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03-30-2011, 10:31 AM   #1
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is this back focus problem?

Dear fellow Pentax experts,

I suspect my camera system (pentax K-x and Tamron 17-50 F/2.8) is having BF problem, and seek your advise.

it gets worse as I get closer to F/2.8

attached 3 photos for your reference.

my questions are:
1. is this back focus?
2. is this a problem with the lens, or with the camera?
3. can k-x hidden menu fix this? or do i have to send the lens/camera in for repair?

thanks in advance


camera settings:
Aperture mode
center focus
pictures taken with both AF lock confirmed and Shake Reduction confirmed

Attached Images
View Picture EXIF
PENTAX K-x  Photo 
View Picture EXIF
PENTAX K-x  Photo 
View Picture EXIF
PENTAX K-x  Photo 
03-30-2011, 10:36 AM   #2
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for clarification:

picture 1, i focused on my dad, but notice lady walking in rear right side is in focus
picture 2, i focused on the boy's face, notice camera is focusing on another boy's pants in rear left
picture 3, i focused on my sister's face, but house across the street is in focus.

my hands were study, and my focus point is dead accurate, and i waited for both AF confirm and SR confirm.
03-30-2011, 10:45 AM   #3
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My suspicion is you actually are using continuous autofocus, and likely the central point. I took the liberty of finding the centre of one of the pics, and as you can see, it is actually in focus. It is backfocused from what you want, but it's likely not the cameras fault.

Make sure your autofocus is set to af.s and once you press halfway down, be sure to keep it halfway down before firing.

Attached Images
View Picture EXIF
PENTAX K-x  Photo 
03-30-2011, 11:57 AM   #4

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too bad the exif is not complete. Otherwise it is usually easy to determine the focus point used. If there is no user error, try the lens with oher camera and other lens on your camera. My personal bet is that the Tammy is misbehaving, it is usually the weakest link.

03-30-2011, 04:05 PM   #5
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If you're using AF.A isn't the camera still guessing what you're trying to focus on? I'd say switch to AF.S, center point, and try some more shots. Inspect your results from there or post them here.
03-30-2011, 10:43 PM   #6
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Set the lens at 50mm and f2.8, AF.S, center point and take a lot of test shots of stationary objects at different distances. With that wide aperture, you should be able to see the point of focus much better. Shutter speed will be really high so camera shake is probably not a factor. Stationary subjects won't introduce motion blur.
04-04-2011, 11:40 AM   #7
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went to a wedding over the weekend, took some pix with AF.S and noticed the following:

at 50mm, pictures seems to be in focus, at 17mm severe back focus problem

attached comparison pix for your reference

note: i focused on the back of the person in the middle in both pictures
Attached Images
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PENTAX K-x  Photo 
View Picture EXIF
PENTAX K-x  Photo 
04-04-2011, 11:42 AM   #8
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are the experts able to determine if this is a problem with the lens or the camera body? i want to contact the appropriate support (Tamron or Pentax) to fix this problem, thanks.

04-06-2011, 10:22 AM   #9
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Have you tried your K-x with any other lens? I suggest shooting a focus chart.
Shooting a brick wall is also an effective way to test focus accuracy.

Based on your picture, I think the lens is the problem, but more tests should be done before drawing any conclusion.
04-06-2011, 11:23 AM - 2 Likes   #10

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Hi Macky,

I think you're experiencing a common situation with Pentax users. I'm assuming you're using the center focus point.

The center focus sensor area ("point") is actually about the size of of the ( ) brackets on your focusing screen, not the red square LED that lights up to indicate focus point. All AF sensors look for contrast borders within the focus sensor area. In many cases, it will pick the wrong border, because the contrast is stronger or the border aligns more to the vertical or horizontal planes.

Superimpose the ( ) brackets on each of your shots posted, and I think you'll be able to see an element within the area that would possibly cause an AF error. Realize that the center focus sensor is shaped like a + that fills the area of the ( ) brackets. It detects contrast borders that intersect the lines of the cross -- anywhere within this area. The closer this contrast border is to either vertical or horizontal, or the greater the contrast, the more likely the sensor will choose this as the thing to lock focus on. BTW, wide angle lenses could be more prone to these focus errors by the camera since the wider FOV probably includes more elements that might confuse the AF sensor.

#1 The stairs in the background -- there are both vertical and horizontal contrast borders that the camera might have mistakenly assumed to be the focus plane.

#2 The strongest contrast border in the focus sensor area is probably the inside of the thigh of the woman holding the child up.

#3 I'd say either the white fence or the intersection of the tree trunk on the white car between the couple is the culprit.

