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Hi Brendan, Haven't seen you for a while.
Contrary to earlier posts, I consider there is no difference between camera lens and telescopes. They are the same. The ratio of the diameter to the focal defines the f stop or focal ratio - they are the same figure. The aperture control on a camera effectively cuts the diameter of the lens, thus reducing the amount of light entering the camera. Same as for a telescope. If you stop it down, you will reduce the amount of light entering which amounts to effectively reducing the focal ratio. You will not change the focal length. That is a physical dimension ground into the lens or mirror. Only the focal ratio can change and that depends as said above, on the ratio of the focal length to the effective diameter. Change the effective diameter and you change the focal ratio - camera or telescope.
Robert
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