Choosing a digital camera for your microscope
John C. Russ, Materials Science and Engineering Dept., North Carolina State Univ., Raleigh, NC
One vital step is to choose a transfer lens matched to your sensor or chip size. Digital cameras come with chip
sizes that range from the size of a 35 mm film negative (36 mm x 24 mm) down to consumer cameras with
stated sizes such as "1/3" or "1/1.8" inch. Note that these descriptions are not actual dimensions! The "1/3"
chips are actually 4.8 x 3.6 mm (and there are smaller sizes out there). Matching a high-quality single lens reflex
body such as the Nikon D80 or Canon EOS 400 to a camera may be an option, given its other potential uses in
the lab, and reasonable cost, but it is important to understand a few things about all digital cameras before mak-
ing a choice. The D80, as an example, has a 10 million photosites (called individual sensors below, since the
word "pixel" has too many different meanings in various contexts) each of which is 6 µm square. By compari-
son, in a 1/1.8 chip with 4 million photosites each would be 3 µm square. (These values also allow for a space
of about one half µm between the individual sensors to isolate them electrically, and this applies to CCD detec-
tors, not CMOS chips which also require two or three transistors for each photosite, which also take up space).
Figure 1 illustrates the relative sizes of different detectors.
Figure 1. Relative sizes of a 25 mm negative, APS chip (used in digital SLR's such as the Nikon
D80), and the "1/1.8" and "1.3" inch chips used in typical consumer cameras.
The role of the transfer lens is to project the image onto the chip. Normally the rectangular area that is captured
is set well inside the circular field of the microscope, to avoid focus and illumination variations near the edge. In
my rather typical setup, with a 10x objective the captured image field is about 1600 µm wide, and with a 100x
objective it is about 160 µm wide. We'll use those numbers again shortly.
The maximum resolutio