Vignetting in photomicrography will be very familiar to microscopy enthusiasts when exploring and optimising homebrew camera/relay lens/adaptor setups. There are two main types.

The first occurs when only part of the circular visual field is presented at the film / sensor plane giving either a complete / partial black circular border or at the corners. This type is likely the least acceptable especially if it limits the amount of subject captured.

The second is when the full visual field is captured but again gives either a complete or partial black circular border. This type when experienced on small format film is likely not acceptable but in these days of predominantly digital cameras it is not necessarily irksome and potentially has some benefits both with image capture and ancillary equipment costs. These aspects are explored below based on my own experiences.

 

Previous experiences setting up a Leica S series stereo (May 2016 Micscape article)
Since the advent of affordable digital camera technology the more modern microscope systems both stereo and compound have been designed primarily around small image sensors for the photo port. This was the case with my Leica S series stereo. Using the first stage reasonably priced photo adapter a typical '1 inch' sensor microscope camera was intended to give vignette free parfocal images (image right below). My camera at the time was a Canon 600D APS sensor DSLR and on hindsight made the mistake of investing in the second stage Leica adapter for full frame use (image left below). It was expensive, produced an impractical, long, vibration-prone extension on the scope and was unsuited for APS where the area capture cf visual was small.

The better and more cost effective solution was to adopt a used Sony NEX 5N with C-mount adaptor directly on the first stage adaptor. It also has an electronic first curtain shutter option for essentially vibrationless operation. This does not have the feature of remote operation from the PC but more recent makers' mirrorless models might. 



Leica stereo, left and middle image. It can be more practical and cost effective to buy a used camera to fit an adaptor (Sony NEX 5N with C-mount adaptor, middle) rather than commit to additional adaptors for a less suited camera (Canon 600D, left). Although both have an APS sensor, the NEX 5N is mirrorless and presents the image at the same plane as a C-mount microscope camera so parfocal. The DSLR with its longer distance to the sensor is a long way off parfocal and poor coverage of the visual field (even if sensor was full frame).

Nikon Eclipse E800, right, photo port on Ergo trino' head.

 

Setting up a Nikon Eclipse E800
When setting up an E800, as with the Leica stereo, it has the common 38 mm ID port designed for a lensless 1X camera adapter for a small sensor microscope camera. The Sony NEX with C-mount adaptor sits well here capturing the full field and almost parfocal at higher mags. The 10X screen zoom allows critical focusing. Example images shown below. The Eclipse system diagram does show relay lens adaptors to fit larger sensor cameras but would be expensive and as shown below for the stereo, little is to be gained and ability to capture the full field lost.

The Leica stereo adapter 2 above does fit on the photo port providing a vignette free image on full frame/APS cameras and well corrected images with the APS sensor but some loss of planarity towards the edges for a full frame (a Canon 5D Mk. II was briefly owned to assess) but neither of benefit to capture full fields.

 

The merits of capturing the full field
Square or rectangular format film/sensors can never be an ideal fit into a projected circular image field. Consumer digital cameras sold in the last few years have a large image pixel count and well beyond that required to capture microscope images and capturing all the resolved detail. After cropping within the vignette the Sony still gives an 8 Mpixel image from the 17 Mpixel sensor, more if crop outside the vignette as for the diatom below.

Many subjects can have an external boundary which fits closer to a circle than to a rectangular or square sensor format. The beetle above and right is an example on the stereo. Capturing the full field allows simple image editing to fill in the vignette areas. Stitching of smaller field image captures can form an image but ensuring exact tonal matches especially of backgrounds can be tricky.

Two classic examples of round subjects almost filling the visual field are cross sections of plant stems / roots likely encountered at low to medium mags and, as shown above, centric diatoms at high mags. A typical camera sensor of any ratio would not capture the complete subject but when the sensor captures the full visual field a tight crop works well. Just the corners need filling in an image editor using either paintbrush if a clean tone or, as here, the clone stamp if slightly structured.

 



An antique slide of Arachnoidiscus ehrenbergii with the 100X/1.3 Plan Fluor objective on the Nikon Eclipse (brightfield with phase 2 ring for enhanced contrast, three image stack in CombineZP).

 

Final thought ... presenting the images in a circular field
Some older publications did sometimes show circular photomicrographs and usually on a white page background. Nowadays, in formal publications it is typical to present rectangular/square vignette free images on a white background but for enthusiasts sharing their work other options are open. The entire visual field with crisp vignette can be presented and these can work particularly well if the page is all black (or if the vignette is bucket-filled with the same colour chosen for the page background). My brother Ian has used this to striking effect on Micscape in his, for example, The Strange and the Beautiful article. He was using a consumer Canon G6 held up to the eyepiece of a Zeiss Axiolab where the lens naturally gives this view unless zoomed in. These sort of cameras for afocal work remain very useful as the typical modest 3X zoom fitted over the eyepiece and allowed either a full visual field or a zoomed in tight crop into the field circle. The Sony P200 with 3X optical zoom that I formerly used was the same.

Down of an eider duck courtesy of Ian Walker, mixed colour darkfield. Unnamed blue papered 19th century slide.

 

Comments to the author David Walker are welcomed.

 

Acknowledgement Thank you to my brother Ian for use of the image from his article.

 

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