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08-24-2010, 07:10 AM   #16
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ur welcome.

here's another good read.

Mastering Wide-Angle

the author also suggests that AF w/ wide angles is well, not accurate.

Last edited by opiedog; 08-24-2010 at 07:22 AM.
08-24-2010, 07:18 AM   #17
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QuoteOriginally posted by jeffshaddix Quote
Wow I was just about to post this same question. I have the same difficulty with the K7 and the 18-55WR. At 18mm only about half my AF shots came out in focus (and yes the shutter-speed+SR should have ensured minimal camera shake).

Is the 18-55WR one that I can use the zoom-in, focus, zoom-out trick?
the only way is to try it.

zoom in to 55, get AF, zoom out, take picture. compare at 100% w/ image taken with AF at 18mm
08-25-2010, 01:50 PM   #18
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QuoteOriginally posted by Marc Sabatella Quote
No way to know for sure without seeing samples - but my guess is that you are simply seeing the effect I described above. And I don't really know of a way to force the AF system to pick the subject you want in these cases - I might give it a couple of chances in AF, then I'll take over myself. I do this often enough that I probably will never buy an AF lens that lacks Quick Shift.
I sold the 16-45 a week after getting it and deleted everything. The very few photos I had were terrible. Just to illustrate, I focused on a group of my friends (3 of them) and focus should be on the middle one, but the one behind him (about 0.7m) was perfectly sharp... And they were all covering their third of the frame so there's no talk about subjects being too small or something.

The Sigma 17-70 was from a friend. The problem was less visible.
DA18-55 is pretty much okay, sometimes it misses but not that often. And it's way to soft to tell if it's slightly misfocused or just soft.

In the end, I think it's my K10D... Guess I'll have to get a new body before buying a kit lens replacement because I heard that the 16-45 I sold works just fine on a K-x...
08-25-2010, 02:21 PM   #19
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QuoteOriginally posted by opiedog Quote
this takes a break from all the Kr and K5 discussion.

i love my kx, but when it comes to using wide angle zooms, the AF is not very accurate. I don't know if this is a pentax thing only, but from what i've read, it's not. Physics spares no one

somehow, i could seldom coax my kx's AF to get it right when i focus subjects at or close to infinity with wide angles. i've tried several lenses, sigma, pentax, tamron. same/similar issue.

for me, i find that zooming in to max zoom, AF, then zooming out works pretty well, even for varifocal lenses.

i know there are many factors that make wide angle AF iffy. obviously, one thing is contrast. when zoomed in, the AF module can get a better contrast.

but is it the camera or the lens or both?

i think tips and experiences from forum members would be helpful.

here are some examples:

subject: Cell phone antenna array

from 28mm

zoomed in then back to 28

subject: radio tower

from 28mm

zoomed in then back to 28
I noticed this same exact problem with my Tamron 18-200. Every time I tried to focus at 18mm the focus ring would indicate the wrong distance. If my subject was 20 feet away the focus ring would be on about 35 feet away. If I zoomed in the focus ring would lock in correctly at 20 feet. I now do the same as you do, zoom in, focus, zoom back out and take the shot. This is annoying but I do not see a way around it.

I believe someone told me that this could also be due to the fact that some zoom lenses are varifocal as compared to something else that I can't remember what the lens was called but they said the varifocal lens was not a "true" zoom and that is why focusing at 18mm is different than focusing at anything zoomed in closer than that.

I actually started a thread on the very subject that has some good info in it, here's the link:

Last edited by KxBlaze; 08-25-2010 at 02:26 PM.

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