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05-24-2021, 09:14 AM   #1
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Problem with focusing with my K70 and DA18-135

About five days ago I started to have a focusing problem with my K70. I found that on auto focus I sometimes got a double image, i.e. on checking the images I found overlapping images of the last two shots. It was intermittent but if happened too much to be an odd glitch. Iím not sure whether I could also get double images if I had been on the manual focus mode since I donít normally use manual. Also, on one occasion, I was successfully focused on an object but, as I zoomed, the image went out of focus and the lens refocused when I depressed the shutter button halfway. I also found that most of my images were not pin sharp as they normally should be.

I am going to check to see if I get consistent behavior when I use manual focusing. Iím on vacation for the next three weeks and cannot go to my backup K50.

Do you think the camera or lens is at fault? Any advice would be very welcome.

---------- Post added 05-24-21 at 01:03 PM ----------

Hi folks, Iím embarrassed to say that I discovered part of my problem. I somehow had set the camera to multiple, instead of, single exposure. How it happened is a mystery since I seldom fiddle with that aspect of my camera. However, I still had the problem of loss of focus when I zoomed my lens although that seems to be a rarer problem. I also donít seem to get fast focusing on many images even though they seem to be easy-to-focusi images. I hate to have a malfunctioning camera while away from home on vacation.

05-24-2021, 10:40 AM   #2
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QuoteOriginally posted by psoo Quote
somehow had set the camera to multiple, instead of, single exposure
Was going to suggest that.
QuoteOriginally posted by psoo Quote
the problem of loss of focus when I zoomed my lens although that seems to be a rarer problem.
I'm not sure on the 18-135 but many (most?) lenses will change focus when zoomed I think the term is varifocal. You have to have a parfocal lens for the focus to stay during a zoom.

---------- Post added 05-24-21 at 10:41 AM ----------

This might help:
Parfocal vs Varifocal Zoom: What is the Difference and Why Does it Matter? | PetaPixel
05-24-2021, 11:41 AM   #3
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Try focusing in Live View to see if you get substantially different results. Live View uses the image sensor directly for focusing instead of the separate focus sensor used for OVF. If photos taken with Live View seem sharper, then you might need to calibrate your lens using the AF fine tune. I also agree with jatrax - the 18-135mm, like most zoom lenses, does not stay in focus when changing focal length. I just did a test to verify with my K-70 and 18-135mm lens.

I think that is why cinematic zoom lenses are so expensive - they are designed to stay in focus through the zoom range.
05-24-2021, 12:00 PM   #4
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The 18-135 is not a parfocal lens. This means that the focus does change when you zoom. It actually changes quite a lot in this lens. This is completely normal behavior for that lens. You need to choose zoom first, then focus without modifying the zoom at all.

If your images aren't as sharp as you expect, check for front/back focus.

05-24-2021, 03:59 PM   #5
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Thanks to you all for the expert advice you gave. It seems clear that my K70 is a lot smarter than its user. I’m delighted that my Pentax is behaving normally and hasn’t let me down.
05-25-2021, 05:02 AM   #6
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You might try a blower with the lens off and mirror up. The PDAF af sensor under the main mirror Might have a bit of dust which could slow down and negatively impact focus. Blowing out off could improve focus accuracy and speed.
05-25-2021, 05:21 AM   #7
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QuoteOriginally posted by UncleVanya Quote
You might try a blower with the lens off and mirror up. The PDAF af sensor under the main mirror Might have a bit of dust which could slow down and negatively impact focus. Blowing out off could improve focus accuracy and speed.
Thanks UV, I didnít know that dust could affect focusing. I seldom use a blower to remove dust, so Iíll give it a try.

05-25-2021, 09:04 AM   #8
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QuoteOriginally posted by psoo Quote
Thanks UV, I didnít know that dust could affect focusing. I seldom use a blower to remove dust, so Iíll give it a try.
Iíve read other user reports of a single hair getting in that area wreaking havoc on focus accuracy. It typically impacts all lenses but from reading what you wrote Iím assuming you have only one lens with you.
12-26-2021, 09:24 PM   #9
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Last edited by jie; 12-30-2021 at 01:02 PM.
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