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09-16-2011, 07:23 PM   #1
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Moonfinity?

Hi, gang -

I was out shooting the moon (so to speak) earlier this week with my brand-spanking-new K-r and 55-300. The pix turned out great, and I'm absolutely loving the camera.

One technical issue that came up was the focus distance. The autofocus nailed it pretty well, which was nice to see. I was surprised to find, though, that the focus distance was *not* fully at infinity. In fact, if I manually focused to infinity (fully turned to the right), the pictures were out of focus.

So my question...
Is this normal for photographing the moon? Or is this resulting from an incorrectly set infinity stop on my lens? (Or is there always some play beyond infinity?)

Thanks!
Ryan

Edit: The ideal focus point was not far from all the way to the right. It was far enough to be easily noticeable, though, and turning all the way to the right definitely took things out of focus.

09-16-2011, 07:24 PM   #2
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That's normal for some lenses. Not every lens will be at exactly infinity when the focus ring hits the end as some do go past. I don't know why (someone else can enlighten us), but it's not abnormal.
09-16-2011, 07:43 PM   #3
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IMHO the far-focus distance (FFD) of a lens shouldn't be called infinity. That's just too vague. (Infinity means "too big to measure". Yeah, that helps.) Depending on the lens construction, 'infinity' may be anywhere from one meter to a million meters or whatever. And because of copy variation, the FFD's of any particular model of lens may vary. So it's safer for a lensmaker to make lenses that "focus past infinity" than to build lenses that won't focus at 100m. So just figure that 'infinity' on a lens is a polite fiction.

Similarly, using AF to find infinity can be troublesome. Some infinities are rather bleak, pale, featureless. AF needs to find differences. An AF system can *probably* find a lunar limb, but will have trouble with a low-contrast blur. So, the trick: Focus on something very, very far away. Call it infinity. Use tape or whatever to mark that point on the lens. Then, when you're shooting 'infinity', go to that mark and switch off AF.
09-16-2011, 07:44 PM   #4
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QuoteOriginally posted by JinDesu Quote
That's normal for some lenses. Not every lens will be at exactly infinity when the focus ring hits the end as some do go past. I don't know why (someone else can enlighten us), but it's not abnormal.
I never gave much thought to it before, but assumed that focusing past infinity allowed for manufacturing tolerances and temperature expansion. I just did a quick bit of reading and learned that autofocus lenses use it to prevent slamming the lens during focusing. It's also more common in zooms than primes.

09-16-2011, 07:49 PM   #5
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Thanks, all...good info as usual.
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