Telescopes vs prime camera lenses (1 Viewer)

doublespinss · Feb 16, 2014

This is a noobie question so apologies in advance.

An apochromatic telescope has 3 pieces of glass in it total, correct? (not counting eye piece, field flattener or TN/TC)

So what does it mean when canon describes their prime telephoto lenses as having "18 elements in 14 groups" or something of the like? Do those big nice lenses have 18 pieces of glass in them? if so, what do they do?

Thanks for reading,
Richard

MPeoples · Feb 16, 2014

Richard,

Some have 4 elements, TeleVue NP101, Takahashi FSQ 106 Etc. Remember telescope have a fitted focal length. Where big lens you can adjust the Focal Lengths.

Mike

doublespinss · Feb 16, 2014

MPeoples said:
Richard,

Some have 4 elements, TeleVue NP101, Takahashi FSQ 106 Etc. Remember telescope have a fitted focal length. Where big lens you can adjust the Focal Lengths.

Mike

Ah, so the objective lens set is 100% the glass in a scope (3 total for triplet 2 for doublet ect)

I was referring to prime telephoto lenses, not zoom lenses. Like this monster:

http://www.usa.canon.com/cusa/consu..._lineup/ef_800mm_f_5_6l_is_usm#Specifications

Paul Corfield · Feb 17, 2014

I presume all that glass helps to achieve focus without changing the length of the lens too much and also to help with the close focus distance down to 4.5m. The focus can be achieved by moving the internal groups which helps keep the lens balanced throughout the focal range. On a telescope you have to externally focus and the length of the system can change quite a lot which will alter the balance. Apart from that a good telescope will beat an expensive prime by a long way.

Paul.

doublespinss · Feb 17, 2014

Paul Corfield said:
I presume all that glass helps to achieve focus without changing the length of the lens too much and also to help with the close focus distance down to 4.5m. The focus can be achieved by moving the internal groups which helps keep the lens balanced throughout the focal range. On a telescope you have to externally focus and the length of the system can change quite a lot which will alter the balance. Apart from that a good telescope will beat an expensive prime by a long way.

Paul.

That makes perfect sense, thanks

DanC.Licks · Feb 17, 2014

Every lens in a scope/lens means one more lens to calculate, grind, polish, coat, and every step along the way offers chances for error. Also each lens adds more glass-air surfaces. It is always a tradeoff. The advantages of a more compact, faster camera lens with AF all go at the price of IQ.

Telescopes vs prime camera lenses (1 Viewer)

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doublespinss

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