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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 10 Sep 2013 (Tuesday) 12:28
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Focus on Green Laser beam

 
alliben
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Sep 10, 2013 12:28 |  #1

I mistakenly posted my question in the photo section and was just made aware of this discussion forum. Wow, looks like a lot of good stuff is here.

With my T2i and 100-400L, I can't even see any stars in live view to focus on and stars are way too dim in the viewfinder. My dark sky area is in a state forest overlooking a swamp so there is no distant light source to focus on.

Will a camera autofocus on the beam of a green laser if the laser is held off to the side? Or can I manual focus towards the end of the laser beam? Anyone tried this?




  
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Steve ­ Winterton
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Sep 10, 2013 12:41 |  #2

Have you tried disabling the ALO?




  
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alliben
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Sep 10, 2013 13:09 |  #3

What is the ALO?




  
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Steve ­ Winterton
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Sep 10, 2013 13:36 as a reply to  @ alliben's post |  #4

Automatic Light Optimizer.

http://www.imaging-resource.com/PRODS/T2I​/T2IA5.HTM (external link)

I have a Canon 60Da and I can auto focus on stars with a 300mm lens at a dark sky site, so when it comes to the T2i i am really just guessing as I don't have any experience with this particular camera.

You may also need to change your metering mode to spot.

If you have dark skies then I cannot see any reason for not seeing bright stars in live view at high magnification




  
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Steve ­ Winterton
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Sep 10, 2013 13:42 as a reply to  @ Steve Winterton's post |  #5

you might also try toggling the exposure simulation setting.




  
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alliben
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Sep 10, 2013 14:05 |  #6

These references seem to be about exposure. My inquiry isn't about exposure.




  
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Steve ­ Winterton
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Sep 10, 2013 14:24 as a reply to  @ alliben's post |  #7

Your enquiry is about the cameras inability to show stars in live view.

You need to see the stars before you can focus on them.

Cheers I am going to bed now.




  
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scottishguy
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Sep 10, 2013 14:33 |  #8

I've tried this on my 550D(same camera, different country), when I did get it to work, the focus was way out for stars though.

So I upped the ISO to 3200, aperture wide open and used live view while slowly moving focus until stars came into view and into focus, then used 5x then 10x live view to get as Sharp as possible.


http://bighugelabs.com …0/ffffff/330826​48@N00.jpg (external link)

  
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alliben
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Sep 10, 2013 15:25 |  #9

This method of opening up the lens all the way and using high ISO works in live view for my 17-55 2.8 lens, but I couldn't get it to work for the slower 100-400L.

My inquiry is primarily about using the laser as an aid to focusing.

Using the online DOF Master, the hyperfocal distance for my 17mm lens at 2.8 is 17.7 feet. I can autofocus on a flashlight lit tree and have the lens focused out to infinity.

But with the 100-400, say at 300mm and f/5.6, the hyperfocal distance is almost 2750 feet, and at 400mm the distance is almost 4900 feet.

If focusing towards the end of the beam doesn't work, I wonder if shining it at a far distant tree would produce a large enough and bright enough disk for the autofocus to work?

Now that I've worked out the hyperfocal distances, I don't think a green laser would do the trick, but I've never owned one to try it.

Perhaps setting the ISO to 6400 just for the sake of focusing the slow lens in live view might help. This I can try.




  
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scottishguy
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Sep 10, 2013 16:17 |  #10

Try using the preview button next to the lens release button whilst using live view.
That might help with the bigger lens.


http://bighugelabs.com …0/ffffff/330826​48@N00.jpg (external link)

  
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alliben
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Sep 10, 2013 19:49 |  #11

Thank you for your tips, Mr. Winterton. My ALO was disabled, probably by default because I've never heard of it before.
On the T2i, the exposure simulation is always active.




  
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hollis_f
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Sep 11, 2013 04:28 |  #12

alliben wrote in post #16285729 (external link)
Perhaps setting the ISO to 6400 just for the sake of focusing the slow lens in live view might help. This I can try.

It's worth a try.

If it don't work you could try taking a test shot with the lens set to the infinity mark, then chimp it. Change the focus slightly, chimp again, repeat until it looks good.

I presume you'll be using a tracking mount with the 100-400.


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alliben
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Sep 11, 2013 06:44 |  #13

I was trying out the 100-400 using a motorized barn door mount on Andromeda and that's how I had to focus, just touching it along the infinity mark.




  
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Nighthound
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Sep 13, 2013 10:49 |  #14

A simple method for focus on stars that has never failed me when shooting with a moderate to low focal length lens.

1) Set lens to AF.
2) Point camera a the brightest star in the sky(not planet) and press shutter half way with viewfinder center focus point on star.
3) Once the star has snapped to focus, reset lens to MF and be sure camera is set on Manual as well.
4) Point camera to area of sky you are imaging. Be very careful not to touch the lens focus ring while composing your shot.
6) Using the camera delay timer or a remote switch(best) trigger your exposure.

This is a quick and effective way to achieve focus without repeated test images and focus tweaking. It's still wise to check focus by zooming all the way in on a test shot to confirm. If there's doubt repeat the process, being careful not to touch the lens(outside of the M/AF switch) after acquiring focus.

At higher lens focal lengths there are other devices and techniques for achieving critical focus. Some swear by live view for focus, some use laptops to assess focus. When you get up higher in focal length there are more devices and methods are available to achieve critical focus. There's an increase of micro adjusting involved at high focal lengths.

I hope this is helpful.


Steve
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Footbag
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Sep 13, 2013 11:38 |  #15

I have had no problem manually focusing on stars with much slower optics. It does help to use a laptop, but if that isn't available you really shouldnt have that much trouble. The T2i does have video crop mode. I suspect that this could be used to boost the viewable signal.

When I image with narrowband filters and a bahtinov mask, I have to use this mode to see the spikes.


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Focus on Green Laser beam
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