#4 The wedding party in the background is the highest contrast border in this one

#5 No problem -- either the highlights in the hair or the man's shoulder against the white background allowed the AF sensor to lock at close to the right distance.

Your job as the photographer is to understand the actual area of the AF sensor, try to make sure that part of your subject is the dominant contrast element in this area, and override the AF sensor when it makes a mistake -- assume it will make mistakes since it cannot know your intentions.

I shoot birds and wildlife, with lenses that have very narrow Depth of Field (DOF), so with them hiding behind tall grass and perching among a lot of branches, I have to make sure that the AF system picks the right thing to lock focus on. I make small adjustments in composing the scene, and sometimes have to make a couple of tries of the AF until I can see in the VF that the subject I want is what is actually in focus. At this point, I hold the AF lock, recompose the scene, then take the shot. the trick is to focus your attention on the ( ) brackets, make sure that the intended subject is in focus, lock, then open your attention to the entire scene to compose, then take the shot.

I hope I explained this adequately.

Here are a few examples -- not people shots, but you can see how they would cause problems. With the deer, the camera wanted to choose the grass. I had to make the top of the doe's head the dominant contrast border, then recompose the shot to include her whole head. With the bird, the camera wanted to focus on the intervening branches, so I had to move the VF so the main contrast border in the AF sensor area was the one between the bird's head and the background.

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PENTAX K-7  Photo   

Last edited by snostorm; 04-06-2011 at 11:31 AM.
04-12-2011, 08:30 PM   #11

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QuoteOriginally posted by macky112 Quote
are the experts able to determine if this is a problem with the lens or the camera body? i want to contact the appropriate support (Tamron or Pentax) to fix this problem, thanks.
You should photograph a stationary object, preferably with the camera on a tripod. I'd suggest something flat with texture (brick wall, wood fence) or a sign with clear lettering that mostly fills the frame. Doing this will also help show any other issues the lens might have (decentering, etc.) I'm surprised you didn't perform that kind of testing immediately after buying such an expensive lens.

Just out of curiosity, I'm wondering if you are experiencing any other anomalies (such as exposure inconsistencies) which seem to be reported by some 17-50mm owners?


Last edited by tibbitts; 04-12-2011 at 08:37 PM.
05-27-2011, 11:16 AM   #12
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Hi All,

Just wanted to provide some feedback regarding Tamron QC and customer service.

1. I emailed same pix on this thread to Tamron, and they were quick to acknowledge the back focus was caused by the lens, but I have to pay shipping to them to have it serviced under warranty

2. Tamron acknowledged receipt of the lens, and told me via email the turn around would be around 2 weeks, but call to make sure. (and I didnt call)

3. I waited a month and then contacted Tamron, but was told that the lens needed special services and was sent to Japan

4. I received the lens after another 3 weeks, BUT...

5. I was unable to verify if the back focus problem was fixed or not because lens now cause camera to display MF (as if I enabled manual focus on camera body)

6. i emailed Tamron about this, but this time they questioned if it may be the k-x body causing this. so i went to Costco and bought a K-r 2 lens kit just to cross verify. (I feel Tamron was being unreasonable here, I told them I did not touch the k-x for about 2 months because i didn't have any other lenses, but they still questioned the k-x)

7. both kit lenses worked fine on my k-x, and Tamron caused MF on the k-r, 'nough said =P

8. Tamron emailed me a shipping label

9. after they received and verified the lens to be defective, they are now sending me a brand new lens.

10. rewind back to Aug 2010, I already did an exchange with B&H when i first bought the Tamron due to image softness.

11. so I guess I am back at square 1 (step 10?) with a new round of Tamron QC/customer service coming?

I am frustrated because I was not only out of lens for about 2 months, but I had to go out to get a k-r kit to convince them that after their "special service in Japan" they completely broke the lens!

By going with the Tamron, it caused me a lot of time and inconvenience, just thought I'd point that out.

05-27-2011, 12:28 PM   #13
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Thats too bad, my 17-50 seemed to be a pretty good copy on my K-r.
05-27-2011, 06:23 PM   #14
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FF or BF means focus is off by a matter of a few millimeters here or there from where the camera tried to focus. What you are seeing is something entirely different: the camera simply chose to focus somewhere other than you intended, because the focus "points" are not point at all, but rather broad areas. You might aim for the head of a person, but the background is within range of the focus sensor too, and sometimes, the camera will focus there (like if the foreground subject is moving, or lower contrast, or there are no clear edges). Sorry it took so lng to get this figured out, but there is no way there was ever a camera or lens problem here.

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