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10-24-2020, 09:47 PM   #1
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Focus Issues?

I love the Pentax K70 which I recently purchased after falling under the spell of some old photos I captured with my once beloved K7. My question is this: I also bought the 31mm limited lens because that was my go to in the old days but I feel like it's not quite performing as I expected or remembered. I still like it don't get me wrong, but I always use center point focus in the middle of my frame and notice it's often not nailing the faces of the people I'm photographing. In fairness, the faces are small (kids) and maybe I have been spoiled by modern focus but is this normal at all? Here is a sampling of photos shot using back button focus, center point af on the face. I've also tested the lens and tried micro adjusting the focus but a) on a tripod shot in live view on self timer, the focus seemed spot on in my tests. However when I tried changing the micro adjustment just to see how it would look with one shot for every increment both minus and plus, I could literally see not one iota of difference between the shots. Am I missing something altogether here?

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10-24-2020, 10:08 PM - 1 Like   #2
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QuoteOriginally posted by acoulter Quote
tried micro adjusting the focus but a) on a tripod shot in live view on self timer, the focus seemed spot on in my tests.
Live view uses a different focusing system. AF fine tuning does not affect it in any way.

One way to test for front / back focus is to set up on a tripod and take a focused image using live view, then one using PDAF (viewfinder) and then one using manual focus. Ideally all three shots will be in focus but in your case I'm sure they will not be.
- Liveview AF is based on the sensor and is not able to be calibrated. It is what it is
- Manual focus depends on the focusing screen and the diopter adjustment
- PDAF focus depends on a separate AF sensor in the camera that can be adjusted using the AF fine tuning procedure

Note that AF fine tuning is extremely sensitive and must be carried out in a precise manner. Otherwise the margin of error is larger than the actual error.
Somewhere on this site is a tutorial on how to handle AF fine tuning but I cannot find it right now. Try a search and you should find a number of different methods.
10-24-2020, 10:09 PM   #3
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QuoteOriginally posted by acoulter Quote
a) on a tripod shot in live view on self timer, the focus seemed spot on in my tests. However when I tried changing the micro adjustment just to see how it would look with one shot for every increment both minus and plus, I could literally see not one iota of difference between the shots. Am I missing something altogether here?
Are you saying that you didn't see any difference when using live view, or when using the viewfinder?
AF fine adjustment only affects PDAF which is what is used with the viewfinder, AF fine adjust will have no effect on live view.
You have to take a lot of care when attempting AF fine adjust, making sure you have an appropriate target that will show the effects of front or back focussing, and using the tripod ensure that shake reduction is off.
If you have followed all the normally recommended procedures, then I would suggest you may have to take a number of shots at each AF fine adjust setting as there is likely to be variation between shots.

I see jatrax has just beaten me to it.


Cheers,
Terry
10-24-2020, 10:12 PM - 1 Like   #4
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QuoteOriginally posted by jatrax Quote
Somewhere on this site is a tutorial on how to handle AF fine tuning


Fixing Front and Back Focus - Introduction - In-Depth Articles

10-24-2020, 10:25 PM - 1 Like   #5
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Thank you!
10-25-2020, 04:29 AM   #6
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QuoteOriginally posted by acoulter Quote
I love the Pentax K70 which I recently purchased after falling under the spell of some old photos I captured with my once beloved K7. My question is this: I also bought the 31mm limited lens because that was my go to in the old days but I feel like it's not quite performing as I expected or remembered. I still like it don't get me wrong, but I always use center point focus in the middle of my frame and notice it's often not nailing the faces of the people I'm photographing. In fairness, the faces are small (kids) and maybe I have been spoiled by modern focus but is this normal at all? Here is a sampling of photos shot using back button focus, center point af on the face. I've also tested the lens and tried micro adjusting the focus but a) on a tripod shot in live view on self timer, the focus seemed spot on in my tests. However when I tried changing the micro adjustment just to see how it would look with one shot for every increment both minus and plus, I could literally see not one iota of difference between the shots. Am I missing something altogether here?
Maybe this helps? "https://testcams.com/blog/2015/09/22/dottune-autofocus-fine-tuning-in-under-5-minutes/". Simple procedure and it works very well.
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