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04-05-2020, 07:14 PM   #1
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AF fine adjustment - clarification please.

I searched forums on explicit instruction on how one is suppose to do this to no avail for a first timer,. Everyone assumes there are no first timers. The K30 manual on page110 gives only the scantest info.

I understand that some prime lenses and other auto focus lenses unrecognized by the camera my require adjustment. Does this mean correcting a lens for infinity?

Is there a step by step example of a lens being fine adjusted so that I can see exactly what to do (and why)?

Marty


Last edited by marty-0750; 04-05-2020 at 07:22 PM.
04-05-2020, 07:30 PM   #2
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Have a look here: Fixing Front and Back Focus - Introduction - In-Depth Articles

And here: https://www.pentaxforums.com/forums/53-pentax-dslr-camera-articles/61385-aut...ent-hints.html
04-05-2020, 07:32 PM   #3
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AF adjustments apply to autofocus only. If the lens cannot manually be focused to infinity, that would need to be addressed by repairing the lens.

I'd only recommend applying adjustments if you are consistently experiencing front or back focus with a particular lens.

Here's the guide on how to do it. One thing to note is that for unrecognized lenses, only the global adjustment will apply:

Fixing Front and Back Focus - Introduction - In-Depth Articles

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04-06-2020, 10:36 AM   #4
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Typically autofocus fine adjustment is to correct slight front or back focusing that can happen between a particular camera / lens combination. It requires tedious and precise testing using perfectly aligned test targets and multiple tests. Unless you have an obvious and repeatable front or back focus situation autofocus fine tuning is not needed. And unless you do the process with exacting precision you can end up worse not better. If done casually the margin of error in the test can exceed the actual error making it nothing more than a guessing game.

The process is explained well in the links posted above but it is not something everyone needs to do.

04-07-2020, 03:50 AM   #5
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Do you need AF fine adjustment on manual lenses (or AF lenses used manual) when the hexagon focus indicator in the viewfinder is used as a confirmation?
04-07-2020, 07:19 AM   #6
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QuoteOriginally posted by Kobayashi.K Quote
Do you need AF fine adjustment on manual lenses (or AF lenses used manual) when the hexagon focus indicator in the viewfinder is used as a confirmation?
Yes, if the AF module in the camera is not accurately reading proper focus for a given lens, it will be incorrect when using autofocus and manual focus when using the hexagon focus indicator. Tuning a camera to correctly focus a given lens will give correct focus in both instances.

What isn't affected is focus on the actual focusing screen (that's when it's up to your eyeballs to decide if it's sharp or not), or when using Live View - which should theoretically provide accurate focus at all times, as you are creating a sharp image on the sensor itself.
04-07-2020, 07:45 AM - 1 Like   #7
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QuoteOriginally posted by Ontarian50 Quote
Yes, if the AF module in the camera is not accurately reading proper focus for a given lens, it will be incorrect when using autofocus and manual focus when using the hexagon focus indicator. Tuning a camera to correctly focus a given lens will give correct focus in both instances.

What isn't affected is focus on the actual focusing screen (that's when it's up to your eyeballs to decide if it's sharp or not), or when using Live View - which should theoretically provide accurate focus at all times, as you are creating a sharp image on the sensor itself.
Thanks. The difference between AF and Manual is that the former uses a feedback loop with the focus motor involved. In Manual the loop is cut open and only the phase detect part from the AF module is used, so that would potentially change its behaviour. I'm going to read the threads provided by @MarkJerling and @Adam to understand it fully.

04-07-2020, 06:03 PM   #8
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Thank you all for the explanations. It was mostly a curiousity question. Noticed a bit of a problem imagining planes in flight at high hand held zoom. Sometimes they are sharp and other times a bit out of focus. This may be the auto focus struggling with a wobbly target against a blank background sky.

I was also concerned about infinity focus on astro photography. Live view focus takes care of that on a sufficiently bright star. The results are crisp clean star trails. I also have a Rikenon prime f1.8 for astro The focus barrel stops dead at infinity and indeed the star trails here are once again crisp and clean. So I assume I have no issue with back/front focus.

Out of curiosity I might try the test on a ruler leaning back similar to the commercial setup.
04-07-2020, 06:31 PM   #9
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QuoteOriginally posted by Kobayashi.K Quote
Do you need AF fine adjustment on manual lenses (or AF lenses used manual) when the hexagon focus indicator in the viewfinder is used as a confirmation?
In general, yes. However, the Green Hexagon focus indicator may be illuminated across a range of focus settings, including front and back focus.

This article provides some insight into achieving sharp focus with manual lenses: Achieving Better Manual Focus with the Green Hexagon - PentaxForums.com

The article might be overly detailed in response to the query from @Marty-0750, but it includes a section that describes the repeatable calibration approach that I use.

- Craig
04-08-2020, 10:11 AM   #10
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QuoteOriginally posted by marty-0750 Quote
I was also concerned about infinity focus on astro photography.
Many lenses will focus beyond infinity. In most cases this is not an issue but for Astro it is. If you find a lens that focuses exactly at infinity at the stop, that's a keeper in my opinion. Many people prepare for an astro shoot by setting a lens at infinity during daylight then turning off AF and taping the focus ring with gaffer's tape to keep it from moving.
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