Photographing coins through a stereoscope is really easier than one might think. Obviously, a good scope is necessary, and I highly recommend a stereoscope. This is a scope with dual eyepieces that will usually have better optics.
The camera will adapt to one of the eyepieces with a microscope-camera adapter. Adapters are usually available from the microscope dealer or from many camera supply stores. The adapter will simply match to the eyepiece and will also attach to the camera body without the camera's lens in place.
In this configuration, the scope is then acting as the lens of the camera. Unlike with macro photos, the microscope will not have settings for the lens opening. The scope will also be the only means of focus that you will have. To focus, you must view through the view finder of the camera and adjust
the scope until the image on the coin is as sharp as possible.
Best results are obtained when the coin is tilted under the optics of the scope. I have built a small, angled table upon which I rest the coins. It's made primarily from cardboard 2 x 2s and covered with a black fabric.
The coin is then placed on this table and positioned under the lens of the scope, so the light will reflect from the coin into the lens. As with macro photos, best results are obtained when the fields of the coin are light, and the shadows fall on either side of the devices. Film and lighting are the same as with macro photos. However, I recommend that film with an ASA rating of at least 100 be used. The lens of the scope is usually rather small and will allow small amounts of light into the camera. You will also discover, by moving the light around the coin, differing results can be obtained. You may find it helpful to move the coin platform around at times rather than moving the light.
I have learned with micro photos that the automatic setting on the camera can generally be used. I will "bump" the automatic setting on the camera to +2. This basically adds two stops to the length of the exposure. Remember that each setting on the lens either doubles or halves the amount of light as the next setting. A bump of two stops actually takes the automatic exposure and multiplies it by 4.
For instance, if a normal reading would force the camera to an exposure of 1/15th of a second, a two-stop bump would force the camera to an exposure of 1/4th second. That equals two stops.
Many cameras have an adjustment that will allow you to use the bump simply by adjusting one dial. This dial will usually have 5 settings: +2, +1, 0, -1, -2. If you set the dial at +2, you are adding 2 "stops" to the exposure. If your camera does not have this adjustment, you can alter the automatic exposure by changing the film setting. If you're using ASA 400 film, set the camera at ASA 100. That will give you the same result as if you had set the bump setting at +2.
Generally speaking, the light that the camera will read comes directly from the coin, and this reading is not altered by peripheral matter, as will the background in macro shots.