Technical question about f stops

[traveller]

I have a technical question which others may shed some light on. Camera lenses have f stops as determined by the square root of 2 1.4, 2, 2.8, 4 etc. But I don't see telescope f numbers following the same pattern. Does any one know why? Is it a cost issue or manufacturing issue? Just curious.
Thanks.

[Steffen]

The "f-stop" or focal ratio is simply the ratio between focal length and aperture of a lens or mirror.

Unlike most telescopes, most camera lenses are designed to be stopped down in steps. The series obtained by multiplying the f-stop number with sqrt(2) gives a sequence of exposure values in steps on 1 EV (all else being equal). This means that stopping down from f/4 to f/5.6 has the same effect as halving the exposure time or the numeric ISO/ASA sensitivity of the film or sensor.

With camera lenses the f-stop sequence starting at f/1 is pretty much hard-wired into the minds of photographers, but it is no more than a convention. With telescopes there is no need for any such convention, so the focal ratio of a telescope denotes nothing more than the ratio of its focal length and aperture.

Cheers
Steffen.

[tempestwizz]

Following on from Steffen, stopping down a terrestrial lens will increase your depth of field making objects nearer and further from the actual focus point also appear to be in-focus. For astro work we are always focussed at infinity. No need for and increase in depth of field.
For astro work we want the widest aperture available, as it allows us to gather more light in a shorter period of time. This minimises issues with tracking/guiding due to shorter exposure times.
Additionally it allows us to better capture deep sky targets such as nebulae without over-exposing the neighbouring stars.
Can be a difficult concept to grasp, but extended objects such as nebulae follow the standard photographic laws of capture, i.e. longer exposure increases the brightness of the image captured.
Stars are different. They are point sources of extremely bright light and will quickly saturate your detector even with only very short exposures. Longer exposures (necessary to capture extended objects) cause the pin-point stars to bloat and bleed and will reveal the imperfections in your optical train very quickly.
At another level, the F number of your astro lens is an indication of the angle of view that can be achieved through it. For two say 8 inch diameter scopes, and F4 will allow a much wider field of view than an F10. If you are looking to image bright objects such as planets ( which are also small) then an F10 would be more appropriate than an F4 lens, but for capturing the nebulosity of say the Rosette nebula, then an F4 would be better.
Hope This Helps

Brian

[traveller]

Thanks Steffen and Brian, and not a reference to illogical number or other pure maths talk :lol:. Having learnt photography on a Pentax K1000 and a Canon AE-1, I manged to grasp most of the concepts you mentioned. At the risk of cross-threading, can either of you shed some light (pun intended!) on my quesiton about 0.5x v f6.3 focal reducer question (Beginners Equipment) ? :help: