[bpv_UW]

Hi all,

Recently was fortunate enough to acquire an F100/2.8 macro, and need your help in sorting out some AF difficulties I'm having. Basically, the AF often refuses to lock on at all. This happens in good light (not low-light, fluorescent light, harsh backlight, etc, but well-lit outdoor conditions), with clearly defined, stationary objects (in other words, the exact conditions where AF should most thrive). It happens at both long and short working distances, with the focus limiter both engaged and disengaged, and happens regardless of aperture setting, focus mode (Auto and SEL), and metering mode (center, evaluative, etc). I'm using a fully charged, non 3rd party battery. I know the issue is not primarily with my camera body, as my other AF lenses (Fa 77, Fa 31, Da 15) work normally (tested at the same time, using the same settings, subject matter, etc). Let me explain in a little more detail what is happening:

When I push the shutter button halfway down, the AF engages and quickly reaches a point near where it should be locking on, but then >50% of the time hunts back and forth for a second or so (it just hunts within a limited range with small movements, it does not hunt through the entire focus throw) and subsequently refuses to lock focus. If I press the shutter button fully at that point, of course nothing happens. If I let my finger off the shutter button and again attempt to focus, I often am met with the same result several tries in a row. Sometimes it will eventually lock on, and others I have to recompose and choose a different focal point (maybe several times) before it will lock focus. Other times (though the minority), when I compose and focus it will lock on the first try. There doesn't seem to be any discernible pattern. It's erratic.

To be clear, the issue here is not AF accuracy. I'm not having problems with front- or back-focus. I haven't even been able to do any focus calibration testing and AF fine adjustment yet, as I just can't get the darn thing to AF successfully at all the majority of the time. So this is not something that will be fixed by AF fine adjustment or focus calibration. I have noticed that occasionally if I'm attempting to focus on an object and it won't lock after multiple attempts, if I quickly turn the camera off and on, then try it again, it will *sometimes* work (maybe 25% of the time). I've also noticed that focus seems to lock more readily in live view (though still not 100% of the time, and of course this is painfully slow).

I'm sure someone will read this and think I'm expecting too much of the lens, or attempting to focus on objects that are too small, or crowding a variety of objects at different working distances near the center AF point when I compose thus making it difficult for the AF system to "decide" what to focus on, or (insert other user-related issue here). I would never claim there is no user-related room for improvement, but I really think there's more going on here. I've used many Da, Fa, F, and 3rd party AF lenses on 3 different Pentax bodies over the past 8+ years, and I've encountered several lenses with focus accuracy issues that needed calibration. I'm also familiar with how different conditions (low light, back light, fluorescent light, moving subject, flat/very low contrast subject, etc) make not only AF accuracy/speed but locking AF at all difficult at times (more so with certain bodies/lenses than others, of course). But I've never encountered behavior like I'm seeing now with the F 100.

Anyway, sorry for the lengthy post, just hoping someone will have suggestions for how to improve this, or has encountered a similar issue and can give me some sense of what's going on.

Thanks in advance for reading and responding.

-Brandon

[Just1MoreDave]

I wonder if you can get a focus lock by manually focusing. And maybe try AF on distant subjects If that works, my uninformed wild guess is that the lens isn't sending complete distance information to the camera because internal contacts are dirty or broken. The guess is based on typical use being close up, and that part of the distance measurement getting a lot of wear. Manual focus bypasses the distance information and distant subjects use less-worn parts.

[Adam]

How's the AF performance in live view? If that can't lock on, there might be something with the optics.

[Dave's clichés]

I have had this issue and it turned out to be a speck of dust on the AF sensor prism. It is the small rectangular prism beneath the mirror, go to the menu and use sensor clean, with a charged battery and raise the mirror, then you can see easily the prism. I use a sensor swab with a tiny drop of the sensor cleaning liquid, but a blower brush will probably do the job, as the prism faces upwards a speck can easily fall on it. The usual symptoms are longer lenses struggling to focus! This is at any rate your first port of call!

[bpv_UW]

Thanks all.

Just1MoreDave - the issue seems to occur regardless of working distance

Adam - Live View seems to be able to lock on the vast majority of the time

Davescliches - I'm going to try your advice of cleaning the AF sensor - just have to get the appropriate equipment first - will let you know how it works

Brandon

[Lowell Goudge]

Are you on a tripod?

If not maybe it is the AF responding to slight movements of the camera

[bpv_UW]

Are you on a tripod?

If not maybe it is the AF responding to slight movements of the camera

I was not on a tripod, but did have the camera sitting on my deck table for much of the time I was testing the AF, so there was no movement.

[pacerr]

Are you asking about when you're focusing at macro / close-up distance? You may simply be inside the focusing range of the lens. (Don't ask how I come to mention this irritating possibility.)

If so, back out a bit an' see what happens.

[Dave's clichés]

Thanks all.

Just1MoreDave - the issue seems to occur regardless of working distance

Adam - Live View seems to be able to lock on the vast majority of the time

Davescliches - I'm going to try your advice of cleaning the AF sensor - just have to get the appropriate equipment first - will let you know how it works

Brandon

Like I said go into the menu choose sensor clean, the mirror rises and from then you can easily see the rectangular prism which sits behind the lens mount, a quick and gentle pass with the blower brush should do it!

[bpv_UW]

Just a follow-up. I tried cleaning the AF sensor with Rocket Blower, and the behavior seems to have improved fairly notably. Still failed to lock a couple times, but was much more reliable than when I initially tested it. Thanks Dave! (and, as an aside, I have a K3 on order, so will get a chance to see in a couple days if the entire issue was indeed the camera or if the lens has something to do with it).

Brandon

[Dave's clichés]

If it still doesn't "always" lock on you could try a wet clean of the prism using pharmaceutical alcohol, lightly applied with lens cleaning cloth!