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05-04-2012, 07:40 AM   #1
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DA* 50-135 focus confusion

Hello everyone,

I realise there are plenty of threads about this already but I hope you can bear with me on my lengthy thread anyway, it will be much appreciated

It's not long ago I bought my first Pentax lens, the DA* 50-135 as you might have guessed. Was excited to get it since it go t good reviews, however, right from the box it did seem to back focus a lot in the beginning. Went out to put it to the test with a mountain bike race and much to my dismay, it could not even focus on the well lit side of a stationary tree, it constantly threw focus in the background, zoomed in or not...
I had a protective filter on which I took off hoping that it was the culprit, sadly it was not, so in a frantic panic attack, I went into AF fine adjustment and put the lens to +10, suddenly the pictures seemed to be correct in focus! I thought to myself that it was a bit high in compensation but whatever it takes, right? Attended a few races more, the occasional back focus picture did appear every now and then, still the lens seemed to be working, the back focus could after all be due to riders coming towards me and me not pressing the button just fast enough for the rider to still be within the focus area.

Then the other day the lens appeared to go haywire with consistent back focusing at +10 AF adjustment, even with a flash on, broke my heart a little. Behaved that way through out the entire day, relentlessly, shade and direct sunlight alike.

Today when I got up, I saw a crow down on the lawn from my apartment. Took several shots and again focus was not right, however this time it was front focusing, ended up with a +5 on the lens in order to have focus in the middle of the focus point, at 135mm f/2.8.

Later, after I got home from exercising, I thought for the hell of it I'd try taking a picture of the wooden model sitting on the window sill and again back focusing reared it's ugly head, as I could see the dirt on the window a lot more sharply than the model I focused on. Several tries at 135mm and 50mm with f/2.8.

My initial take on it is that focus seem to lay very differently depending on light amount and the light source type, like sunlight vs artificial light.

Is this normal behaviour for a DA* lens at f/2.8?
I keep comparing it to my Sigma 70-200 HSM APO EX and it doesn't seem to suffer from the same focusing issues, I don't use AF fine adjustment with it.

As I'm writing this I realise I should go back and check the aperture of all the good race pictures I took previously to see if they were generally higher than f/2.8, sorry for not having done this already, still I'm think focus should be more consistent even at f/2.8.
I have already had my K-5 body replaced once due to focusing issues so but I'm sure it can't be the body once again.

Any thoughts?

05-04-2012, 09:05 AM   #2
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I just got my 50-135 last week and for my first outing it was frequently back focused. After running through some tests, +6 seems to be a good setting for mine. Although at 50mm it is still very slightly BFing, but good enough; rest of the range seems pretty spot on.

What I discovered however is that, sometimes, my focusing issue is because K-5's center focusing area is much larger than I expect it to be. It can pick a focus at any point within the entire center marker in the OVF, instead of an area that is only slightly larger than the center red dot. So if I happen to point at something that has a hole within the focusing area, or at edge of an object, the camera can decide to pick a focus point that's outside of my intended object. I wish the debug mode would allow me to shrink the area but it only seems to allow me to enlarge it... I would play around with it to see if you are getting similar results.
05-04-2012, 09:12 AM   #3
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If the hardware is ok, the 2 standard explanations would be: 1) the camera uses other focus points than you think, 2) it takes the picture before the AF has finished its work.

I would try to take pictures (fully open) where everything is at the same distance, for a range of distances. Not from too near, this is not a flat-field lens.
You should try this in different ways:
- Using AF only
- MF using the green hexagon
- MF using the screen
- MF using LV
And, of course, with a tripod and SR switched off.

With your description, it's a mystery. As Pentax DSLRs verify AF after focusing to the first calculation (making AF a bit slower than some other brands), a loose mechanism inside the lens should not result in front or back focus, but in hunting forever.
05-04-2012, 10:11 AM   #4
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QuoteOriginally posted by ruggiex Quote
What I discovered however is that, sometimes, my focusing issue is because K-5's center focusing area is much larger than I expect it to be. It can pick a focus at any point within the entire center marker in the OVF, instead of an area that is only slightly larger than the center red dot. So if I happen to point at something that has a hole within the focusing area, or at edge of an object, the camera can decide to pick a focus point that's outside of my intended object. I wish the debug mode would allow me to shrink the area but it only seems to allow me to enlarge it... I would play around with it to see if you are getting similar results.
I know the AF points are pretty big, so I have also considered if it is just the K-5 focusing on something else.

This picture is an example, if I remember correctly, it's 58mm focal and f/3.5. If you look at the picture,
you can see the tree behind the rider is sharp and she is a bit blurred: All sizes | IMGP1253 | Flickr - Photo Sharing!

05-04-2012, 12:46 PM   #5
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Just got my DA* 50-135 back yesterday evening from C.H.R.I.S. (2nd SDM failure...this one cost me $225). and it is perfect. Took it out this morning for a trial, shot Otis.....straight from the K5 with only cropping and resizing. This is how yours should work, fast and accurate, it is how mine has always worked ...excepting the SDM failures. I can't diagnose your problem, but it doesn't sound right to me. Mine is dead on with no adjustment, and no hesitation.

