[tibbitts]

I'm sure this question must have been asked and answered, but of course most discussions about autofocus adjustments that turns up during a search are about in-body adjustments, so...

When Pentax/Precision/whoever adjusts AF on a Pentax lens, what exactly happens to the lens? In other words, your lens focuses just fine using live view AF; with viewfinder AF, you need to set your body to +14, which Pentax conveniently fails to provide (seriously - I hope that's been fixed in the K3iii.) You get your lens back, remarkably without sending your body along with it, and sure enough, it now does viewfinder AF perfectly at +0. What did the service facility do to your lens? Can anybody besides Pentax service facilities (Precision, etc.) accomplish the same thing?

Just to complicate things, let's say your zoom lens now does viewfinder AF perfectly with +0 at the longest focal length, which is where you objected to needing +15 the most, for obvious DoF reasons. At other focal lengths, say at the opposite end of the zoom range, viewfinder AF is close but still not quite correct. Are all AF lenses capable of being adjusted independently for 2/3/infinite different focal lengths? Again, not with regard to in-body adjustments (multi-range adjustments are possible with some other manufacturers' in-body adjustments), but strictly by adjusting the lens.

Would the answers to these questions likely apply similarly to Tamron or Sigma Pentax-mount AF lenses, or would they be completely different?

[stevebrot]

When Pentax/Precision/whoever adjusts AF on a Pentax lens, what exactly happens to the lens?

Good question. My understanding is that it involves tweaks to individual element alignment and spacing with assistance from special instrumentation.


Steve

[tibbitts]

One interesting consideration is that whatever process is involved, live view AF results apparently aren't affected. So any physical (or electronic, for that matter) adjustments would have to apply somehow to one type of AF but not another.

[stevebrot]

One interesting consideration is that whatever process is involved, live view AF results apparently aren't affected. So any physical (or electronic, for that matter) adjustments would have to apply somehow to one type of AF but not another.

That is because the two are done quite differently. Live view says focus is good when contrast at the focus point (measured at the image sensor) is highest. That is why there is no need or feature for adjustment. PDAF (viewfinder AF) is much more complicated (greatest similarity is to a split-image rangefinder) and is sensitive to ambiguity in edge rendering.


Steve

[UncleVanya]

I would suggest liveview may be impacted but not in a way you would see. So if I change the absolute focus point of a lens so it goes from needing +14 to 0 the absolute point of focus in liveview may well have changed but unlike the viewfinder this change is masked from the user. It just focuses at the right distance even if that distance has changed.

[Mooncatt]

That is because the two are done quite differently. Live view says focus is good when contrast at the focus point (measured at the image sensor) is highest. That is why there is no need or feature for adjustment. PDAF (viewfinder AF) is much more complicated (greatest similarity is to a split-image rangefinder) and is sensitive to ambiguity in edge rendering.


Steve

The way it was explained to be is that when using viewfinder focus, you are focusing with a separate sensor. So when the focus locks, it's locked based on what the focusing sensor sees, not what the imaging sensor sees. If those two are out of alignment, that's where the in body adjustment comes into play. Because live view focuses with the imaging sensor, what you see is what you get, eliminating the need for adjustments.

As to the original topic, I have no idea.

[jcsnyc]

Just got a new DA 300mm and I have to adjust to -10 for focus (maybe needs -11).

Thoughts: Do I keep it or send it back or swap it out? Seems pretty good at 10 on K3 II - will test on K1.

Using for birds and so focus is pretty critical and I shoot wide open when lighting is low…

Will test more but would hate to be without a new lens and expect I risk a worse problem on a swap! (And I am heading out of town tomorrow)

[andy271]

i got my k-1 calibrated it cost £120 and was worth every penny the difference in focusing is astonishing like a new camera i am very very happy

[tibbitts]

Just got a new DA 300mm and I have to adjust to -10 for focus (maybe needs -11).

Thoughts: Do I keep it or send it back or swap it out? Seems pretty good at 10 on K3 II - will test on K1.

Using for birds and so focus is pretty critical and I shoot wide open when lighting is low…

Will test more but would hate to be without a new lens and expect I risk a worse problem on a swap! (And I am heading out of town tomorrow)

When I bought a 60-250 new it came with needing +10 (or -10, whatever, 10 at least.) I kept it because of my own experience with trying to get properly centered lenses (and reports here on PF of decentered 60-250s.) And it was on sale when I bought it, so I couldn't just re-order at the same price, and keep the best one. Maybe I could have worked something out but it would have been complicated. Anyway I thought Pentax was far more likely to be able to adjust the AF than fix decentering or some other problem. I even did the debug mode thing to preset 5 so I could test for 10, 11, 12, etc. It was a tossup between 10 and 11. Incidentally I had access to another rental 60-250 and it was fine at 0. Well, honestly I mostly tested at 200-250mm and don't remember if it was exactly right at other focal lengths. Subsequently I've found that especially on my K-5 vs. K-5ii AF seems to be somewhat focal-length dependent. Not as bad as on some of my other zooms maybe, but close. Anyway it came back from Pentax (Precision) dead-on at 0 for the longer focal lengths at least. I have other experience with lenses that have been 10 off (2 of them) and one that's more like 12 off, unfortunately. And that's why I dislike Pentax being so stingy with the adjustment range. And if you can send a brand new lens in without a body along with it and have it come back needing 0 on your body, that tells me there's a serious quality control problem with the lens - not just a general tolerance issue.

[andy271]

im extra careful when i out my lens in my bag and dont bump it etc i allso have the fx button assigned to shake reduction so when i carry camera and lens i can turn it off and on