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I have a kit lens which has no markers. What would be a recommended way to find infinity focus on this lens?

I plan to shoot objects at night -- not necessarily stars or the moon. So, adjusting the focus using the viewfinder may not be an option.

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If precise focus is important, the markers won't help much anyway. – mattdm Apr 26 '12 at 0:02
up vote 4 down vote accepted

I'd go to the aperture you want, go to live view, zoom in, and manually focus on the moon. For all practical purposes, it should be infinite. Mark it on the lens.

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Can I manually focus on some other object which is quite far away during the day, and hope to use that setting at night? Just wondering if there is an inexact science. – publicRavi Apr 25 '12 at 14:59
Generally, yes. But if conditions like temperature change too much, it could be slightly off. See this answer -… . – rfusca Apr 25 '12 at 15:02

Another possibility is to make a hartman mask for the lens. This method is used to find infinity focus for astrophotography. Use this link to create the dimensions and position of the holes:

In use, you'd point it at the night sky so that you'll see trails of stars. Slowly rack the focus and take a long exposure to test that setting. As you approach infinity, the two star trails will merge into one line.

Be sure to look at a single and not a double star!

Once you find focus, mark that point and note the local temperature as this will change depending on the materials the lens has and its focal length.

You may want to bring some gaffer tape to secure the focus ring once you've found the right spot so that it won't move if you touch the camera.

Hope this helps!

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Bahtinov mask would be better in that case I believe. – rfusca Apr 25 '12 at 16:35
The problem with the Bahtinov mask with camera lenses is that it gets very hard to see the spikes. I've tried these with a 200mm and 300mm focal lengths and it's quite difficult. It gets better if you have a computer screen to see the image at 100%. The Hartman mask works down to 50mm focal lengths. It's best to orient the Hartman mask so that it's at right angles to the star trail to see the gap in the lines. – smigol Apr 25 '12 at 19:45
Interesting, I've never had a problem with my Bahtinov on my 50mm. – rfusca Apr 25 '12 at 19:52

A site called Catching the Light has a wealth of information on focusing. Here is the link for 17 methods of focusing to infinity.

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