Quote:
Originally Posted by JA
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. 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
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Thanks Ja. It is a Takahashi fsq-85 f/5.3 with a 40mm image circle I believe.
Just checked - it is actually 44mm, it reduces to 40mm with the reducer.