You can estimate the image size of the lunar disk by dividing the focal length of the zoom lens in use by 110. The result will be the size of the disk in millimeters at the film plane.

In the case of a total solar eclipse you want a disk size small enough to allow for the corona yet large enough to show details. Previous experience has shown that a size of about 12mm for the lunar disk produced wonderful photographs on 35mm film.

Example: 35mm film is measured across (from corner to corner) and results in an image capture area about 25mm high. A 12mm disk is just under half (12/25) of the vertical image area of film. This leaves a lot of room for the corona in pictures.

For digital cameras you can use the same calculation to obtain the relative size of the lunar disk in your images. Take the effective focal length (EFL) in MM and divide by 25 for an approximation of the lunar disk image size as a percentage of the image. The effective focal length may be more than the actual lens if you are using a digital SLR with a smaller chip size. Consult the manufacturer information for the lens multiplier to apply or locate the actual size of the chip to create your own estimated factor.

Example: Canon digital Rebel has a multipier of 1.6 to be applied to focal length of any lens attached.

Of course it is strongly recommended that you practice using the camera and lens set up by taking pictures of the moon. The practice shots will reveal the actual image size of the moon. Remember that we are not as interested in the lunar details as what surrounds it in the sky (the corona and prominences) when photographing a solar eclipse.

Prominences are about the size of large craters on the moon and the corona extends several lunar diameters away from the center. Sometimes selecting the right lens to use results in using multiple cameras - a practice to be avoided by most eclipse chasers since the enjoyment of the eclipse is greatly diminished.