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09-17-2014, 07:03 AM   #1
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Not focusing on far away subjects

Hi all,

I have a Pentax K-x with a DA-L 18-55 and DA 55-300. Recently I have been having some issues with focusing with the 18-55 only. The issue usually happens when I try to take photos of landscapes, such as mountains. I have noticed it the most at 18mm.

I look through the viewfinder, hold the shutter half way down to obtain focus lock but the little light indicating a lock will just flash on and off. Sometimes I need to pan around for about 5 seconds, and I have missed a few shots because of this. In order to gain focus, I need to pan around a little bit of the same mountain in order to get a lock.

Focusing on closer objects (<20m at a guess) is no issue, focusing at longer focal lengths is no issue.

My 55-300 is working fine. Conditions are clear, and there are no reflections etc.

Is this normal? I don't recall this happening before (I don't use my camera all that much)


Last edited by maxwolfie; 09-17-2014 at 08:07 AM.
09-17-2014, 08:16 AM   #2
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Well I'm not sure what could be causing that, but just manual focus on infinity subjects if your lens can't get focus.
09-17-2014, 08:20 AM   #3
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OP - to be clear, you are finding it difficult to lock focus at 18mm on the DA 18-55 on very distant targets, but only in that specific scenario, correct? Your 55-300 is fine, and locking focus at 55mm on the 18-55 at infinity targets is also fine, correct?

If so, this is not abnormal - when you are shooting at 18mm, the details of objects really far away become very small, and the autofocus sensor may not be precise enough sometimes to perform it's job. An advice I could give is to find a contrasty subject (like the horizon line, or a big building) to center your focus on. Alternatively, you can zoom in to 55mm and focus on that distant object, then zoom out. While I'm not sure if the 18-55 is parfocal, the DOF at 18mm should be deep enough to keep that subject in focus.
09-17-2014, 08:37 AM   #4
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Try setting it to f/8 and focus on the edge of the mountain or the edge of the horizon.

09-17-2014, 08:45 AM   #5
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Thanks - is the auto focus sensor in the lens or body?

Would upgrading either fix the issue?
09-17-2014, 08:49 AM   #6
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QuoteOriginally posted by maxwolfie Quote
Thanks - is the auto focus sensor in the lens or body?

Would upgrading either fix the issue?
The autofocus sensor is in the body (stupid me said lens). Upgrading to a k-50/k-5II/k-3 may improve the chances of it locking focus on a super distant object at 18mm, but I won't guarantee it. I haven't shot wider than 24mm in a while lol.

It's just the way things work - the AF sensor needs to take incoming light and split it to determine the phase difference to get the focus correct. If the incoming light isn't contrasty enough, the sensor cannot do its job. So a super distant object will obviously lack contrast, or just be too small for the sensor to figure out what it is looking at. This is going to be true for all DSLRs at very wide angles. The solution often-times is to find a contrasty object like the horizon, or where a building cuts into the sky, or mountain, etc. Some feature that will help the camera find contrast.

Last edited by JinDesu; 09-17-2014 at 10:09 AM.
09-17-2014, 09:04 AM   #7
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QuoteOriginally posted by JinDesu Quote
The autofocus sensor is in the lens.
Huh???

The AF sensor is in the body.

It was not clear from the original post which light is blinking (red focus point indicator or green (hexagon) focus attained), either way w What the camera is telling you is that it cannot find an element of the frame on which to lock focus. Usually this is caused by lack of contrast in the subject (compare to attempting focus on a blank wall). This behavior is more prevalent at shorter focal lengths and with lenses that have inherently somewhat lower contrast. I have never had this problem with the 18-55 kit lens, but can imagine that AF might fail under hazy or foggy conditions.


Steve

Last edited by stevebrot; 09-17-2014 at 09:43 AM. Reason: Forgot that the K-x does not have focus point indicators
09-17-2014, 10:09 AM   #8
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QuoteOriginally posted by stevebrot Quote
Huh???

The AF sensor is in the body.

It was not clear from the original post which light is blinking (red focus point indicator or green (hexagon) focus attained), either way w What the camera is telling you is that it cannot find an element of the frame on which to lock focus. Usually this is caused by lack of contrast in the subject (compare to attempting focus on a blank wall). This behavior is more prevalent at shorter focal lengths and with lenses that have inherently somewhat lower contrast. I have never had this problem with the 18-55 kit lens, but can imagine that AF might fail under hazy or foggy conditions.


Steve
Woops, huge mistake there haha. Corrected it, was rushing away from the computer.

09-17-2014, 10:24 AM   #9
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Instead of trying to focus on a distant infinity object try calculating the lens hyperfocal distance and focus using that distance.
09-17-2014, 01:02 PM   #10
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Thanks - I am still a bit concerned that something may not be working as intended, solely because I have never had (or at least noticed) this problem before.

Is there any "test" to check?
09-17-2014, 01:08 PM   #11
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QuoteOriginally posted by maxwolfie Quote
Thanks - I am still a bit concerned that something may not be working as intended, solely because I have never had (or at least noticed) this problem before.

Is there any "test" to check?
Try focusing on clouds, easily available infinity subjects
09-17-2014, 01:16 PM   #12
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QuoteOriginally posted by maxwolfie Quote
Thanks - I am still a bit concerned that something may not be working as intended, solely because I have never had (or at least noticed) this problem before.

Is there any "test" to check?
Are you trying to check if the lens/camera will definitely focus on an infinity object? Try looking for a very large building (for example, where I am I have the Empire State Building) against a blue sky.
09-17-2014, 02:46 PM   #13
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QuoteOriginally posted by JinDesu Quote
Are you trying to check if the lens/camera will definitely focus on an infinity object? Try looking for a very large building (for example, where I am I have the Empire State Building) against a blue sky.
It does not even have to be that far away. A hundred meters is probably far enough when using the 18-55 kit.


Steve
09-22-2014, 03:06 PM   #14
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I have done more testing and I am convinced there is a problem.

I have had issues focusing on buildings only 200-400m away, on a clear day. Most of the time I still need to half depress the shutter and "look around" (runs small circles around the building) in order to get a focus lock. I do not recall having these issues before and I am sure I would if I was as it is blatantly obvious and really makes taking photos a PITA.

I guess the bottom line is, whether it's working as intended or not, it simply isn't working for me so I am going to need to upgrade either my body and lens or body only (assuming that the body is the issue here).

Last edited by maxwolfie; 09-22-2014 at 03:13 PM.
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