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01-15-2014, 04:46 AM   #1
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AF problems with K5 II + Pentax smc FA 31mm F1.8 Limited

Hi guys,

I was wondering whether some of you with more experience could help me with the following problem.

In partucular, my problem is that in 40-50% of my shoots taken through AF there is an obvious lack of sharpness. However, when I take the same pictures using the Live View option all images are perfectly sharp. All my
pictures are taken using a very good tripode and a remote control. So this is not related to any accidental shakes. All pictures are taken with wide aperture F2.8, and shuter speed 30-50.

As I mentioned above, 50-60% of pictures are sharp (similar as when I take them via Live View) even when I use the AF, which shows that the lens is perfectly calibrated (I have calibrated the lens - AF Fine Adjustment +5 gives the best possible sharpness in my case).

Consequently, my question is why the K5 II does not produce the appropriate sharpness in 40-50% cases when I use the AF and not the Live View? Is this problem relateed to my camera or to my lens?

I have addressed this question to UK Pentax tech support but have not recived any answer.

Please kindly help me to find out the possible cause.

Many thanks in advance.

01-15-2014, 07:40 AM   #2
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QuoteOriginally posted by GTS Quote
Hi guys,

I was wondering whether some of you with more experience could help me with the following problem.

In partucular, my problem is that in 40-50% of my shoots taken through AF there is an obvious lack of sharpness. However, when I take the same pictures using the Live View option all images are perfectly sharp. All my
pictures are taken using a very good tripode and a remote control. So this is not related to any accidental shakes. All pictures are taken with wide aperture F2.8, and shuter speed 30-50.

As I mentioned above, 50-60% of pictures are sharp (similar as when I take them via Live View) even when I use the AF, which shows that the lens is perfectly calibrated (I have calibrated the lens - AF Fine Adjustment +5 gives the best possible sharpness in my case).

Consequently, my question is why the K5 II does not produce the appropriate sharpness in 40-50% cases when I use the AF and not the Live View? Is this problem relateed to my camera or to my lens?

I have addressed this question to UK Pentax tech support but have not recived any answer.

Please kindly help me to find out the possible cause.

Many thanks in advance.
OMG! You have the best camera/lens combo on the planet :-P

Sorry, can't answer your question but as a k5ii owner, I'm interested to find out the issue. I have a k5ii with FA35 and at F2.8 things are sharp. However, you may want to make sure you have turned off aids like SR so check all your settings.
01-15-2014, 11:17 AM   #3
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there's still several variables not covered in your statement:

are there any aspects in the images that are sharp? the K5 line PDAF has documented issues with locking focus on areas that are not the intended subject.

if entire image is blurry, have you gotten focus lock before taken picture? (probably, but in low light it's not always a guarantee, that's why I still like the focus lock beep. if you have that turned off, you may not actually have a lock)

when shooting into the sun, the k5 (I have k5iis) will sometimes lock focus at min distance (because of flare?) I've had this happen a few times with very strong light sources.
01-16-2014, 04:06 AM   #4
GTS
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QuoteOriginally posted by nomadkng Quote
there's still several variables not covered in your statement:

are there any aspects in the images that are sharp? the K5 line PDAF has documented issues with locking focus on areas that are not the intended subject.

if entire image is blurry, have you gotten focus lock before taken picture? (probably, but in low light it's not always a guarantee, that's why I still like the focus lock beep. if you have that turned off, you may not actually have a lock)

when shooting into the sun, the k5 (I have k5iis) will sometimes lock focus at min distance (because of flare?) I've had this happen a few times with very strong light sources.
Thanks nomadkng, gbeaton for your thoughts. In my case I am sure that I managed:

- I have got focus lock before taken picture.
- I have had lock beep as well.
- I was not shooting into the sun.
- I have turned off aids like SR

It seems that the problem is with a body and not the lens. AF appears to have some sort of issues. I am waiting for final advice from UK Petax tech support. If eventually the problem is with the body then I will be very disappointed, due to the fact that I bought this camera brand new from AMAZON.DE couple of months ago. Brand new item with this sort of problem seems to be a real trap for customers. Do not they have any quality control?!

01-16-2014, 04:01 PM   #5
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QuoteOriginally posted by GTS Quote
my problem is that in 40-50% of my shoots taken through AF there is an obvious lack of sharpness. However, when I take the same pictures using the Live View option all images are perfectly sharp.
My K-30 is exactly the opposite.
Taking picture through the Optical Viewfinder, center weighted, center AF.= SHARP
Take picture through Live View = soft edges throughout the image.
01-16-2014, 05:39 PM   #6
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@ GTS: I was going to ask if SR was off, but your later post indicated it was. SR is not designed for the sort of vibration frequencies that are created on a tripod and can create blur rather than eliminate it. Do you use the 2 sec delay function. I do when my cameras are on a tripod, this automatically turns off SR, it also allows any vibrations to settle before the actual exposure. You may be doing this with the remote control but your post does not mention it. The fact that images are sharp on liveview is a possible clue here - in Liveview the mirror is already up and would reduce/modify vibration patterns vs using the optical viewfinder. Re your quality control comments, AF is a very finely tuned beast - I'm impressed it works as well as it does for a consumer product given the micro-millimetre tolerances involved. Do you see this behavior on any other lenses, or is it confined to the FA31?

