How to focus a lens manually to infinty for astronomical shots

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F-Stop

Senior Member
So...last night I tried some star shots with my D5100, 35mm f1.8 G lens. I set it to manual exposure, f1.8, and a 15 second exposure and got some nice captures. But my old eyes couldn't see to manually focus stars in the viewfinder and I did the best I could in Live View. Autofocus wasn't working consistently. The question is this: Shouldn't I be able to rotate the focus ring to the extreme position toward infinity and have good focus for a star field? With live view, it looked to me like the stars came into focus and then went slightly out at the end of the focus ring's travel. The result was that what looked good in the Live View screen ended up out of focus when I looked at the pictures this morning. Where am I going wrong? Should I have turned the focus ring to the limit of it's travel?
Thanks everyone, for making this forum a great resource. I've read a lot more than I've posted since joining and hope to be able to contribute constructively in the future. I'm new at the DSLR art but years ago I was into photography quite a bit...it's really cool what we can do with digital these days!
 
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Don Kuykendall_RIP

RIP :(
ON the auto focus cameras/lenses the focus has to go through the focus to work correctly so it will go past infinity. You can not turn it all the way to get to infinity. Play with it sometime in bright sun to see where infinity is.
 
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madcapmagishion

Banned
Yes I have heard that you can go .... "To Infinity and Beyond" ... I have basically the same gear as you do except I have the 50mm f/1.8 and not the 35mm like yours.

Almost all AF lenses actually go past the Infinity setting, this is designed into the lens itself to prevent fast AF lenses from hitting the "brick wall" so to speak when adjusting focus. When they reach infinity the sensor will stop the AF function, but the lens has that built-in "cushion" range just to avoid possible damage to the lenses focusing system. When a lens is brand new it is not really an issue, but as the mechanism wears there is the possibility that some play will develop and allow for that damage so the manufacturers allow for that. Smart thinking in my opinion.

You can do as tracsoft suggests above and "Play with it sometime in bright sun to see where infinity is"
 
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F-Stop

Senior Member
Thanks, guys. I actually just did that and it's true with all three of my lenses, so I guess it's just technique on my end. Autofocus sure didn't seem to work well at all. On the positive side, I was astonished as to the detail the photos reveal in the sky. There are a lot more stars up there than my naked eye could see. It was really cool. I'm going to go back out this evening and shoot some more. I'm using a tripod and the IR remote shutter release to avoid any camera movement from tripping the shutter.
 
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Mike150

Senior Member
I always try to manually focus the lens in the daytime to an "infinity" object.
Then mark the lens and focus ring with two small piece of painters tape that will show the proper position for infinity.
Once it gets dark and I can set the camera on the tripod, I check the lens with a flash light to make sure the tape is aligned and shoot away.
Oh Yea. I use painters tape because it doesn't leave a sticky mess when it's removed when you're done. I never leave it on the lens for long just in case.
 
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