Several years ago, at a time when digital single-lens reflex (SLR) cameras were still financially unobtainable, I used slide film to document my microscopic observations. These slides are now sitting, more or less nicely sorted, in a folder, doing pretty much nothing. I don’t even have a slide projector to look at them. Evidently the slides need to be digitized so that the resulting images can be used more widely.
There are several ways to digitize the slides:
- Using a slide or film scanner: This is the method of choice if you want to retain image quality. These devices are connected over USB to a computer. On the down side, scanning takes a long time and a film scanner is also not cheap. Some better slide scanners have a dust removal system.
- Use a flat-bed scanner: This is possible, if the resolution of the scanner is high and if there is a background lighting. Some flat bed scanners come with an appropriate slide holder. I found this system too time consuming, however.
- Get the slides scanned by a company: I did this once, it was expensive, but the quality was good. This is probably suitable for a smaller number of slides
- Photographing slides with a dedicated slide duplicator: This duplicator is directly mounted on the camera, instead of the existing objective. There is a slide/film holder attached. The slide duplicator that I initially tried was designed to reproduce 36mm slides again on 36mm analog systems (or digital cameras with a large sensor – the “full-format” systems). My digital camera’s sensor is smaller than film size. As a consequence it was not possible to fit the whole slide on the image and I always had added magnification. The objective allowed me to zoom in, but not zoom out (what I would have needed.) There are objectives like this that are specifically made for digital SLR cameras with a smaller sensor. So watch out if you get one of these devices.
- Photographing with a duplicator in front of the objective: This system is mounted in front of the camera’s existing objective. It contains extra lens elements to magnify the slide. This is the system that I used, and it worked well. The adapter is screwed into the filter threading of the camera’s original objective, so be careful that they are compatible (or use an extra adapter ring). One possible problem may be, that there are now many lens elements between the slide and the camera’s sensor. The image quality may suffer because of this. For my purposes, this was perfectly fine. Considering the generally low resolution of microscopic images, the quality loss was negligible. This duplicator also allows me to zoom in. This way I can take overlapping pictures of the slide and assemble them (“stitch” them) using panorama software. This way it is possible to reproduce the slide with an extremely high total resolution – but it’s time consuming (and it’s questionable if the slide / microscopic image has the necessary resolution in the first place.)
About exposure time
It’s very important to rest the camera body as well as the objective (whatever system is used) solidly on a stable surface. The objective should not be able to vibrate in relation to the camera body. If both are stable, then the optimum exposure time (to minimize vibrations) should not be too critical. Because I am in no hurry, I set the exposure to about 2 sec. The whole system will have vibrated out (and be steady) for the most part of the exposure. Long exposure times are more important when the camera is mounted on a microscope. In this case the effects of vibrations are much more evident. To minimize vibrations even more, I use the mirror-lock up feature of my camera.
About white balance
My camera allows me to adjust a custom white balance. I first take a blank picture of the white screen (the adapter system has a white screen) of the and use this as a reference image. The camera will then automatically adjust the white balance of all images that are taken. If your camera does not allow for the use of a reference image, then you should set the white balance manually based on the actual light source used. It’s not a good idea to use auto-white balance, as there is a color drift. Depending on the algorithm used, the camera may assume that the brightest spot on the image represents white (or a shade of grey, if darker), which may not be the case.