[opiedog]

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

[Adam]

When zoomed out, there's almost no contrast in the image, so the AF system won't be able to tell a difference between the tower and what's behind it, especially when there's little light. Your best bet would probably be to focus to infinity, use a faster lens, or do as you did- zoom in and then back out again (note that this approach doesn't work for all lenses, as it's dependent on the construction of the lens).

It's probably a combination of both, as the lens is likely also at fault- wide-angle to tele zooms tend to be lacking in quality at the longer end.

[jjdgti]

Adam: Thanks for the explanation. I experienced some of that while shooting with wide angle lenses and focus to infinity.

[opiedog]

these were shot at high noon using center point only.

both the cell tower and radio tower were smack in the middle of the center AF module.

maybe the contrast wasn't enough?

for the radio tower, maybe so since it was many miles away, but for the cell tower, that was only a few hundred feet

what i'd like to know is what's the algorithm used by the kx in situations like these and how to best coax the AF.

[Mr. B]

My K10D has this problem too. Sometimes with the 18-55, more with the sigma 17-70 at 17mm and all the time with 16-45 at 16mm. It seems to get worse as it goes wider. Now I can't even buy a decent wide angle zoom because of this...
Everything at daylight, at the central point, plenty of contrast. I'm sick of this but as have no money yet to sell Pentax and switch to something else, I'm sticking with it. Will see how this turns out...

[opiedog]

unfortunately, switching brands is not the solution.
physics applies to nikon and canon as well.

see this from nikon forum:
Autofocus and wide angle are not compatible.: Nikon D90 - D40 / D5000 Forum: Digital Photography Review

an interesting read:
http://www.leongoodman.com/d70focusnew.html

[johnbee418]

first thought is to mount a manual lens and zone focus or set the focus at infinity.

[Marc Sabatella]

There's also the issue that when viewing a distant scene wide angle, most objects in the scene are much smaller than the AF sensor. So it's usually impossible to guarantee the camera will choose to lock on to your intended subject as opposed to the background. For example, in your cell tower example, the tower itself is not as wide as the sensor, so the camera may well have chosen to focus on the background. Now, in this case, both are close enough to infinity that it really shouldn't have mattered, and indeed, it's not at all obvious where focus actually is here. Nothing seems even close to in focus in that second example, making wonder if there isn't something actually wrong with the lens. One would need to do more tests to know for sure.

[opiedog]

marc,

good explanation.

so is there any way to "coax" the AF to pick the tower?

sometimes if my subject is not at infinity, i would point to a subject at infinity to force my kx to refocus, then jump back to my subject. sometimes it works, sometimes not.

what i find works best is to zoom in and getting a bigger subject/better contrast then zoom back out.

tnx
jordan

[Mr. B]

unfortunately, switching brands is not the solution.
physics applies to nikon and canon as well.

Well, what is? I'm curious... I can't get anything in focus below 18mm - outdoors, focus test charts... Is it my camera?

This does not happen on "small" subjects but on normal ones: people, buildings, whatever, the wider I go, the more back focus I have.

[Marc Sabatella]

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.

[timo]

I've noticed this with several cameras and wide-angle lenses. With my Sigma 17-70 at 17mm, if I focus at a very distant object three times, the lens will end up at three different places. My former 12-24 (now sold) did the same, if anything more so. My (now sold) 16-45 often had difficulty locking on anything at all at 16mm.

Having established via live view that infinity is more or less where the lens marking says it is, I often use MF for wide-angle, long-distance scenes. The very large DOF makes it generally non-critical and MF works fine. I think it's just something you live with if you are shooting distant shots at wide angle, as others have said.

Tim

[jeffshaddix]

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?

[simico]

This is even worse with a wideangle fast prime - you can't zoom in, focus, zoom back I've just purchased a Sigma 30 F1.4 and focusing on anything farther away than 2-3 meters is a hit and miss, mostly being a miss. If my wife is 3-4 meters away then his entire head fits into the AF sensor and I have no idea (and no control) what it will focus on: her nose, eyes, ears, hair or something behind her? And sometimes focus is totally off, i.e. nothing is in focus.
FA 50 F1.4 is almost just as bad, but the 20mm FL difference helps a bit.

[leadfoot]

unfortunately, switching brands is not the solution.
physics applies to nikon and canon as well.

see this from nikon forum:
Autofocus and wide angle are not compatible.: Nikon D90 - D40 / D5000 Forum: Digital Photography Review

an interesting read:
My Photography

Thanks for these links, very interesting.