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Old 20-10-2010, 07:33 PM
The_Cat (Jeremy)
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Canon 85mm Lens

I don't know if this is the right place to post this inquiry but here goes:

There are more than 8000 people who subscribe to this forum so there will certainly be someone who can help me !!

I use an 85mm F1.2 lens for low light terrestrial photos and for controlling DoF. Now you would think that this is an amazing lens for astro work but I think not. Here are two pics :-

Crux in 15 Secs:

http://lh6.ggpht.com/_nmcM5hLJalE/TH...9_sec_9544.jpg

LMC in 20 sec.

http://lh3.ggpht.com/_nmcM5hLJalE/TH...0_sec_9535.jpg

Both pics taken on a 5D with a fixed tripod.

Notice the CA on stars that blow out?

Can anybody please offer any explanation?

TIA and ANY answers will be much appreciated.

Jeremy.
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Old 20-10-2010, 07:51 PM
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Octane (Humayun)
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All lenses exhibit chromatic aberration wide open, especially when they're wide open at f/1.2.

These lenses weren't created with astrophotography in mind. Stop down to f/4 or so, and you'll have much better results.

H
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Old 20-10-2010, 08:25 PM
The_Cat (Jeremy)
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Quote:
Originally Posted by Octane View Post
All lenses exhibit chromatic aberration wide open, especially when they're wide open at f/1.2.

These lenses weren't created with astrophotography in mind. Stop down to f/4 or so, and you'll have much better results.

H

Of course stop down but then I cannot go as deep on a fixed tripod - get star trails etc.

But there may be something else, When stars blow out using a long exposure (1000 Sec) on my refractor scope there is no CA at all. The 85mm shows no CA as long as the stars don't blow out.

Jeremy
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Old 21-10-2010, 03:33 PM
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Having used dozens of lenses over the years, I can say that those images represent very good performance from a lens wide open at f1.2.

Your long focal length refractor is probably f5/f6 at best right? That bears no comparison to a (relatively) wide lens at f1.2!! In the real (affordable) world, you’re never going to get perfect stars without halos etc at f1.2.

There’s a reason why equatorial mounts are pretty much a requirement for astrophotography, although you could try stacking a series of 15 second exposures on a tripod at say f2.8-f3.5 and see how you go. Even if the individual frames are underexposed, you should be able to pull something better out of a stack of images.


Phil
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Old 21-10-2010, 04:02 PM
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As Phil says, every lens will have CA, more or less..
Have a look at results I obtained with Canon 100mm F2.8.
It is OK at F4, but anything wider and longitudinal CA is there.
http://www.iceinspace.com.au/forum/s...ad.php?t=58629
That is why we are using mounts with tracking.
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Old 21-10-2010, 07:01 PM
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RB (Andrew)
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Quote:
Originally Posted by The_Cat View Post
But there may be something else, When stars blow out using a long exposure (1000 Sec) on my refractor scope there is no CA at all. The 85mm shows no CA as long as the stars don't blow out.

Jeremy
Jeremy I've used my 85mm f/1.2 for astro.
The thing I've found is that it's absolutely critical to get focus spot on else you get this happening.
I see it when using live view and it's very very difficult to get focus spot on.
Also focus can shift throughout the session as temps drop so you need to re-check it now and then.
I also stop it down a couple of stops.
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Old 21-10-2010, 07:25 PM
The_Cat (Jeremy)
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Quote:
Originally Posted by philiphart View Post
Having used dozens of lenses over the years, I can say that those images represent very good performance from a lens wide open at f1.2.

Your long focal length refractor is probably f5/f6 at best right? That bears no comparison to a (relatively) wide lens at f1.2!! In the real (affordable) world, you’re never going to get perfect stars without halos etc at f1.2.

There’s a reason why equatorial mounts are pretty much a requirement for astrophotography, although you could try stacking a series of 15 second exposures on a tripod at say f2.8-f3.5 and see how you go. Even if the individual frames are underexposed, you should be able to pull something better out of a stack of images.


Phil
Thank you Phil much appreciated.

Re: ... Having used dozens of lenses over the years, I can say that those images ...

I know your work so this comment is really well appreciated and believed!!

My refractor (TOA-150 now replaced with a Mewlon 250CR) had incredible performance.

Last paragraph is a "damn good idea" I shall try it.

Jeremy.
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Old 21-10-2010, 07:27 PM
The_Cat (Jeremy)
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Quote:
Originally Posted by RB View Post
Jeremy I've used my 85mm f/1.2 for astro.
The thing I've found is that it's absolutely critical to get focus spot on else you get this happening.
I see it when using live view and it's very very difficult to get focus spot on.
Also focus can shift throughout the session as temps drop so you need to re-check it now and then.
I also stop it down a couple of stops.
Wonderful Andrew. I'll try a mini Bhatinov mask. Stop it down ? Yes (second person to mention this) . Stopping down and using Phils' stacking idea might just do the trick.

Jeremy.
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Old 21-10-2010, 07:29 PM
The_Cat (Jeremy)
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Quote:
Originally Posted by bojan View Post
As Phil says, every lens will have CA, more or less..
Have a look at results I obtained with Canon 100mm F2.8.
It is OK at F4, but anything wider and longitudinal CA is there.
http://www.iceinspace.com.au/forum/s...ad.php?t=58629
That is why we are using mounts with tracking.
Laziness on my part for not wanting to use a mount - consider this my attempt at "Grab and Go" astrophotography.

Jeremy.
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Old 21-10-2010, 07:38 PM
The_Cat (Jeremy)
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Quote:
Originally Posted by bojan View Post
As Phil says, every lens will have CA, more or less..
Have a look at results I obtained with Canon 100mm F2.8.
It is OK at F4, but anything wider and longitudinal CA is there.
http://www.iceinspace.com.au/forum/s...ad.php?t=58629
That is why we are using mounts with tracking.
Just inspected the images and guess what? I see and take the point.

Jeremy.
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Old 02-12-2010, 12:49 AM
The_Cat (Jeremy)
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Quote:
Originally Posted by RB View Post
Jeremy I've used my 85mm f/1.2 for astro.
The thing I've found is that it's absolutely critical to get focus spot on else you get this happening.
I see it when using live view and it's very very difficult to get focus spot on.
Also focus can shift throughout the session as temps drop so you need to re-check it now and then.
I also stop it down a couple of stops.
Testing the advice - good stuff!!

At last a clear night. Yes! focusing on these lenses is critical. This is my latest attempt: The LMC in 15 sec. exposure at iso200

http://lh6.ggpht.com/_nmcM5hLJalE/TP...s576/comet.jpg

http://lh5.ggpht.com/_nmcM5hLJalE/TP...8/s512/lmc.jpg

and Comet Hartley 103P in 10 sec at iso 200.

Last edited by The_Cat; 02-12-2010 at 12:50 AM. Reason: Added comet picture link
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