BioImages - Notes on Tethered Digital Camera - Macro and Microscope

The following relates to the Pixera Pro system which I used from May 98 to August 02. It is now only of historical interest.

A tethered digital camera is permanently attached to the computer, so is restricted to lab work. They are generally used with microscopes and give superior results to a video camera with image capture board which some amateurs favour, although it isn't a lot cheaper.

I use the Pixera Pro digital camera.

The Pixera Pro has a C-mount (actually CS-mount - the "S" means short and a C/CS adaptor ring is required otherwise you get a spot of light in the centre of the image.) It can be fitted to any microscope with a c-mount fitting.

The camera is driven from the computer. The view-finder is a window on the computer screen. In practice this gives a larger and clearer view finder than the miserable little LED screens on consumer digital cameras. As you take pictures they are downloaded to the capture application via a proprietary card in the computer. It takes a few seconds per image. The images can then be saved to hard disk in a variety of formats.

The camera itself is similar to a video camera and has some of the same automatic features: there is automatic exposure, automatic gain control (AGC) and automatic white balance (AWB). The automatic exposure works well, but the manual control for increasing or decreasing brightness doesn't seem to work - perhaps it is overridden by the AGC? However, on the few occasions I've wanted it I've been able to get around it by introducing white backgrounds.

The automatic white balance seems to work like "autolevels" in PhotoShop and burns out all the pale parts of the image. On Macintosh I have to do it manually. The Windows version of the software supports semi-manual white balance where you calibrate it by photographing something white (eg a blank part of the microscope slide); this works really well.

The camera is built round a 640 x 480 ccd chip, but the software allows image capture at a variety of resolutions, up to 1280 x 1024. This high resolution is achieved by combining four consecutive frames taken through some fancy optics which bend the light path by half a pixel in each direction between the consecutive frames. The resulting image benefits from the smoothing effect of combining four frames as well as the enhanced resolution. Obviously it isn't good as a 1280 x 1024 chip would give but it's far superior to what the camera gives at 640 x 480. Paler subjects come out better than very dark ones which tend to be a bit stripey.

The automatic exposure has a working range and you need to be alert to the effects of exceeding it. Too much light will cause the viewfinder window to cycle, fluctuating lighter and darker. I've seen other video cameras do this too. If there is insufficient light, dark parts of the image will trail greenish-brown along the scan lines, although for some reason this only appears after image capture, not in the viewfinder.

I use the Pixera on a Meiji trinocular microscope, which is a good general purpose machine. Although the oil immersion objective could be better, the other phase contrast objectives are excellent.

I also use the Pixera with bellows and an old Olympus 50mm macro lens. This works well for macrophotography of subjects from 1cm down to a few millimetres - which includes the majority of insects. A second-hand mobile stage from a microscope has been modified for use with this apparatus. The subject is placed on a microscope slide suspended half a centimetre above a square of grey card (Jessops "Grey Card"). This is illuminated from both sides to give a shadow free image. I use fluorescent desklamp striplights on angled arms so they can be positioned to show the desired features. The set up is mounted on a copy stand.

I save Pixera images in TIFF format (a lossless format) then colour correct and scale down to half-size before saving as (lossy) JPG. You can also push up the contrast to make faint details more clearly visible. Once saved as JPG I try never to change the image again as repeated saving as JPG degrades the quality.

For larger images or subjects which require extra depth of focus, multiple images can be combined in PhotoShop. The mobile stage allows several overlapping shots with the same orientation to be taken. These can be carefully superimposed in PhotoShop, generally before colour correction. Similarly images taken at different levels can also be superimposed, cutting out the parts that are in focus in each consecutive image. Most macro images on the site are based on at least two shots, some up to eight or ten. Yes, this does take quite a long time, but the results can be spectacular!

The above text relates to my own personal experience and is offered in good faith. It does not constitute a recommendation.