I’m doing photography with an eyepiece, using my camera with and without a lens. What are the f-numbers of my setup for the afocal and eyepiece projection methods?

The afocal photographic method uses an eyepiece projecting through the camera’s lens and can be used with fixed lens (point and shoot) as well as removable lens (SLR) cameras.

For afocal work the effective f-number is the focal length of the camera lens divided by the focal length of the eyepiece times your original scope f-number. Let's say you have a 50mm camera lens with your 15 mm eyepiece and f/10 Schmidt-Cassegrain (SCT) scope, In this example your afocal f-number would be 50/15 times 10 or f/33.3 for your scope.

The eyepiece or positive projection method uses an eyepiece to project an image directly onto the film or chip inside your SLR camera without its lens attached.

For eyepiece projection, it's the magnification factor times the scope f-number. The magnification factor is the distance between the film or chip and the eyelens of the eyepiece divided by the eyepiece focal length, minus 1. Choosing 150 mm as the chip-eyelens distance with the same scope as above, it would be 150/15 - 1 or 9 for the magnification factor. 9 x 10 or f/90 is the equivalent focal ratio of this setup.

Equivalent focal lengths can be calculated by multiplying these f-numbers by the aperture of your telescope. For an 8” SCT, the two worked examples above give 33.3 x 8 or 266.4 inches (6767 mm) for the afocal method and 90x 8 or 720 inches (18288 mm) for the eyepiece projection method.

Either method can be used to achieve large image scales, which is why both are popular for planetary photography.

See Michael Covington's excellent astrophotography books for more information on this topic.

Updated 12/27/13