A zoom eyepiece allows a wide range of magnifications for visual use. The same is true when using one for photography, where the use of a zoom is a kind of eyepiece or positive projection. This method uses an eyepiece to project an image directly onto the film or chip inside your SLR camera without its lens attached.
The great advantage of model #93232 is the built-in T-thread under the eyecup allowing you to directly screw your camera with T-ring to the eyepiece. This is extremely convenient for projection photography. A huge plus for Newtonian reflectors: the eyepiece as a projection lens brings the focal plane out far enough to overcome the short back focus, allowing you to focus your camera to infinity and take a clear picture.
For any scope it can act either as a relay lens (prime focus T-adapter at unity power), not changing the effective focal length or f-number of the scope, or as a magnifier giving you larger images. To calculate the magnification, f-number and focal length with a zoom eyepiece, just use the same formula as for projection with a fixed focal length eyepiece for the different focal lengths you are using.
For eyepiece projection the magnification factor is the distance between the film (or chip) and the eye lens of the eyepiece divided by the eyepiece focal length, minus 1.
For the 8-24mm zoom, the chip-eye lens distance is about 50mm for most camera/T-ring combinations. For the fully zoomed out position at 24mm, the magnification factor is 50/24 – 1 or essentially 1.
The new projection f-number is the magnification factor times the scope’s f-number This means that at 24mm zoom the f-number is basically the same as the f-number of the scope itself.
Equivalent focal lengths can be calculated by multiplying these f-numbers by the aperture of your telescope. At 24mm it’s nearly the same as the original focal length of your scope.
For an actual example, the Newtonian reflector Omni XLT 150 is f/5 and has a 150mm aperture and 750mm focal length. At 24mm zoom its f-number is 50/24 – 1 or 1.08 x 5 = f/5.4. Its equivalent focal length is 5.4 x 150 = 810mm.
At the other end, 8mm zoom gives 50/8 – 1 or 5.25 x 5 = f/26.25. The scope’s equivalent focal length is 26.25 x 150 or 3938mm.
Thus, you can use the eyepiece to achieve infinity focus with your camera using almost any Newtonian or other telescope. Fully zoomed out, you get the original optical system’s photographic speed and focal length. Fully zoomed in, you get larger image scales useful for close-up nature or planetary photography.