Rarity. There were only approximately 20 of these now out of production lenses ever made. When they were in production they sold for about $90,000 (US). Due to the time needed to grow the large fluorite crystal used in the 3rd element of the lens, once ordered they took about 18 months to produce.

Autofocus Capability. These lenses include auto focus capability. Moving focus elements as large and heavy as this requires mechanisms that are both robust and extremely precise. Moving them fast enough to be used to photograph sporting events means they must also be very powerful.

Maximum Aperture The EF 1200mm f/5.6 L requires an entrance pupil of 214mm (8.4 inches). A 1200mm f/8 telescope needs only a 150mm wide objective. When you compare the areas of a 214mm circle to that of a 150mm circle, you see that it takes at least twice as much material to create a lens 214mm wide compared to 150mm wide. And that is before you consider that the larger lens element must also be thicker in the center to maintain the same amount of curvature on the surfaces.

Optical Image Quality While things like coma and chromatic aberration are expected at the edges of the field of view of a large refracting telescope, they are not as acceptable in a camera lens. And the larger the diameter of a lens is, the more correction must be applied to obtain the same image quality. This requires more elements in the lens, and these additional elements are almost always made of exotic materials with a higher refractive index and lower mass density than normal optical glass. They must also be precisely shaped to almost insane tolerances to perform as designed.

Image Circle Diameter A telescope need only to cast an image circle the size of a human eye's pupil: approximately 8mm in diameter. A camera lens intended for use with a Full frame camera must cast an image circle approximately 44mm in diameter.

Minimum Focus Distance Many telescopes are designed to focus only at longer distances, some even only at infinity. Camera lenses such as the EF 1200mm f/5.6 L can focus at infinity but are expected to focus at shorter distances as well.