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Old 02-10-2013, 09:21 AM
anders_lange (Anders)
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Which telephoto lens to go for...?

Hi everybody

I'm in the market for a decent telephoto lens for my Canon EOS 650D, which I will use together with my iOptron Skytracker, in order to take "deep" sky astro photos.

I been looking at the following lenses:

http://www.digidirect.com.au/camera_...sm_camera_lens

http://www.digidirect.com.au/camera_..._if_macro_lens

http://www.digidirect.com.au/camera_...dg_os_hsm_lens

I'm leaning most towards the Sigma, as it seems to be the best, the Canon lens is a bit slow being only f/4.

What is your thoughts on this matter? And yes I know it would probably be better to go for a fixed focal length lens, but this single lens would give me somewhat more flexibility

Regards
Anders
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Old 02-10-2013, 10:00 AM
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Octane (Humayun)
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f/4 is plenty fast for deep space work.

Why not get the Canon EF 200mm f/2.8L II USM, a dedicated 200mm lens, or if you need the versatility, go for the Canon EF 70-200mm f/2.8L USM (non-IS).

H
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Old 02-10-2013, 11:34 AM
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dannat (Daniel)
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the 70-200mm f2.8 designs are complex lenses with many groups -the simpler the better for astro work, as H says the 200/2.8 or even one of the 135mm primes will fit the bill. even a sigma 150mm macro will also do a good job

what other uses will you use the lens for?
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Old 02-10-2013, 12:08 PM
anders_lange (Anders)
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Thanks for your replies, and for bringing me back to reality.

I will also be using the lens for regular daytime photography.

But the main reason for getting it is to be able to do more "deep" sky work with my current setup including the iOptron skytracker. The latter which I would like to utilize as much as possible before moving on to buying a big EQ-mount and a real telescope for the really serious stuff :-)

I think I will be aiming for the Canon EF 200mm f/2.8L II USM Camera Lens.

What's your opinion on perhaps slapping on a canon x1.4 extender to get close to 300mm focal length, when and if I ever would like to do that. Would this destroy the ability to achieve focus at infinity? and what about the focal ratio.. I gues it will drop to 3.92 right?

Regards
Anders
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Old 02-10-2013, 12:47 PM
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Octane (Humayun)
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Hey mate,

On your camera, the 200mm lens will yield 200x1.6 field of view equivalent. That is, 320mm.

If you bang on a 1.4x extender, that will take you to approximately 450mm, at f/4.

It won't destroy your ability to focus at infinity -- autofocus will work. I can't remember if I have put the 1.4x II extender on my 200mm f/2.8L II USM before, but, I have definitely with the 70-200mm f/2.8L II USM, and it works just fine.

Here's one from four years ago that I shot with the 200mm and a modified 40D. I can't find a link to a bigger version of it, but, this was stopped down to f/4 (hence the diffraction spikes): https://lh6.googleusercontent.com/-V...d_Orion_GP.jpg

H
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Old 02-10-2013, 01:26 PM
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I agree with H. I'd go the 200mm F2.8 prime. Bound to be the better lens.

Sigma 70-200mm 2.8 gets good reviews but its getting pretty large and heavy and zooms are often a compromise. For astro work you really want little chromatic aberration wide open and no coma wide open as an ideal.

You can use stop down rings to achieve a higher F ratio and not get diffraction spikes.

Cheaper choices are Pentax 67 300mm, 200mm. I use a Pentax 67 165mm F2.8 and its spectacular wide open on a huge 16803 chip. A cheap $30 ebay adapter and you are away. The 165mm lens costs hardly anything on Ebay.

Greg.
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Old 02-10-2013, 01:44 PM
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dannat (Daniel)
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keep in mind at 200mm your alignment with the skytracker will need to be pretty good to get 30sec exp
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Old 02-10-2013, 09:29 PM
anders_lange (Anders)
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Quote:
Originally Posted by Octane View Post
Hey mate,

On your camera, the 200mm lens will yield 200x1.6 field of view equivalent. That is, 320mm.

If you bang on a 1.4x extender, that will take you to approximately 450mm, at f/4.

It won't destroy your ability to focus at infinity -- autofocus will work. I can't remember if I have put the 1.4x II extender on my 200mm f/2.8L II USM before, but, I have definitely with the 70-200mm f/2.8L II USM, and it works just fine.

Here's one from four years ago that I shot with the 200mm and a modified 40D. I can't find a link to a bigger version of it, but, this was stopped down to f/4 (hence the diffraction spikes): https://lh6.googleusercontent.com/-V...d_Orion_GP.jpg

H
Your Orion photo is fantastic I'm going for the 200mm f/2.8 Canon lens
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