A webcam’s primary purpose is to create streaming video images for Internet use. Small file size is a necessity for reasonable transfer speed; therefore, 320 x 240 pixels is the typical, and 640 x 480 is the highest chip resolution available.

In terms of image resolution these are not dream specifications, but 640 x 480 is comparable to the resolution of a conventional TV image which serves many microscopists well. Of course, by choosing a high enough magnification one can resolve anything the optics deliver as long as one is willing to sacrifice field size in return. So, it seemed reasonable to try a webcam as a companion to my microscope.

I chose the Logitech QuickCam^® for Notebooks Pro. Its CCD is of the quarter-inch type (3.5 x 2.6 mm) with a 640 x 480 array of square pixels. The driver allows one to increase the image to megapixel size by interpolation, which does not increase the resolution, but reduces pixelation.  As far as webcams go, this is as good as it gets.

The camera is connected to the computer with a USB cable, any PC with Windows 98SE or later should work.

The QuickCam^® has a focusable objective of f 4.9 mm, which, in terms of the viewing angle, is similar to a 50 mm lens on a 35 mm camera. It focuses down to about 6 inches.

For micrography, the camera, with its lens set to infinity, is mounted on top of an eyepiece.  Some experimenting is needed to find the distance where the lens lies exactly in the plane of the eyepiece’s exit pupil to prevent vignetting. My holder is made from scraps of plywood, with wooden alignment rails glued to both sides of the camera; it fixes the distance between eyepiece and camera at 7.6 mm (Figure 1). Double-sided tape holds the holder and the eyepiece together.

click image to enlarge (70K) Figure 1

The choice of eyepiece is important. To fill the area of the chip, the product of magnification and field number must be at least 210. So, with a 10x/18 eyepiece the corners will be cut off. I used a Leica 15x/14 eyepiece (Leica product number for a pair: 13591010) which will fill the chip’s active surface with just a hint of vignetting.

The magnification in the plane of the chip is the product of the optical components in its path:

 M_objective x M_magchanger x M_eyepiece x M_camera

M_camera is calculated by dividing into the focal length the conventional viewing distance of 250 mm. Thus, M_camera = 4.9 mm / 250 mm = 0.0196.