When a C8x eyepiece is used in a raised position to function as a projective, the results will be poor. If the intermediate ring is fitted, an excellent projection eyepiece is created. C-eyepieces have no plan correction, so the results with plan objectives will be slightly less good than with the optics of a Kpl eyepiece. But for objectives without plan correction, this projective is perfectly usable.
When using objectives without plan correction, this projective appears to give good results, as shown in the image below. Here I used a more recent (black) Zeiss 25/0.45 achromat and focused both in the center and on the edge of the viewfield.
Far left: conversion of a C8x eyepiece to a projective in which the eyepiece is extended with the ring of a Kpl10x eyepiece. The other two images show a stage micrometer and Arachnoidiscus photographed with the modified eyepiece. Focussed in the center (middle photo) and at the edge (right photo). Camera: Olympus PEN E-Pl1. Objective: Zeiss 25/0.45.
A method that is often used to make a normal eyepiece function as projective is to raise the position of the eyepiece in the phototube. The results of this are very variable and it strongly depends on the type of eyepiece how the results are. Especially with simpler eyepieces, the results can be very poor. That also applies to the Zeiss C8x eyepiece. I used this eyepiece as a projective by raising it about 5mm in the tube. I compared the image I got from the raised eyepiece with the image from the modified eyepiece.
Arachnoidiscus photographed in phase contrast with Neofluar 40/0.75 Ph2. On the left, the C8x eyepiece was extended by 10.5 mm and used as a projective. On the right, an unmodified C8x eyepiece was raised appoximately 5mm in the tube. The right image is severely degraded due to distortion and chromatic aberration.
Normal eyepieces can easily be converted into good quality projectives by increasing the distance between the field lens and the eye lens. Even simple eyepieces such as the Zeiss C eyepieces can be modified in this way to be useful as a good projective. Zeiss C-eyepieces are easy to find at low prices on the second-hand market, making it attractive for users of 160 mm Zeiss microscopes to use them for focal photography.
Note: when mentioning this method at other locations please refer to this site.