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07-12-2011, 05:45 AM   #1
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Focus Adjustment

Hi all

well ive had my k-7 for some time now and have been a little frustrated with some lenses because i would often get out of focus (or very soft) images when i would have expected them to be sharp.

Many times i had mucked about with the micro focus adjustment but to be honest had come to the conclusion that it did nothing.......until today

Somewhere on here i read that someone had found that even after entering the adjustment value and saving it into the camera they had to turn the camera off and then on again before it had any effect - and blow me down i found the same thing.

So what to do next - well i have at this stage adjusted the focus on my lenses (at least 3 of them) using a method that i found on another site which uses live view and contrast detection to find the focus point then switch to 'normal' focus method and see if the focus point on the lens matches.......if it doesnt adjust the micro adjustment until it does (the assumtion is that the contrast detection method is very accurate when using a focus chart )

Well sure enough i found that the there was a difference in the two methods in particular for my sigma 18-125 lens. I had to set this to a +7 to get the two focus points aligning......

Well ive yet to go and do some shooting since doing this adjustment but i would be interested in hearing if others have done the same and found a noticable difference.

What i also found interesting was the my 50-135 lens didnt seem to be particularly effected by the mirco adjustment value - this didnt bother me too much as i have always found it sharp anyway but i was curious as to why it didnt seem to be affected

Adrian

07-12-2011, 06:25 AM   #2
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QuoteOriginally posted by milesy Quote
Hi all
Somewhere on here i read that someone had found that even after entering the adjustment value and saving it into the camera they had to turn the camera off and then on again before it had any effect - and blow me down i found the same thing.
This is not my experience. I've tested by making extreme changes to the per-lens adjustment, and the effects are definitely noticeable immediately.

QuoteOriginally posted by milesy Quote
So what to do next - well i have at this stage adjusted the focus on my lenses (at least 3 of them) using a method that i found on another site which uses live view and contrast detection to find the focus point then switch to 'normal' focus method and see if the focus point on the lens matches.......if it doesnt adjust the micro adjustment until it does (the assumtion is that the contrast detection method is very accurate when using a focus chart )
Is it this? Contrast Detect vs. Phase Detect Adjustment Method - What is the best way to micro-adjust a lens? - Photography - Stack Exchange

07-12-2011, 08:07 AM   #3
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What you're describing sounds like you are not controlling exactly where the camera is trying to focus well enough. If the camera is simply choosing the wrong target, then the small adjstments mde in camera aren't going to change that. It might help to post some of your tess.
07-12-2011, 12:51 PM   #4
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hi marc

well you could be right (about not controlling where the lens is focussing) however with the 50-135 it always hits the targert so i can only assume that im diong it right :-)

I'll go and play on the weekend and see if i think things have improved with the lenses in question

i can post a sample if your interested


Adrian

07-12-2011, 02:13 PM   #5
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just thinking - do you agree that the two focus methods focussed at the same focus chart from t he same tripod etc should end up with the same focus point on the same lens??
07-12-2011, 02:33 PM   #6
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QuoteOriginally posted by milesy Quote
just thinking - do you agree that the two focus methods focussed at the same focus chart from t he same tripod etc should end up with the same focus point on the same lens??
The live view AF is extremely accurate on my K-7 for all my lenses, which it had better be, considering how long it takes. In broad strokes the idea that you should try to match this Live View AF precision by making microadjustments to the regular phase-detect autofocus that is active when the mirror is down SOUNDS good, except as stated in that article there are some drawbacks.

The most serious one is that the focusing behavior of your lens will almost certainly vary based on distance to your subject. Most focus charts and whatnot want you to keep your lens wide open and at its minimum focus distance to the chart. You can spend an hour or more tweaking to your heart's content only to find that though you can now focus like a champ at 10 inches, you're suddenly way off at 3 feet. Again as the article says, it's a bit of a compromise - choose a distance you think is actually representative of where you'll be using the lens at and make the adjustments to correspond to that.

The sad reality is that, in the Pentax system at least, traditional phase-detect autofocus is not nearly as accurate and consistent across a range of conditions (distance to subject and lighting) as the contrast-detect methods used during Live View. When the mirror is up, as it is in Live View, the camera's imaging sensor is exposed and the onboard computers have way more (and more relevant) information to play with.

Some people maintain (in their heads) a list of AF Microadjustment values for their lenses to use in different settings. For instance a +7 outside on sunny days, but a +3 indoors with the same lens. This is just the way the cookie crumbles.

Fear not though -- one day mirrors will be gone altogether and the computers will be so much faster that you'll get that scary accurate focus with all your lenses every time right off the imaging sensor with contrast-detect or whatever replaces it. Assuming the sensor is looking at what you think it is.
07-12-2011, 05:33 PM   #7
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its enough to give you a headache.........

gut feeling tells me that if you are using the focus chart fixed on a tripod that both methods should focus to the same postion........

anyway i appreciate your replies and i will let you know if i feel what i have done has improved anything at all while i wait for the mirrorless camera that can use all my lenses!!
07-12-2011, 07:07 PM   #8
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Some good points made here in this thread.

QuoteOriginally posted by milesy Quote
gut feeling tells me that if you are using the focus chart fixed on a tripod that both methods should focus to the same postion........
Yes, provided you are using the chart as a flat target. If you use it in a tilted fashion, you need to make sure that there is no ambiguity as to where the contrast-based and the phase-based AF approaches are latching on.

I have compiled a list of AF adjustment hints which you may find useful.

Matt, I just included a link to your approach.

07-12-2011, 09:40 PM   #9
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QuoteOriginally posted by Class A Quote
Some good points made here in this thread.


Yes, provided you are using the chart as a flat target. If you use it in a tilted fashion, you need to make sure that there is no ambiguity as to where the contrast-based and the phase-based AF approaches are latching on.

I have compiled a list of AF adjustment hints which you may find useful.

Matt, I just included a link to your approach.
Class A, the thread you linked to here is incredibly useful, thank you! I still have some microadjustment decisions to make on my new 43Ltd later, so I think I will look more carefully at the tips and resources you have compiled.
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