@OldNoob: I'm wondering if the liveview focusing is choosing a different focus point? I'd be looking very carefully at the distance scale focusing on the same scene using optical vf, then liveview. Is it settling in a slightly different place? Maybe use spot focus with the optical viewfinder, and then in liveview, move the little focus box to perfectly align with what you used to focus on with the optical viewfinder. That should eliminate subject selection differences as a possible cause. What happens if you manually focus in liveview? Also, is aperture being held constant between optical and liveview images? I ask only to the extent that metering differences might vary the aperture and therefore edge sharpness.
01-16-2014, 05:43 PM   #7
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QuoteOriginally posted by southlander Quote
@ GTS: I was going to ask if SR was off, but your later post indicated it was. SR is not designed for the sort of vibration frequencies that are created on a tripod and can create blur rather than eliminate it. Do you use the 2 sec delay function. I do when my cameras are on a tripod, this automatically turns off SR, it also allows any vibrations to settle before the actual exposure. You may be doing this with the remote control but your post does not mention it. The fact that images are sharp on liveview is a possible clue here - in Liveview the mirror is already up and would reduce/modify vibration patterns vs using the optical viewfinder. Re your quality control comments, AF is a very finely tuned beast - I'm impressed it works as well as it does for a consumer product given the micro-millimetre tolerances involved. Do you see this behavior on any other lenses, or is it confined to the FA31?

@OldNoob: I'm wondering if the liveview focusing is choosing a different focus point? I'd be looking very carefully at the distance scale focusing on the same scene using optical vf, then liveview. Is it settling in a slightly different place? Maybe use spot focus with the optical viewfinder, and then in liveview, move the little focus box to perfectly align with what you used to focus on with the optical viewfinder. That should eliminate subject selection differences as a possible cause. What happens if you manually focus in liveview? Also, is aperture being held constant between optical and liveview images? I ask only to the extent that metering differences might vary the aperture and therefore edge sharpness.
Thanks , ill check that out
01-16-2014, 06:20 PM   #8
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Just curious, are you letting the camera to select your focus point, or you select the focus point? Like other suggested, if you select the focus point and still not getting sharpness, then you may need to use fine-adjustment to correct it. I have the 5IIs and FA31, and I don't find any problems with mine.

01-28-2014, 12:55 PM   #9
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Just wanted to update. I sent my k5II with my AF Pentax 31 lens to Pentax tech support UK and received the following response:


"We have inspected your equipment. Please see update from the technician -

Auto focus check results -

Checking your K5ii body focus with your AF fine adjustment on.
The resulting images are front focusing images are not sharp with our master lens and your lenses.

Changing your K5ii body’s focus back to factory spec ( no adjustment on).
Checking your K5ii body with our master lenses. This has passed the checks on our focusing chart.
Also we have checked with your 31mm prime lens, the resulting focus is sharp.

Live view focus checking results -

All resulting images are fine.

We think that the focusing problem is due to the fine AF adjustment which has been set up."


Well, it looks like these guys are not interested in providing any serious support. I spent weeks checking my camera, befor sending it to them. And now they are telling me that it is fine with no adjustments whatsoever!!!? Well, I am a new in Pentax world, but I am not stupid and I am not a beginner in photography. Maybe it was a bad choice - Pentax!? Do not want to believe in that though.
01-28-2014, 01:14 PM   #10
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Have you got your camera back? If they bench tested the body with their master lens and and your fa31... and found both lenses needed to have AF adjust reset...

Please wait until you get your camera back... I had set one of my lenses up to a +7 when I first got my K20d... I was sure it needed it. Later on, I was struggling with that lens again and reset it to 0 to start a new focus chart test... It was spot on on the focus chart... when I reset to +7 the chart showed a front focus issue...

I have no idea why I needed the +7 with that lens at first... But I never had to adjust it again.

Give it a try...


PS the same lens ended up being spot on on my KX when I got it later that year...
01-28-2014, 01:51 PM   #11
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In my experience, the accuracy of PDAF has its limits, especially with wide open fast lenses. It's fast but it's not as accurate as CDAF (live view) will be. One easy thing to try when you get camera back:
1. with camera on tripod, auto focus on something big (wall) using LV, don't even need to take picture
2. without touching the lens, turn off live view and auto focus on the same thing using viewfinder. Observe if the focus ring moves at all.

Repeat a few times.

If the focus ring moves in the step 2, I usually take it as the indication there's significant front/back focus issue. If it doesn't move on most occasions, most likely any inaccuracies fall into margin of error for PDAF and there's not much you can do. You may still be able to fine tune it slightly if you do exact calibration, but that will only somehow increase percentage of perfectly sharp shots and it shouldn't need much correction anyway.
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