[m42man]

Hi,

I decided to buy a used K-3 after my K-50 got the dreaded lurgy.

Testing with a DA 18-55, I was intrigued to find that, at the wider zoom settings, sharpness increased somewhat as you moved away from the centre of the frame (several apertures). Focusing problem?

So, I thought I'd try my DA 35/2.4, on a distant subject. Wall-to wall blur (at f5.6)! Alarmed, I tried Live View (and manual focus), and all was OK. I then experimented with AF microadjust, and found that sharpness was achieved by setting + 8, 9 or 10. Taking repeated shots, the focus mechanism seemed to remain in a set position until I hit +8, when it shifted audibly - and I achieved sharpness.

This doesn't bode well for third-party lenses, or zoom lenses in general, I suppose. (Though I did briefly try a couple of known-quantity zoom lenses, and all seemed pretty much OK).

Is this normal behaviour (I've never had this kind of problem before)? I can post images, but I can say that the blur before microadjusting was gross, when viewed at 100%.

[BigMackCam]

At wider focal lengths (anecdotally, 18-25mm) the DA18-55 exhibits a degree of field curvature. If the centre was out of focus because AF fine adjustment was required, it's possible that without it, the centre would be blurred but the outer parts of the frame could be in focus, primarily at wider apertures and close range (which, I imagine, describes your testing situation).

Regarding the DA35 f/2.4... Mine requires -3 AF fine adjustment on my K-3 and K-3II (most of my DA Limited lenses require zero adjustment).

What is your testing setup and process, may I ask (including distance from camera to target)? Are you using a suitable target and controlled procedure to ascertain whether you've achieved critical sharpness?

[aaacb]

My limited 40 needs +8. My 35/2.4 needs +5. Of the two, the limited is noticeably more consistent. To make sure af moves the lens before each test shot, I either move the focus ring (on a quick shift lens) or try to focus on something closer before focusing on the target (put my hand in front of the lens).

[m42man]

At wider focal lengths (anecdotally, 18-25mm) the DA18-55 exhibits a degree of field curvature. If the centre was out of focus because AF fine adjustment was required, it's possible that without it, the centre would be blurred but the outer parts of the frame could be in focus, primarily at wider apertures and close range (which, I imagine, describes your testing situation).

Regarding the DA35 f/2.4... Mine requires -3 AF fine adjustment on my K-3 and K-3II (most of my DA Limited lenses require zero adjustment).

What is your testing setup and process, may I ask (including distance from camera to target)? Are you using a suitable target and controlled procedure to ascertain whether you've achieved critical sharpness?

Thanks for the replies, guys. Maybe it's not too uncommon to need such a large AF adjustment. I wonder if the K3 is less tolerant of lens variations than, say the K-50 etc. etc.?

Yes, I must say, after noticing the somewhat OOF centre with the 18-55, I was thinking about AF accuracy. At least the AF error was in the right direction! (It sharpened towards the edges at the expense of the centre.)

Regarding test setup, this was nothing special - about 15 feet and f2.8 for AF setup/tests with a small target, and 220 feet and f5.6 for general across-frame sharpness evaluation. Nothing too scientific at this stage I'm afraid.

Assuming this is fairly normal behaviour, then I'm not really concerned, but am I right in assuming you can't assign AF adjustments to third-party lenses on an individual basis?

[BigMackCam]

am I right in assuming you can't assign AF adjustments to third-party lenses on an individual basis?

You can, actually; so long as the lens is in Pentax's lookup tables in firmware such that it's uniquely identified (and that covers most modern Sigma and Tamron lenses - I think some Tokina too, perhaps), then it'll work just fine. As an example, I have my Sigma 17-50 f/2.8 set to -4 individually...

[Na Horuk]

Eh, its not a big problem, as long as it works. Especially since your camera allows you to remember different AF adjustments per lens.
Just make sure your method is correct. You have to take into account DoF field curvature. If you take a photo of s straight brick wall, you have to make sure it is absolutely parallel to the sensor (no tilt up down, no rotation left right). Next, you have to use low ISO, 2 sec timer (or remote trigger), and a low f-number. Even if focus is achieved on the AF point (which may be bigger than the AF overlay in the viewfinder), the rest of the image might not be fully in focus due to field curvature - the in-focus area is not parallel to the sensor (or wall), but is kind of like a wedge. So make sure AF is right on the AF point.
Finally, there are other possible issues like focus shift (some lenses have this, I don't think the DA 35mm suffers from it) or even actual problems like lens decentering (where a glass element is slightly off, which can cause part of the frame to never be sharp, or ruin the whole image, cause odd CA, flares, etc.). I think there are a couple threads about AF adjustment, as well as blogs that even sell AF focus charts with instructions for various methods of calibrating AF

[m42man]

Thanks for the reassurance folks. And it's nice to know that I'll probably be able to fine-tune (if necessary) the popular third-party lenses too. Isn't phase-detect AF a black art though!

I am aware of focus-shift, but I must say, I'm assuming it'll usually be quite small. And as for decentred lens elements, most of my (very-consumer grade) zooms have this to some degree or other (I've even got a vintage Cosinon nifty-fifty which seems to suffer from it!).

[BigMackCam]

Isn't phase-detect AF a black art though!

It's certainly fun and games!

I am aware of focus-shift, but I must say, I'm assuming it'll usually be quite small.

You might be surprised For a fast lens - say f/1.4 - with AF fine adjusted so that focus is spot on at very close distances with the lens wide open, you might find that stopping down to f/2.8 - f/4 results in a shift of critical focus to a point quite noticeably behind your target - far enough behind that your target is more out of focus than you can live with. It's worth reading the following article: What is Focus Shift?

[clackers]

Is this normal behaviour (I've never had this kind of problem before)?

There is normal variation in how they put the PDAF module into your camera in the factory.

When you buy a body, test it - if your lenses don't achieve focus even at +10 or -10, send it back.

[Edgar_in_Indy]

I've wondered before why Pentax does not allow more correction. I've heard many instances of lenses needing a judicious amount of correction for proper calibration.

[Na Horuk]

I've wondered before why Pentax does not allow more correction. I've heard many instances of lenses needing a judicious amount of correction for proper calibration.

Does any camera brand allow much more than this? Its kind of amazing that pretty much the whole Pentax DSLR lineup allows this. Some years ago special debug menus had to be accessed for this kind of stuff, and it would only affect the global value. Then you could set it, but for all lenses. Now you can set different values for a certain number of lenses, plus a global value.
Things are improving, but they are still not quite perfect yet. Total, perfect AF calibration is really difficult if not impossible, even for a pro with the right tools.
Many pros actually take their gear to a shop to get it checked and calibrated at a certain regular interval. The shops might be able to calibrate things mechanically, using precise tools.

Bottom line is AF is just one tool. You have to learn to use it, master it, and then use it to get what you want. Its not something you can just blindly rely on, its not a magic button.

[clackers]

I've wondered before why Pentax does not allow more correction. I've heard many instances of lenses needing a judicious amount of correction for proper calibration.

All brands, Edgar.


As I understand it, a human has to be involved in this step during manufacture, so you have variation right there.

[Edgar_in_Indy]

There is normal variation in how they put the PDAF module into your camera in the factory.

When you buy a body, test it - if your lenses don't achieve focus even at +10 or -10, send it back.

They should allow you to adjust it to 11. Everybody knows that 11 is better than 10.

[BigMackCam]

They should allow you to adjust it to 11. Everybody knows that 11 is better than 10.

As Nigel Tufnel would say, "These go to 11."

[clackers]

As Nigel Tufnel would say, "These go to 11."

Still stands up, that film. 😊