How do I calculate the f stop of a fixed power spotting scope using only the objective lens diameter and the magnification of the device?

Answer 1

We don't think you can do it with that information.

'f-stop' = (focal length of the objective lens) divided by (its diameter)

Magnification of the scope = (focal length of the objective) divided by (focal length of the eyepiece)

Looks like in order to calculate the 'f-stop', you need to estimate or measure the focal length

of either the objective or the eyepiece.

Here's an idea: If you can temporarily separate the objective from the tube, use the

objective to focus an image of the sun on the sidewalk. (Not on anything flammable.)

Measure the distance from the lens to the sharpest image.

With the 'object' at infinity, the image is at the focal length.

Answer 2

To calculate the f-stop of a fixed power spotting scope, you can divide the diameter of the objective lens by the magnification of the device. The formula is f-stop = objective lens diameter / magnification. This will give you the f-stop value which represents the aperture opening of the spotting scope.