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Are fixed focal length lenses & refractors objectively better than zoom lenses?

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#1 Branithar

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Posted 23 January 2021 - 06:22 PM

I've been window shopping for a nice lens, to go with my purchase of a star tracker. I was looking at the Rokinon 185mm and the RedCat 51. But today I found a Sigma zoom lens and Canon brand lenses that are f/5.6 when zoomed to 400mm. I've seen examples of both used for astrophotography, and people like them. If I want a great lens, without getting sucked into throwing money and time at buying/researching lenses, is a fixed focal length refractor, like the RedCat 51 the way to go?

#2 bobzeq25

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Posted 23 January 2021 - 06:30 PM

As a general rule, fixed focal length is definitely better than zooms, although a great zoom can be better than a poor fixed focal length.

 

Reasons.  Zooms have too many elements trying to do too many things, because they need to focus in the near field, AND change focal length.  That causes more aberrations, and stars are very demanding re that.  Zooms are typically slower than fixed focal length camera lenses, high F number.

 

So a refractor, optimized for targets at infinity, is best.  The Redcat is an excellent choice.  The Rokkinon 185mm is second choice.  The zooms bring up the rear.


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#3 Kevin_A

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Posted 23 January 2021 - 07:27 PM

I have both Rokinon 135 and a Sigma Sport 70-200 f2.8 zoom and both are great lenses. The 135 is sharper while the Sigma gets me lots of photons and more reach. The zoom lens has slight issues with larger stars while the 135 gets tighter star images. Both are fast lenses so there is an advantage when you use them on so so mounts. Zooms can also creep so make sure any zoom lens has a stiff zoom ring so it won’t move when Aimed high in the sky.



#4 chanrobi

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Posted 23 January 2021 - 10:04 PM

I've been window shopping for a nice lens, to go with my purchase of a star tracker. I was looking at the Rokinon 185mm and the RedCat 51. But today I found a Sigma zoom lens and Canon brand lenses that are f/5.6 when zoomed to 400mm. I've seen examples of both used for astrophotography, and people like them. If I want a great lens, without getting sucked into throwing money and time at buying/researching lenses, is a fixed focal length refractor, like the RedCat 51 the way to go?

Yep, buy prime unless you already own a high quality zoom like 70-200 f2.8 etc.

 

And there is no 185, i'm assuming you mean 135mm. In that case make SURE you buy somewhere like amazon with a GREAT return policy. I bought 3 now and all 3 were duds.

 

I'd keep going with the exchanges but i'm going to a longer focal length anyways. 135 is pretty much too wide for 99.999% of the sky. Even at APS-C


Edited by chanrobi, 23 January 2021 - 10:04 PM.



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