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Old 15-03-2012, 02:23 AM
luigi
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Scope vs Lens at same focal length

Hi All,

I have a question about using a 560mm scope vs using a 560mm lens with a DSLR.
Which setup would give me better image quality? Is it different for the moon or for stars ?

I'm asking because I own a Canon 400mm F5.6 lens with a 1.4x extender it's a 560mm F8 lens. And somebody offered me a Stellarvue 80mm that is a 560mm F7.1 scope.

What do you think about this battle?

Thanks for the feedback!
Luis
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  #2  
Old 15-03-2012, 09:04 AM
Poita (Peter)
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Well, the lens is more versatile as you can use it as a camera lens.

However, we have some supposedly wonderful, and very expensive Canon camera lenses (and they are great for standard photography) that perform quite poorly for astronomical work. I can't imagine a tele-extender would do it any favours either.

You already have the canon, how does it perform for astro photos?
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Old 15-03-2012, 09:40 AM
luigi
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Quote:
Originally Posted by Poita View Post
Well, the lens is more versatile as you can use it as a camera lens.

However, we have some supposedly wonderful, and very expensive Canon camera lenses (and they are great for standard photography) that perform quite poorly for astronomical work. I can't imagine a tele-extender would do it any favours either.

You already have the canon, how does it perform for astro photos?
Hi Peter ty for your feedback!

Let's forget about the advantage of using the telephoto as a camera lens. I'm only interested in IQ for astro.
Let's also forget the extender, it works really well, you can consider it's not there.

The 400 5.6 works really nice, on APS-C with a 1.4 extender is good for moon photos. And there are some deep sky images made with this lens that are also good.

I'm curious about moon photos and if a scope of the same focal length as the lens will have better or worst IQ.

Which one would win?
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Old 15-03-2012, 10:07 AM
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Any decent APO scope will beat a lens rather easily.

2 reasons. Scope optics are simply a triplet, lenses often have 9 or more elements. Every air to glass surface needs to be figured perfectly to prevent aberrations.

There was an interesting post on the AP Yahoo Group about 3 months ago. A pro photographer there compared city scape photos of a Canon 500mm lens ($14000 or more) to an AP130 (about US$6750).

The AP was noticeably better. So much so he was planning to use his AP scope for that type of imaging. You could make out a guy smoking on a balcony from something like 3 miles away with the AP130.

The best lens images I have seen are from Marco here using a Pentax 300mm 67 EDIF F4 lens. The next best are from Wolfgang Promper using a Zeiss 300mm.

The Pentax 67 300mm EDIF F4 is a rare lens but they come up very occassionally for about US$1500-2000.

I use my TEC110FL for long telephoto type images at 613mm focal length.

It is way beyond any other lens I have although admittedly the longest I have is 300mm Pentax 67 non ED.

The humble ED doublet Orion ED 80 makes a great telephoto as well. It is nice and light so it is relatively portable.

The main advantage of these expensive Canon lenses would be autofocus and image stabilisation. But if you are using a tripod (which you virtually have to) the stabilisation is no advantage. Autofocus is not much either as my scope has fine focuser knob and it snaps to focus better than any other optic I have including an AP scope.

Greg.
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Old 15-03-2012, 10:13 AM
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mplanet62 (Michael)
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In scope vs lens battle scope is a clean winner. Larger (in most cases) aperture, less lenses that reflect or absorb light,lens coating customized for astronomy, much lesser number of moving elements (think auto-focus and optical stabilization - both useless in astrophoto). All of this will contribute to better image from equal focus length telescope as compared to photographic lenses. I don't say lenses can't be used - they may be pretty good, too.
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Old 15-03-2012, 10:30 AM
luigi
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Quote:
Originally Posted by gregbradley View Post
Any decent APO scope will beat a lens rather easily.

2 reasons. Scope optics are simply a triplet, lenses often have 9 or more elements. Every air to glass surface needs to be figured perfectly to prevent aberrations.

There was an interesting post on the AP Yahoo Group about 3 months ago. A pro photographer there compared city scape photos of a Canon 500mm lens ($14000 or more) to an AP130 (about US$6750).

The AP was noticeably better. So much so he was planning to use his AP scope for that type of imaging. You could make out a guy smoking on a balcony from something like 3 miles away with the AP130.

The best lens images I have seen are from Marco here using a Pentax 300mm 67 EDIF F4 lens. The next best are from Wolfgang Promper using a Zeiss 300mm.

