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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.

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

  • 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 - photo.stackexchange.com/questions/2408/… . – rfusca Apr 25 '12 at 15:02
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Another possibility is to make a hartman mask for the lens. This method is used to find infinity focus for astrophotography.

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!

  • Bahtinov mask would be better in that case I believe. en.wikipedia.org/wiki/Bahtinov_mask – 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
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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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Unless your camera and lens are mismatched, the infinity ∞ focus position is pre-set by the lens/camera maker. A lens is focused by physically moving the lens barrel towards or away from the camera body. This action is happing, perhaps undetected by you. So take time and observe the lens barrel as you focus and you will see it move. The infinity ∞ position will follow when the lens barrel is as close to the camera body as the mechanism will allow. If your lash-up does not follow this protocol then focus on a distance building or mountain or cloud in daylight. This action will set the lens to the infinity ∞ focus position. If the lens barrel is not scribed for infinity ∞, perhaps you can make two marks. One stationary on a part of the lens barrel that does not move and another juxtaposed on the part of the barrel that moves. However, I believe all this will be unnecessary as the lens you own will hit an infinity stop position as you are focusing on a distant subject. Just for the record, objects at a distance of about 300 times the focal length are at infinity ∞. As an example if you mount a 55mm zoom and set it to max magnification objects 55 X 300 = 16,500mm away will be at infinity ∞. That’s 16.5 meters or 54 feet downstream from the camera.

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