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12-19-2014, 10:56 AM   #1
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Focus inconsistency/accuracy - Part II

I apologize in advance to all that might have felt, or may feel "insulted", as stated by some members, but it's something that still puzzles me...

The 35 is going back - meaning that I will be losing quite a chunk of money, from my perspective - and am now looking for an alternative.
I found a good deal on a F 50/1.7. After reading some reviews, most of them state 'focus inaccuracy'.

Here's a 'copy-paste' of one of them: "...once you know that 'this lens is dialed in at 10 feet at f1.7' you can work accordingly...
Read more at: https://www.pentaxforums.com/lensreviews/SMC-Pentax-F-50mm-F1.7-Lens.html#ixzz3MMnSJNHe".

Exactly what am I getting into? Is this something that happens on a regular basis? Is it a Pentax 'thing'? Is it lens fault, or camera fault? Is this a way of Pentax 'forcing' one to own more than one lens?... "So, this 35 focuses at 5 feet, this 50 at 15 feet, this other 50 is spot-on at 25 feet, while this yet another 50 is great at infinity, etc..."
Is there something I'm missing? Am I reading this wrong? What is going on here? Have never heard this about any other manufacturer... Micro-adjustments, especially on a prime, should just work; not work at a given distance and be a bit off at others...

Please, I really need some enlightenment on this matter, because I really like the feel of the camera, and it does a good job at B&W, my favourite output...


Last edited by Flugelbinder; 12-19-2014 at 11:30 AM.
12-19-2014, 11:41 AM   #2
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You have already had more issues with one lens and body than I have had with 4 Pentax bodies and 30 or so lenses.

You got a lot of good advice in the other thread which you generally chose to ignore.

It is not a Pentax thing it is not a lens thing. It is an auto focus thing.

If you want to learn how to use your gear and take pictures there are plenty of people here willing to help. But if you do not want to listen there is little anyone can do to help you.

Sorry but perhaps you would be happier with a different brand. Nikon for example is generally well respected for their autofocus. Pentax is not.
12-19-2014, 11:51 AM   #3
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I have been reading some user experiences about the new Canon 7D Mk. II, which is supposed to have extremely advanced AF. Still, some people have trouble. The internet experience is similar to your threads:
Is it the camera?
It couldn't possibly be the camera.
Here's what I'm seeing.
Here are all the things you are doing wrong ever.
Well, I still have focus issues.

It doesn't take a lot for internet discussions to go bad.

I like manual focus, and manual focus issues are almost all user error. Once the focus screen is shimmed correctly, you either turned the focus ring to the right place or you didn't. I also know from some lens testing that even the most careful focusing on a tripod is not perfect every time. I do it three times and hope to get two out of three good results.

Maybe that's a little enlightenment. I seem to get satisfactory AF from my K-5 IIs and an FA 35/2, so I can't help a lot with your issues.
12-19-2014, 01:09 PM   #4
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Hmm... I think it is a pretty well-known fact by Pentaxians that Pentax's AF system is not really that fantastic.

From my own experience and understanding, this is mainly due to the AF point coverage which is by and large almost as big as a dinner plate.
This basically means the actual AF area for any given points are much, much larger than supposedly indicated by the red light in the viewfinder.

For example, try a middling portrait distance of a subject with ~35mm lens, where the center AF point is on the eye and the AF point beside it lies slightly outside the ear.
Now, when you intend to focus on the subject's eye, what the central AF point covers would be the are from the nose bridge to the edge of the ear (something like that, if you get my drift).
The AF system would then decide which has more contrast within that said area, which is pretty huge!
And that's the problem..

With that said, I suspect Pentax actually knows of this issue and therefore provided a couple of "solution":
1) quick-shift (yea... They market it as "fine-tuning" your focus point.. *pfft~* stupid because modern focusing screen only accurately shows **f/2.8 DOF)
2) use smaller aperture / stop down for more DOF (also, reason why the 55/1.4 is the only and last modern fast lens)
3) Pentax K3*

* The K3 has 27(!) AF POINTS, of which 25(!) lies within the SAME frigging central area as covered by all models prior to it, including the K5's, 9 AF points. This basically means the actual AF point coverage area is no longer dinner plate sized and the actual AF coverage arer is much more true to the individual red light indicated in the viewfinder.

