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02-19-2012, 08:22 AM   #1
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Help! 50-135 2.8 AF Adjustment

I recently purchased a 50-135mm 2.8, and I've been testing the focus accuracy on my K-7. It seems to focus reasonably accurately at 135mm, but at 50mm, it quite severely back-focuses. Is there any way to fix this?

Here are two sample shots. I added the SD card, just to make sure I knew where the camera was focusing. These are both shot on a tripod, no changes except the focal length of the lens.

135mm


55mm


As you can see, at 135mm, the camera focuses accurately enough, with the front edge of the card nice and sharp. At 55mm, the focus is way off, with the card blurry. Looks like the focus is around the 5, way behind what I wanted to focus on.

How do I fix this?


Last edited by arnie0674; 02-19-2012 at 11:25 AM.
02-19-2012, 12:49 PM   #2
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Has anyone experienced this? is this something fixable using the K-7 AF adjust?
02-19-2012, 12:57 PM   #3
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Take away the SD card and focus on the black line under it, i know you added the SD card to make it lear where it focu but actually it doesn't make it clear...

Anyway if you still have the same problem adjust the lens on the 135mm value and see how it reacts on 50mm.
It is btw not that strange that the lens focus differently
02-19-2012, 01:09 PM   #4
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QuoteOriginally posted by Anvh Quote
Take away the SD card and focus on the black line under it, i know you added the SD card to make it lear where it focu but actually it doesn't make it clear...

Anyway if you still have the same problem adjust the lens on the 135mm value and see how it reacts on 50mm.
It is btw not that strange that the lens focus differently
The focus was just as bad without the sd card, but I wasn't sure if the AF system was focusing on the black line or not. WIth the SD card, I'm pretty sure I know its attempting to focus on the card.

I've see others report on different focus adjustments at different focal lengths. But what can I do about it? It doesn't really need any adjustment at 135mm (maybe +1,) but at 50mm it is way way off.

02-19-2012, 04:02 PM   #5
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You don't. The card is a very larege object with very little contrast. The camera won't focus o the card itslef but perhaps on the edge between the card and paper - but then, you don't know which edge it will choose. Ipor if it will ignore the card entirely and find something on the paper to focus. By covering up the easy target that the chart was designed to give, you just made it impossible to know where the camera would try to focus.

It's easier to mess up a focus test than to conduct one properly.
02-19-2012, 04:21 PM   #6
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What Marc says, whit the first it actually seem to focus on the front edge of the card but in the second on the edge in the back.
02-19-2012, 05:17 PM   #7
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QuoteOriginally posted by Marc Sabatella Quote
You don't. The card is a very larege object with very little contrast. The camera won't focus o the card itslef but perhaps on the edge between the card and paper - but then, you don't know which edge it will choose. Ipor if it will ignore the card entirely and find something on the paper to focus. By covering up the easy target that the chart was designed to give, you just made it impossible to know where the camera would try to focus.

It's easier to mess up a focus test than to conduct one properly.
Yep, I'm starting to realize that.

I'm using the chart from here: Jeffrey Friedl's Blog , but the lens has to be pretty far away, so i have no idea what the camera is focusing on.

What would be the best way to test this?
02-24-2012, 08:36 PM   #8
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I have the exact same issue with the same lens I just received today. I posted over at dpreview. Hopefully someone can shed some light on the situation.

02-24-2012, 08:40 PM   #9
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QuoteOriginally posted by arnie0674 Quote
Yep, I'm starting to realize that.

I'm using the chart from here: Jeffrey Friedl's Blog , but the lens has to be pretty far away, so i have no idea what the camera is focusing on.

What would be the best way to test this?
Take away the sd card, that's all and do it in day light

PS. I just read the instruction on the site you post, you do know all these things are explained there...
02-24-2012, 09:36 PM   #10
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02-24-2012, 10:30 PM   #11
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Not useful as the lens correctly focuses at 135mm
02-24-2012, 11:37 PM   #12
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QuoteOriginally posted by kiwizz Quote
Not useful as the lens correctly focuses at 135mm
Send it to Pentax, they are the ones that can fix it.
02-25-2012, 04:53 AM   #13
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Looks like I will have to do just that. Haven't even had the lens a day and it needs to be sent off for repairs. Not much going for Pentax quality control these days.
02-25-2012, 05:43 AM   #14
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QuoteOriginally posted by kiwizz Quote
Looks like I will have to do just that. Haven't even had the lens a day and it needs to be sent off for repairs. Not much going for Pentax quality control these days.
Exchange it. I had to when I bought mine a few years ago. 2nd one is fine.
02-25-2012, 06:24 AM   #15
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Pentax Japan have told me they do not replace lenses. Sending it back to B&H would cost more.
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