Quote:
Originally Posted by OneCosmos
All I want to know is what camera lens is the equivalent of an 85mm f/5.3 refractor in terms of light gathering/image size?
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Hi Chris an 85mm f/5.3 refractor is a refractor with an effective 85mm front objective and a focal ratio of 5.3, that means it has a focal length of ...
85 x 5.3 = 450.5 mm,
so
an equivalent camera lens in camera parlance would be a 450mm f/5.3.
So the focal length as stated will be 450mm, but there is one further proviso that's not quite as obvious and that relates to the image circle size. For the 85mm f/5.3 telescope and the the 450mm f/5.3 lens to be equivalent in light gathering terms they must have the same image circle size. It is often the case that telescopes have 40-43 mm image circle diameters, but some smaller refractors have ~30mm image circle diameters. This compares with the typical full frame camera lens which has a 43.27mm or so image circle to support a 24x36mm sensor diagonal.
To be sure of the light gathering equivalence you need to know the image circle size (in other words that you are comparing optical devices designed to support the same system / sensor).
Presuming your 85mm f/5.3 telescope has a full-frame image circle it is equivalent in focal length and light gathering to a 450mm f/5.3 Full frame camera lens.
So the question to you is,
if this a real comparison, what does the manufacturer specify as the image circle of the 85mm f/5.3 telescope you are interested in?
Best
JA