Afocal digital photography

Fuji MX2700 digital camera	Grabbed video image
Figure 1. A micrometer in a microscope, viewed at the same magnification (!) with a digital still camera and a video camera. Click on the thumbnails for original image sizes.

Many cameras have lens designs that cause heavy vignetting (Table 1), no matter what zoom setting and eyepiece one use. Usually, this vignetting occurs because the front lens diameter of the camera is much larger than the image the eyepiece projects into the camera. If you are looking for a digital camera for afocal photography, this vignetting is the first thing you should worry about, in my opinion. Any vignetting will reduce the effective resolution of the camera, potentially turning your costly 3 mpixels camera into a <1 mpixels camera.

Some eyepieces may aggravate the vignetting. Eye relief translates to camera relief. With a short eye relief it may not be possible to get the camera close enough to the eyepiece. The result is like looking through a keyhole from a distance. Finally, eyepieces with narrow apparent field of views may cause vignetting in the image (but of a different kind).

Table 1. Images showing the degree of vignetting in the cameras I have tried, at minimum and maximum optical zoom. The testing was in no way standardised, since different targets were imaged, and different combinations of eyepieces/microscope/telescope have been used. The "sharp-edged" vignetting in rows one, four, five and six, is the edge of the visible field of view in the eyepiece (the field stop), and has nothing to do with the vignetting introduced by the cameras with larger lenses. Click on the thumbnails to see original image size. Warning: some of these images are quite large.

Camera	Lens diameter	Eyepiece	Min zoom	Max zoom
Fuji MX2700	9.5 mm	Olympus 10x microscope eyepiece, 22 mm eye relief		n/a
Fuji MX2700	9.5 mm	Minolta 10x50 binoculars		n/a
Fuji MX2700	9.5 mm	Televue 7 mm Nagler, 10 mm eye relief		n/a
Olympus C3020Z	23 mm	Olympus 10x microscope eyepiece, 22 mm eye relief
Nikon CoolPix 950	16 mm	Olympus 10x microscope eyepiece, ca. 15 mm eye relief		-
Epson PhotoPC 850Z	?	Olympus 10x microscope eyepiece, 22 mm eye relief
Olympus C1400L	43 mm	Meade 26 mm super plossl, ca. 10 mm eye relief
Olympus C1400L	43 mm	Pentax 5.2 mm, 20 mm eye relief
Olympus C2500L	43 mm	Olympus 10x microscope eyepiece, 22 mm eye relief

The camera should be placed perfectly centered and perpendicular to the optical axis, and not moved during exposure. An adapter will be helpful (links below). Some excellent web pages with tons of information about digital cameras:

Digital camera astroimaging FAQ
dpreview.com
dcresource.com
steves-digicam.com
imaging-resource.com

Adapters:
The ultimate adapter page