Telescope Reviews: f/ratio myth

“Forget about the camera, sampling and pixels for now and consider the virtual image.”

“The number of photons that are collected from any object is entirely determined by the intrinsic brightness of the object (usually measured in magnitudes), the collecting area and efficiency of the optical objective (lens or mirror), and the length of time used to create the virtual image. Note that focal length (FL) and thus f-ratio are irrelevant because they have no effect on the any of those determinants.”

What the above statements say to me in terms of a proof are as follows.

A + B + C = D However, we are going to forget about A right now and just look at B, C, and D.

B + C = D Therefore, A must = 0 and therefore Focal Ratio has nothing to do with exposure at all.

I actually laughed when I read that article. If he wants to rewrite physics then I say go right ahead. It might actually be a best seller in the Fiction section.

Here is a better proof to explain this in words.

If you keep the front aperture the same and add a barlow you will increase the focal length and decrease the exposure(assume a non-point source object). If you increase the front aperture and keep the focal length the same you will increase the exposure. Focal Ratio is the Focal Length divided by the Front Aperture.

Both Focal Length and Front Aperture affect exposure as seen in the barlow example. Focal Ratio is just the ratio of those two values therefore exposure varies with Focal Ratio.

A / B = C, 2A / B = 2C, A / 2B = 1/2C, and Exposure = 1/C, Therefore an increase in A(Focal Length) will decrease exposure while an increase in B(Front Aperture) will increase exposure.

When you vary the aperture in a camera lens you are keeping the focal length the same but you are essentially varying the front aperture by closing the aperture blades. He got that part right. However, his theory negates the fact that you can vary the focal length, keep the front aperture the same, and still affect the exposure.

This error is demonstrated in his pictures which seem to negate the fact that for the aperture to remain the same and the focal ratio to decrease the focal length must change as well. The focal length did not appear to change at all in his pictures.

In addition he shows a faint object and a bunch of stars. For a point source like those stars he is correct. The focal length will not affect the brightness of those point sources. The front aperture is the only significant factor for those.

Edited by mpgxsvcd (08/07/12 01:45 PM)