Thread: F-ratio myth
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Old 22-02-2018, 08:52 AM
JA
.....

JA is offline
 
Join Date: Oct 2016
Location: Melbourne, Australia
Posts: 2,945
Quote:
Originally Posted by multiweb View Post
Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether?

100% on the money - That it the crux of it!
Only rays that are within the field of view of the telescope or lens will make their way through to the focal plane (unless limited by a field stop/iris). In the case of a telescope these rays are near parallel to the lens axis, typically no more than a few degrees down to fractions of a degree off-axis at higher focal lengths. For much lower focal lengths typical of camera lenses say 14 to 300mm, the field of view is larger and so the angle off-axis is much larger.

The diagram I drew earlier was a general case for a convex lens, but in essence holds true whether a lens or telescope. All that differs is the field of view.

For example for a 36mm x 24mm full frame, the horizontal field of view may be:

104 degrees FOV for 14 mm focal length
40 degrees FOV for 50 mm focal length
6.9 degrees FOV for 300 mm focal length
1.8 degrees FOV for 1150 mm focal length
1 degree FOV for 2000 mm focal length

Consider these field of views in relation to the Convex Lens ray diagram, in terms of the issue you raised of rays "near parallel to a closed tube optical axis".

In terms of the closed tube you mentioned it's internal diameter will (should !) always allow for the telescope's field of view, otherwise vignetting would result.

Best
JA

Last edited by JA; 22-02-2018 at 09:19 AM.
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