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Old 08-07-2019, 08:47 PM
poider (Peter)
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focal length

G'day
Is there a website that can tell me what focal length matched lenses....
For example, I have a 150-500mm lens and I am wondering what the equivalent telescope would be in a refractor
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Old 09-07-2019, 11:50 AM
Wavytone (Nick)
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Focal length has same meaning for lenses and telescopes, ie 500mm focal length in a telescope will give exactly the same results as a 500mm lens.

What is to some extent confusing is that the photo industry always refers to lenses by focal length - eg a “500mm lens” whereas telescopes are generally referred to by aperture - eg “100mm refractor” means a scope of aperture 100mm which will be somewhat larger in comparison and may have a focal length anywhere between 500 and 1500mm.

Focal length = aperture x focal ratio.

Last edited by Wavytone; 09-07-2019 at 12:40 PM.
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Old 12-07-2019, 07:09 PM
poider (Peter)
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Thank you Wavytone
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Old 12-07-2019, 08:58 PM
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Ukastronomer (Jeremy)
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Yes surely 100mm is 100mm

A1 Nick
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Old 12-07-2019, 10:30 PM
poider (Peter)
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So taking into account that 100 mm is 100mm and 500mm is 500mm what role does the eye piece play, do I assume that with a 100mm focal length and a 20mm eye piece the magnification would be 5x ?
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Old 13-07-2019, 02:03 AM
astro744
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For some basic advice on eyepieces see http://www.televue.com/engine/TV3b_p...n=Advice&id=79

and

http://www.televue.com/engine/TV3b_page.asp?id=154

Camera lenses have a variable disphram hence focal length and minimum f stop is usually the reference. Telescopes do not have a diaphram and maximum aperture is usually referred to.

e.g. A 60mm f6 telescope (Tele Vue 60) has a focal length of 360mm. If this was a camera lens it would be referred to as a 360mm f6 with f6 being its minimum f stop and assuming it had a diaphram could then be stopped down further.

Focal length is the same term for either instrument.
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Old 13-07-2019, 10:53 AM
poider (Peter)
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thank you astro
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