Solution for you (either or all)?
1) Get a K3 (for the more "honest" indicated AF points)
2) Stick/use only modern DA limited lenses (for quick-shift and compensatory larger DOF from their small widest aperture)

** For me, i just took the solution of changing my focusing screen to the Canon S screen from focusingscreen.com so that i can see the real focus points, via manual focusing.


Last edited by SyncGuy; 12-19-2014 at 01:24 PM.
12-19-2014, 01:28 PM   #5
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QuoteOriginally posted by Flugelbinder Quote
Exactly what am I getting into? Is this something that happens on a regular basis? Is it a Pentax 'thing'? Is it lens fault, or camera fault? Is this a way of Pentax 'forcing' one to own more than one lens?
1. Hard to say

2. No

3. No

4. Probably not, damaged mechanicals may make it harder to establish focus lock, but usually do not cause imprecision or inaccuracy unless the focus shifts after lock. Complaints about a particular lens having poor AF performance is usually a matter of unreasonable expectation from the AF system. This is particularly true when shooting at wider than f/2.8 maximum aperture. Another problem is when a lens is soft overall at maximum aperture making precise and fast AF all but impossible.

5. Quite possibly, though hard to demonstrate. The key is to do systematic tests on a simplified system to isolate the fault and determine the nature of the problem.

6. Absolutely not.


Steve
12-19-2014, 01:32 PM   #6
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So, basicly, the solution is to shoot at 2.8 or smaller if using auto focus, or just manual focus?
12-19-2014, 01:34 PM   #7
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QuoteOriginally posted by jatrax Quote
You have already had more issues with one lens and body than I have had with 4 Pentax bodies and 30 or so lenses.
I agree. The OP's experience is perhaps the most complicated of any user's since I joined this forum in 2007.

QuoteOriginally posted by jatrax Quote
Sorry but perhaps you would be happier with a different brand. Nikon for example is generally well respected for their autofocus. Pentax is not.
I suggested the same thing, though I am not sure what the OP will do with the K-5 IIs. I know that I would not knowingly buy it unless it had been given a clean bill of health by a competent repair facility.


Steve
12-19-2014, 01:39 PM   #8
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QuoteOriginally posted by stevebrot Quote
I agree. The OP's experience is perhaps the most complicated of any user's since I joined this forum in 2007.



I suggested the same thing, though I am not sure what the OP will do with the K-5 IIs. I know that I would not knowingly buy it unless it had been given a clean bill of health by a competent repair facility.


Steve
Again, maybe I'm just too picky - demanding...

Man, I wish I have heard this before I bought this...

---------- Post added 12-19-14 at 01:44 PM ----------

Any idea how much Pentax charges for a calibration?


Last edited by Flugelbinder; 12-19-2014 at 01:48 PM.
12-19-2014, 01:47 PM   #9
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QuoteOriginally posted by Flugelbinder Quote
So, basicly, the solution is to shoot at 2.8 or smaller if using auto focus, or just manual focus?
No, the solution is to realize that the system is not able to reliably detect out-of-focus at a maximum aperture wider than f/2.8 and then only with the center AF point. The rest of the AF points have a focus sensitivity of f/5.6. The result is lack of precision with fast lenses. Your DA 35/2.4 did not fall into that category. This is not a Pentax issue, but is present in ALL PDAF cameras currently on the market, most of which get by with only f/5.6 AF sensors.

As for shooting at narrower than f/2.8, the taking aperture is not a consideration except for DOF and missed focus is missed focus.


Steve
12-19-2014, 01:51 PM - 1 Like   #10
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QuoteOriginally posted by Flugelbinder Quote
Or is Pentax a manual focus optimized system?
Hardly.
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