FOV Calculation

Hi,

I'm curious as to how the focal ratio for a binocular would be calculated for a given true FOV (not apparent FOV)?

For instance, if I wanted a binocular that would provide a fixed magnification at 30x and provide a FOV of 3 degrees, what would my focal ratio have to be and what role would the eyepieces play in this calculation?

Thanks,

Eric

Hi Eric,

Since I happpened to be reading when you posted your question, I'll have a go at it.

The simplified calculation for true field of view (TFOV) is to divide the apparent field of view (AFOV) of the eyepiece by the magnification (the exact calculation requires knowing the field stop diameter). Thus, a 90° AFOV eyepiece would be required at 30x to achieve 3° TFOV (90/30=3). To my knowledge this is at the absolute maximum of what has ever been done. The popular TeleVue Nagler EP's are 82°. I believe there was a report of one binocular (Docter?) that achieved 90°.

The focal ratio merely determines the required focal length of the eyepiece to achieve your 30x. For example, f/6 100mm binoculars would have a 600mm focal length (6x100=600), meaning that you would require 20mm EP's to achieve a magnification of 30 (600/20=30). The focal ratio only becomes a limitation when it is so large (called a "slow" focal ratio) that the resulting long focal length of the EP does not allow the desired AFOV due to design constraints.

Hope this helps - Ed Zarenski please feel free to expand.

Clear Skies,
Milt