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10-01-2012, 01:24 PM   #1
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18-135mm: How to AF Fine Adjustment over the full zoom range?

Hello

Last week we bought the Pentax K-30 Kit that comes with the smc DA 18-135mm f/3.5-5.6 ED AL [IF] DC WR. This Sunday we had a first longer test run with it and we noticed out-of-focus problems despite good light conditions (it was around 2pm and sunny). So we printed Yvon Bourque's AF adjustment chart to use the AF fine adjustment of the K-30 to calibrate the lens and body. But it seems that we have a front focus effect at 18 mm and a back focus effect at 135 mm (have a look at https://picasaweb.google.com/102461996326133585052/LensFocusTest ). The AF fine adjustment has only one value so we can't use it as it seems that we need different values depending on the focal length of our 18-135 lens.

Did you experience something similar with your "zoom"-lenses and what did you do?

stefan

10-01-2012, 01:58 PM   #2
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The AF adjustment has only one setting so for zooms you must either set it for the middle focal length, average things out the best you can, or set it for the focal length you use the most.

I have no idea what your experience level is, so don't take this wrong but there are many, many other things to check before jumping into using the AF fine adjust. In the majority of cases the 'errors' are either user technique error or not running a properly setup test. The amount of change we are talking about here is very small and unless the tests are done very precisely the margin of error in the test itself exceeds the potential error from the hardware. If you are experienced with this type of test, sorry no offense and I'm sure you already understand.

If you have not done this before, it requires a sturdy tripod, precise alignment of the target both horizontally and vertically and repetitive testing to average any errors. Without doing it properly it is far too easy to do it wrong and just make things worse.
10-01-2012, 02:07 PM   #3
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QuoteOriginally posted by jatrax Quote
Without doing it properly it is far too easy to do it wrong and just make things worse.
Seconding this.

I'm not preuming anything about your level of camera experience, so please don't be offended, but one of the issues of camera technique to get right before you jump into AF adjust is to understand and work with depth of field to perhaps get better focus results, particularly as it applies to your lens. This depth of field table may be useful, as may this DOF tutorial. Another article worth looking at is this guide to understanding auto-focus

Last edited by rawr; 10-01-2012 at 02:17 PM.
10-02-2012, 02:38 PM   #4
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I've had the same issue with the notorious Tamron 17-50mm F2.8. I sent mine in for service and it was given to Tamron Japan for more extensive service work

Came back, still requires adjustment if I want precise focus at all focal lengths but at least its not FF by like 5-10cm regardless of camera settings like it was before.

If the 18-135 is new, part of the bundle I suppose you could try and get another one - thats reasonable if it really isn't focusing well.

Can you put up some test images?

If you can hit the focus mark at all lengths, but just not ideally distributed (between ff/bf of the focal point) then I would just live with it probably.

10-06-2012, 06:31 AM   #5
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Thanks a lot to everyone for your quick responses! We performed the measurements doing the following:
  • using a tripod
  • manual careful horizontal and vertical alignment towards the test sheet
  • verification of alignment using the K-30 electronic level
  • repeated the test photos at daylight and at night using artificial light
  • 2 sec. self-timer to avoid accidental movement
You can see the results and a photo of the setup at https://picasaweb.google.com/102461996326133585052/LensFocusTest?authuser=0&feat=directlink

But I think FruitLooPs is right, I was expecting too much and for lenses that have a "large" focal range I need to accepted to have a small FrontFocus and small BackFocus effects at the extreme focal range settings if I have only one value to calibrate the lens. We will test it further, also do more real live tests as the reason why we started to measure the focus was that we had problems during our first trip with the K-30 into nature (for example a snail at a tree was always out of focus, even when we manually told him to use only the middle focus point. Once we focus at the bark of the tree the snail was in focus...).

stefan
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