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If you take a look at a few of long exposure sky photos at Flickr, I think the context and content of the photo is okay, they are just all blurry. I'm using a tripod, 15 second exposure with a 18mm lens with the focus cranked all the way back, 1600 ISO.

Why is the sky out of focus? Anyone know any tips to help me out?

marked as duplicate by Michael C, Philip Kendall, MikeW Apr 25 '16 at 19:22

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15 second exposure with a 18mm lense with the focus cranked all the way back

When you say "cranked all the way back" it sounds like you just turned the focus ring to the far limit. That's not the right way to focus at infinity because the focus adjustment for most lenses goes past infinity. You need to focus either with autofocus, or by looking through the viewfinder or at the screen and adjusting until the image is sharp.

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Unlike many older manual lenses, especially primes, that have a hard stop at a lens' infinity focus setting, most autofocus lenses do not. Instead they allow for the focus mechanism to be turned past infinity. There are several reasons why this is so, but the main one is to avoid the focus motor slamming against a hard stop when focusing the lens at infinity.

For more, please see: Why do some lenses focus past infinity?

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