Foacl ratio of binoculars [Archive]

steve55

Thursday 12th May 2005, 21:37

iporali

Thursday 12th May 2005, 22:25

ok i'll keep this quick and simple.

im digiscoping with binoculars and i want to work out the specs so...

Does anyone know how to calculate the focal ratio for a pair of binoculars.
I know its focal length divided by aperture but i dont know the focal length...
Steve,
A scope or the binoculars which have the objective and the eyepiece (ocular) do not have a focal length - they are afocal - they only have magnification. In digiscoping you add a camera lens to the system and to get the total fl you just multiply the focal length of the camera lens by the magnification power of the afocal telescope.

The focal ratio is also slightly different from normal photography. You actually get the focal ratio or f-number directly from the camera if the exit pupil of the binoculars exceeds the "aperture" of the camera lens.

If you have eg. 8x32 binoculars, they have exit pupil of 4 mm. The Coolpix 4500 lens is 7.85-32mm with f-ratios of 1:2.6-5.1. At wide angle setting the maximal aperture diameter is 3.02mm, which means that the binoculars are not limiting the light and the f-ratio is what you have on the camera. When you zoom the camera the values change, but in principle you can use the camera's f-ratios.
HTH :t:

Ilkka