Afocal photography

Afocal photography, also called afocal imaging or afocal projection is a method of photography where the camera with its lens attached is mounted over the eyepiece of another image forming system such as an optical telescope or optical microscope, with the camera lens taking the place of the human eye.

Afocal photography works with any system that can produce a virtual image of parallel light, for example telescopes and microscopes. Afocal photographic setups work because the imaging device’s eyepiece produces collimated light and with the camera's lens focused at infinity, creating an afocal system with no net convergence or divergence in the light path between the two devices. In this system the device is focused on the object and the camera is placed above the eyepiece as close as possible. The drawback is the system will have a high focal ratio, with a correspondingly dim image, and some vignetting. A high focal ratio also means the field of view will be narrow. Field of view can be calculated using:

Focal field of view/angle of view:

One method of afocal photography is to mount a camera with its lens attached behind the eyepiece an Keplerian optical telescope, the combination giving the photographer a long focus lens. Historically afocal photography with 35 mm SLR or large format film cameras was a very difficult method of photography. With film cameras the bulk and mechanical shake had to be taken into consideration, with some setups employing a separate tripod for the camera (adding the complexity of setting up the camera in relationship to the eyepiece). The general difficulties of focus and exposure with film cameras, along with the detailed mathematical calculations, combined with the time lag of waiting for the film to be developed, meant film afocal photography could be pretty hit and miss.

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