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01-20-2010, 08:32 PM   #1
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Kit lenses and Infinity

Both of my K-x's kit lenses are focusing "past" infinity. The 18-55 focus stops closer to infinity but still goes past it. The 55-300 is terrible, you can keep turning the focus ring maybe 1/4 inch past where it should stop for infinity.

Is this normal, because they're super cheap lenses, or should I be contacting Pentax about it?

I noticed it first trying to shoot the moon, and chalked it up to having to contort myself to see the view finder... but recently trying to photograph distant objects on land, I find the same issue.

It doesn't seem to affect auto focus, but the camera probably doesn't rely on the stops like a human does.

01-20-2010, 09:15 PM   #2
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It's perfectly normal. In fact, all the high-end lenses I know of do it. I'd rather have it focus past infinity than before it...
01-20-2010, 09:27 PM   #3
Ash
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A basic provision for a lens to allow the AF motor to run past infinity and permit accurate AF locking at infinity distance.
01-20-2010, 09:47 PM   #4
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Just a small point of clarification...

Are you complaining that the infinity mark on the lens does not match the index when the lens is properly focused on infinity or are you saying that the lens is not focused at infinity when the focus ring is at end-of-travel? Both?

I have the 18-55 v1 and the infinity mark does not exactly match the index mark at end-of-travel. I just stepped out side and manually focused on a lamp pole a 100 yards away with my Katz Eye split image. The focus ring was a fraction of a degree from end-of-travel for that distance. I then went and did a quick tally of my other lenses. With the exception of the FA 35/2, all had minor misalignment of the infinity symbol with the focus index.

Something to consider:
  • Even wide open at 55mm, your kit's hyperfocal is at 139 feet (acceptable focus from 70 feet to infinity)

Steve


Last edited by stevebrot; 01-20-2010 at 10:12 PM.
01-20-2010, 11:10 PM   #5
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I think Ash's explanation nails it for me... the motorized system would need a little play rather than a hard stop right at infinity.

QuoteOriginally posted by stevebrot Quote
Just a small point of clarification...
Are you complaining that the infinity mark on the lens does not match the index when the lens is properly focused on infinity or are you saying that the lens is not focused at infinity when the focus ring is at end-of-travel? Both?
Steve,

The lens is not focused when at the end of travel. It comes into focus and then goes back out as I turn the focus dial. My kit lenses do not have the distance marks on them.

I should have given more time for a "cool down" period before posting. Thank you to everyone for explaining it to me without judgment

I was trying to take some astro photos, and was rather frustrated having to bend over and peer through the view finder (which was pointed at the ground), in ~18F weather. After warming my hands and switching to lithium batteries, my second attempt brought along a stool, and a new strategy. I aim at the brightest star I can find, and fine tune the focus using live-view. Then careful not to touch the lens again, I line up my target and take pictures. I'm still not able to focus the kit lens as well as my old primes, but I think I just need more patience.
01-21-2010, 03:14 PM   #6
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Thanks for asking. I had noticed the same myself but never bothered to ask the question why.
01-21-2010, 03:42 PM   #7
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Actually, there's another reason that is sometimes cited too, although I've heard it mostly (exclusively?) with regard to mirror lenses - thermal expansion. If a lens just barely focuses to infinity at one temperature, it might not quite make it there at all at another temperature. So better to err on the side of letting it focus past infinity at some temperatures.
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