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PostPosted: Mon Sep 24, 2007 1:10 am 
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Hi everyone, one of the biggest challenges with astro-photography is getting the focus right. AF systems rarely work with astro-subjects and turning the manual focusing ring all the way round actually takes you beyond the required 'infinity' position. So you just have to peer through the optical viewfinder and try to figure out when a tiny dot is sharp - it sure is tricky!

With the advent of DSLRs sporting Live View though, it's got a whole lot easier as you can magnify a portion of the image to check the focus. It really works too and we have a full demo using a Canon EOS 40D and a Televue Genesis telescope. Hope you enjoy it!

So the next time someone says 'Pah! Live View on a DSLR? It's pointless!', just show them this video and they'll understand how useful it can be to specialist photographers...

Cheers!

Gordon


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PostPosted: Mon Sep 24, 2007 5:36 pm 
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Pah! Live-view to focus on infinity? I have this nice marking on my lens!
And I know that no star is closer than infinity :) :D :wink:

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PostPosted: Mon Sep 24, 2007 10:30 pm 
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Hi Thomas, seriously, try manually turning your focusing ring until the marking is at infinity, then take a photo of a star with the aperture wide open and zoom in on the picture afterwards - I almost guarentee it won't be in focus!

Expansion of materials along with inprecise markings means these distance marks are guidelines only, and cannot be relied upon. So in the past you'd need to make test shots, but now Live View lets you confirm before the shot.


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PostPosted: Thu Sep 27, 2007 8:03 am 
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Aha! I thought this was only a prob, when you encounter very high temperature differences with the camera/lens. But anyway. On my old Oly OM2n there was no such thing as "infinity+".
But I understand that when shooting with the telescope with the T-adapter there is no defined infinity.
I was quite astonished to see the whole caboodle vibrate violently, when you turned the focus-knob and also how much distance the tube travelled when turning a little on the knob. That must be a helluva challenge to get the focus right. Reminded me of professional microscopes where you have special dials for finer focus adjustments. Do you have these on telescopes as well?
Also, I assume that in-body anti-shake is of no use here!?
Are there telescopes with IS built in? I think, I've seen binoculars with IS...

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PostPosted: Thu Sep 27, 2007 9:20 am 
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There are binocs with IS, but only because you generally handhold them. Telescopes are designed for use on solid mountings, so - as long as you're not touching it - there'll be no wobble.

But as you can see, at higher powers, even the slightest touch can cause an uncontrollable wobble - although to be fair, what you were seeing was a 10x close-up on a 'lens' which was already working at 1728mm!

Built-in IS is an interesting proposition and it could be cool to see if it helps while composing in Live View (I'd be using an Olympus then). But since you lock the mirror up and use either a self timer or a cable release to take the shots through a telescope, it wouldn't be that useful on the final pictures themselves.

As for an infinity position, yep, there isn't one on a scope, but I'd still say look at your modern lenses and notice how there is a position slightly 'beyond' infinity. You're right, in the old days of manual focus lenses, there was a stop position which we assumed was at infinity, but modern lenses can go a bit beyond.

Either way, you can't assume just turning the focusing ring all the way to the end, or carefully lining up the infinity mark will give you a sharp result on stars - you simply have to take test shots or use things like Live View to check.

Back to the wobbling though! They do offer motorised focusing attachments for many scopes, and it's something I've long wanted!

Gordon


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PostPosted: Thu Sep 27, 2007 12:36 pm 
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As for an infinity position, yep, there isn't one on a scope, but I'd still say look at your modern lenses and notice how there is a position slightly 'beyond' infinity. You're right, in the old days of manual focus lenses, there was a stop position which we assumed was at infinity, but modern lenses can go a bit beyond.


Gordon

Perhaps in the 'old days' for manual focusing, there is no need for that 'extra infinity' as people controls the focusing but in this modern age where,
the motor or AF mechanism does the focusing and it does this in a series of plus (+) and minus (-) ? adjusting itself.
Perhaps I am wrong but this is what I observed whenever the camera auto-focus.

So if there is no extra room for it to focus past the infinity the motor would end up bashing the lens after some time of usage ? As it can't move past infinity ?

So some compromised made for the sake of advancement :?:


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PostPosted: Thu Sep 27, 2007 2:05 pm 
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Nice point made David!
But I can hear the motor bang the lens at the closest focus point when the objekt is too near! So why give the controlls some leeway at infinity but not on close-up :?

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PostPosted: Thu Sep 27, 2007 2:22 pm 
Good one Thomas :!:

Perhaps Some more compromise were made :?:
or 99.9% of the users Do not focus closest ?

I mean most people , me included, when trying focus closest , if cannot normally we just zoom out or in a bit and try again. Or move one step back or lean backwards.
Why :?: Perhaps we think we have exceeded the lens min focus distance ?

But for 'infinity' you can't move any more :?:

Just a thought, as I am not an expert.

Gordon ? your expert opinions ?

DavidL


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PostPosted: Thu Sep 27, 2007 2:40 pm 
tombomba2 wrote:
On my old Oly OM2n there was no such thing as "infinity+".


Ahh, would this be why my 18-200mm VR focus ring moves to a point slightly past infinity? To allow for this?


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PostPosted: Thu Sep 27, 2007 2:52 pm 
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Looking at the lenses I own, there is no obvious way to go past infinity on my 10.5mm and my 50mm, but on all other lenses there is. Be it the 10-20mm or the 105mm or the 500mm or the 18-200mm.
So obviously "infinity+" is not limited to zooms or long lenses :?

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PostPosted: Fri Sep 28, 2007 1:32 am 
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Well this infinity plus is there to cater for things like expansion, but it is funny it never used to be there on older lenses... I'll ask around and see what I can discover!


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PostPosted: Fri Oct 05, 2007 5:44 am 
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Hello everyone, we've had some requests for samples of the images taken during this video, so here are a few...

Image

Above: Uncropped shot of the Moon taken with EOS 40D and Televue Genesis scope.

Image

Above: 100% crop taken from original image. No retouching.

Image

Above: 100% crop taken from original image. No retouching.

Image

Above: On the left, the uncropped original of Jupiter - it looks tiny! Top right, a 100% crop of the dot, revealing the dot as Jupiter. Below that a blow-up of the 100% crop, scaled in Photoshop to twice it's size. I reckon it holds up quite well!

Gordon


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PostPosted: Fri Oct 05, 2007 6:15 am 
Gordon
I love those shots.
So nice to live in a place that's have very much less air-pollution - no clouds!
I am beginning to understand why a half-moon is better than a full moon shot.

BTW , did you managed to find the answer to infinity + ?


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PostPosted: Fri Oct 05, 2007 6:48 am 
I share the same problem as you David! I don't have the equipment for astrophotography anyway.

Those are nice shots Gordon.


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PostPosted: Fri Oct 05, 2007 7:08 am 
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Thanks guys!

Actually, the air quality wasn't great that evening and Jupiter was shimmering all over the place - I think there may be some troublesome heat rising from a house just behind us too... But it does show that you can grab an image of Jupiter at prime focus!

As for infinity +, I'm still looking into it, but I'm pretty sure it's to cover for expansion. But why it wasn't on older lenses, I'm not sure yet...


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