The Pentax 67 300mm EDIF F4 is a rare lens but they come up very occassionally for about US$1500-2000.

I use my TEC110FL for long telephoto type images at 613mm focal length.

It is way beyond any other lens I have although admittedly the longest I have is 300mm Pentax 67 non ED.

The humble ED doublet Orion ED 80 makes a great telephoto as well. It is nice and light so it is relatively portable.

The main advantage of these expensive Canon lenses would be autofocus and image stabilisation. But if you are using a tripod (which you virtually have to) the stabilisation is no advantage. Autofocus is not much either as my scope has fine focuser knob and it snaps to focus better than any other optic I have including an AP scope.

Greg.
Thank you Greg, this really helps.
Very good points and thanks for the feedback and experience too!
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Old 16-03-2012, 04:49 PM
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Oh I realised I said 2 reasons then only posted 1!

The other reason (and I don't know if this is true in some of the high end lenses) is that camera lenses because they are not magnifying so hard as in telescopes, they are not figured to the same accuracy. If they were then we would have $10,000 lenses for a decent lens and they are often under $1000.

So telescopes are usually made to a much more demanding standard (at least I think so). It seems to bear out that way as most lens shots are usually showing less detail than APO telescope shots with perhaps Marco's images being the exception.

Greg.
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Old 16-03-2012, 10:45 PM
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Quote:
Originally Posted by gregbradley View Post
Oh I realised I said 2 reasons then only posted 1!

The other reason (and I don't know if this is true in some of the high end lenses) is that camera lenses because they are not magnifying so hard as in telescopes, they are not figured to the same accuracy. ...

Greg.
Well, they do magnify the same - as there's no difference belongs 1000mm focus distance to telescope or lens it's still 1000mm. But scopes are collimated much better - in part because they have less elements in optical path. Especially movable elements. I would think that you are right in your conclusion.
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Old 17-03-2012, 06:21 AM
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Well I would partially disagree with this as I have had a few long Canon Lenses and one can use it quite well for astro work.

The 500mm F/4.0 would auto focus on any star, i would than turn it to manual focus and leave it there, it would be crisp and clear, and equivalent to the FSQ-106 which was also on the set up.

As the night went on, and the temp went down all that was needed was to auto focus again and turn back to manual and so on.

I took some awesome images i might add.

Leon
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Old 17-03-2012, 06:32 AM
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Quote:
Originally Posted by mplanet62 View Post
Well, they do magnify the same - as there's no difference belongs 1000mm focus distance to telescope or lens it's still 1000mm. But scopes are collimated much better - in part because they have less elements in optical path. Especially movable elements. I would think that you are right in your conclusion.
Oh right. Thanks.

Its amazing how they can work out how all those lens elements interact.
I guess that's the power of computers.

Greg.
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Old 20-03-2012, 11:41 AM
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There are a couple of deficiencies with Canon's super telephoto lenses. They can never be as well baffled as an astro APO. They also suffer from vignetting with a full frame sensor camera.

Where they leave astro APO's for dead is that they have a flat field. They also have a single crystal of Magnesium Fluoride as a lens element plus two ED glass elements in the major objective lens group. A Super APO in my opinion.

I have used a 300mm F2.8L Canon lens on a full frame camera and once optimised nothing would come close at this focal length.

At f/2.8 only the very bright stars showed residual aberrations. Limiting the the aperture by an external aperture at f/3.6 even bright stars showed very little secondary aberrations. The other trick is to limit the amount of sky the lens actually 'sees' to not let in any light that does not contribute to the image from being scattered inside the lens and so lower contrast. This can easily be done by extending the lens hood. There are also processing tricks to overcome the lack of perfect focus with the visible spectrum and HA.

See this thread

http://www.iceinspace.com.au/forum/s...ad.php?t=80863

Bert

Last edited by avandonk; 20-03-2012 at 05:40 PM.
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  #12  
Old 01-04-2012, 09:31 AM
luigi
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Here's a quick compare:

400mm F5.6 lens + 1.4x extender on the left (FL: 560mm)
80mm ED refractor F7 560mm on the right

From my tests it was clear that the scope was sharper and provided better contrast.

Thanks for all the feedback!
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Old 01-04-2012, 11:03 AM
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Thank for doing that Luigi. Its quite convincing.

Greg.
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