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08-27-2022, 01:38 AM   #1
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AF problems with Pentax 645z

Hi,

I've tried a Pentax 645z camera with a 55mm lens and there were some AF problems.







I focused on the eye but the camera focused on a much closer plan on the right.

These focus errors occurred mostly at f2,8.

I tried the AF Fine feature following the Pentax instructions, there was just a very very slight difference for which I moved the adjustment from 0 to +2. But it was an almost imperceptible difference, incomparable to the focus problem I am talking about.

Above all, these focus errors occurred irregularly and randomly, because after a few seconds and with the same identical focus point, the first shot the focus was correct and the second shot the focus completely wrong. Therefore, since they occur in a random and unpredictable way, any type of correction is impossible - assuming that it was possible to correct such a focus mistake in some way, I don't know.

In short, I returned the camera to the seller.

But, since I would still be interested in buying this 645z, two questions for some 645z expert users.

1 _ Are these focus defects to be attributed to the camera or lens?

2 _ Are these AF problems possible or even common with the Pentax 645z, or instead, in this case, is it just a not properly working camera or lens?

Many many thanks for your help,

Enrico

08-27-2022, 04:59 AM - 2 Likes   #2
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A couple of generic suggestions for how this could happen:

Dirt on the AF sensor lens: The AF sensor sits under the floor of the mirror box and has a lens that looks up into the mirror assembly. Dirt such as hair, threads, etc. on that lens will cause unreliable AF. That AF sensor lens can be cleaned with air (e.g., a little bulb blower) when the mirror is up for main sensor cleaning .

Loose mount: Any looseness or play in the lens mount (of the camera or the lens) can cause significant random shifts in the plane of focus. For example, if 55mm lens on a 645Z focused to 5 meters will shift to focusing at about 4.6 meters with just a 50 micron (the breadth of a human hair) shift of the lens-sensor distance.
08-27-2022, 05:48 AM   #3
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The Live View mode will use different focusing technology than through the view finder, Did you try that and compare?
The comments from Photoptimist are right on - dirt is the enemy.
08-27-2022, 06:13 AM - 1 Like   #4
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Was focus confirmation turned on? It's always on in my Z so I can see whether the AF is focusing on what I want it to focus on. AF is great, but I don't blindly trust it, ever.

08-29-2022, 07:21 AM   #5
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I didn't check focus confirmation or live view mode, but it wouldn't be useful as I usually use the viewfinder and because I would need a a fast camera, reliable even in case there is no time to check focus confirmation.

Mostly, I need to know if these AF problems are possible or even common with the Pentax 645z, or if it's just because a not properly working camera or lens.

Many many thanks for your answers,

Enrico
08-29-2022, 10:15 AM - 1 Like   #6
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QuoteOriginally posted by enricob Quote
I didn't check focus confirmation or live view mode, but it wouldn't be useful as I usually use the viewfinder and because I would need a a fast camera, reliable even in case there is no time to check focus confirmation.

Mostly, I need to know if these AF problems are possible or even common with the Pentax 645z, or if it's just because a not properly working camera or lens.

Many many thanks for your answers,

Enrico
In my experience autofocus will favour vertical elements. I found the most reliable to use the 9 points (auto) focus or sometimes spot focus. I switch off the sound confirmation and switch on the visual confirmation which is the small red box in the viewfinder that shows where the camera focused (Menu C14 & Menu page #4-Button customization & AF dial just left of view finder set to "C"). Another helpful change I made is to switch on back button focus and removed focusing from the shutter button. Then switch to continuous focus. That way you can focus when you want and simply release your thumb from the button when you want to lock the focus. With a bit of practice I found it does not slow me down and I can lock focus and move around to follow the target or recompose and press the shutter button without messing with focus again.

Although with a target such as a bird sitting amongst branches, the most reliable is the same procedure above but with spot focus, otherwise the camera will select one of the vertical branches and focus on that. Tracking flying birds the 9 point is very reliable, A bit annoying is that the continuous focus will sometimes stop tracking and you have to release the back button and press it again. I am so used to this now I press and release the back button as needed without waiting for it to stop.

Hope this helps.
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