1/800 f2.8 123mm ISO 3200 Otis
[IMG] [/IMG]

I plan to use it to shoot a wedding on the 13th, it is near prime quality...... with the ability to avoid changing lenses for most shots. A stellar lens, yours should be too...have it checked out, it is too good a lens not to enjoy.
Best Regards!
05-05-2012, 01:50 AM   #6
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I have the opposite problem to the OP. My 50-135 is as near as dammit to perfect , but the sigma 70-200 I bought yesterday is so far FRONT focussed that even a -10 adjustment cannot correct it. I have spent most of the day testing and fine tuning all my 'good' lenses and the Sigma was the only one that was unable to focus correctly after adjustment. For what its worth the 50-135 is my favourite zoom of all time (closely followed by the DA* 60-250)

By all means check your lens, do some tests on a tripod, make sure that the AF mechanism is focussed on what YOU decide to focus it on, and if it is still unable to be corrected, return it. Thats what I'll be doing with the Sigma.
05-06-2012, 02:23 AM   #7
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to the OP

i seriously can undestand your frustration.After spending a lot of money and seeing all those great pictures taken from this lens by others and then to find out that its not focusing right must be really heart breaking.well i have the same problem i bought one few weeks back here in japan that had back focus issues .I took it to the service center they said it was due to the body and so they corrected it and at that stage it seemed fine. but yesterday i took it out for a real life shooting and must say even though the pics are ok they are not great i still feel there is still a focusing problem in my camera lens combo specially when you are focusing at mid to long distances.

in your case also may be the problem lies on the body. so why not look for a friends k5 or kr and check it out.
and also try seeing whether you can focus correctly at short to mid range focus distances. hope this helps
05-06-2012, 11:39 AM   #8
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QuoteOriginally posted by mikknu Quote
Went out to put it to the test with a mountain bike race and much to my dismay, it could not even focus on the well lit side of a stationary tree, it constantly threw focus in the background, zoomed in or not...
no possible camera or lens defect would explain this. Errors in focus calibration are measure in millimetea, not meters. This is a simple matter of the camera having *chosen* to focus on the background for whatever reason - either because didn't pick an appropriate focus sensor or becuase ou understimaed the size/score of the selected sensor. There is no point even considering any other possibility here.

QuoteQuote:
My initial take on it is that focus seem to lay very differently depending on light amount and the light source type, like sunlight vs artificial light.
While light type also affects focus by a matter of millimeters, again, ou actual examples speak to a mich different effect - the camera simply not readi your mind about exactly where you wanted it to focus, so it chose something else within range of the selected focus sensor.

05-06-2012, 12:29 PM   #9
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Used the DA* 50-135 for a family wedding yesterday (bit of a gamble) but there were both shots in focus and out of focus, which for me could indicate that it could be the camera focusing different from what I am expecting. I find it a bit odd that there is no indication of the actual focus area size, which could help determining if that is the reason behind the back focusing.
If this is the case, it frustrates me that the K-5 can pick the background over an object that fills out, what appears to be, the entire focus point. Is there a chart out there somwhere with a more specific indication of the actual focus area?
05-06-2012, 12:38 PM   #10
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QuoteOriginally posted by mikknu Quote
Used the DA* 50-135 for a family wedding yesterday (bit of a gamble) but there were both shots in focus and out of focus, which for me could indicate that it could be the camera focusing different from what I am expecting. I find it a bit odd that there is no indication of the actual focus area size, which could help determining if that is the reason behind the back focusing.
If this is the case, it frustrates me that the K-5 can pick the background over an object that fills out, what appears to be, the entire focus point. Is there a chart out there somwhere with a more specific indication of the actual focus area?
The K-5 AF sensors are big. Supposedly, if you want to be sure of what you are focusing on with center point focus, look at the entire center circle (#2).

From:https://www.pentaxforums.com/forums/pentax-k-5-forum/177572-all-fuss-about-au...oint-size.html


From:https://www.pentaxforums.com/forums/pentax-dslr-discussion/181885-analysis-pe...provement.html
05-06-2012, 08:24 PM   #11
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QuoteOriginally posted by mikknu Quote
Used the DA* 50-135 for a family wedding yesterday (bit of a gamble) but there were both shots in focus and out of focus, which for me could indicate that it could be the camera focusing different from what I am expecting.
This is what is happening to me as well

QuoteOriginally posted by JinDesu Quote
The K-5 AF sensors are big. Supposedly, if you want to be sure of what you are focusing on with center point focus, look at the entire center circle (#2).
Is the other AF point are as big as the center AF. IF not its worth manually selecting a different focus point
and try to focus at different subjects to see whether they are in focus.
05-07-2012, 08:58 AM   #12
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QuoteOriginally posted by JinDesu Quote
The K-5 AF sensors are big. Supposedly, if you want to be sure of what you are focusing on with center point focus, look at the entire center circle (#2).
Thank you for the chart JinDesu, if it is correct, the K-5 does seem a bit inferior compared to the Nikon D7000 AF system. Will have to try and be more meticulous next time I'm out taking pictures.
05-07-2012, 12:00 PM   #13
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QuoteOriginally posted by mikknu Quote
Is there a chart out there somwhere with a more specific indication of the actual focus area?
it's easy to see for yourself. Put a black dot on a piece of whie paper, aim you camera at it, a d try to focus. If you succeed, then the dot is in range of a focus sensor. Try it with the dot at various offcenter positions and you'll have direct evidence of what is in range and what is not.
05-07-2012, 02:34 PM   #14
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QuoteOriginally posted by nirVaan Quote
This is what is happening to me as well


Is the other AF point are as big as the center AF. IF not its worth manually selecting a different focus point
and try to focus at different subjects to see whether they are in focus.
I believe they are as large. Also - it appears that the Pentax AF system likes to focus on the LARGER object, as opposed to the NEARER object that other AF systems will do.

Or it could be the other way around, someone please correct me
05-07-2012, 05:23 PM   #15
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It's not that predictable. The system usually picks the "easier" target, but that could mean higher contrast, harder edged, more stationary, etc.
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