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Thread: K5 Auto Focus - Some questions

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    Senior Member Jim Radcliffe's Avatar
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    K5 Auto Focus - Some questions

    I have noticed that many times the focus of my K5 is more accurate when using Live View than it is when I use the normal, through the viewfinder method. This is bothersome to me. I really don't wish to use Live View that much but I do appreciate it on certain occasions.

    Can someone explain the differences between the LV and non-LV AF and possibly explain why one would be more accurate than the other? Shouldn't both modes be equally accurate?

    Live View certainly has its place and reasons for use but I really prefer looking through the viewfinder. I know the AF area is larger than what is depcited in the viewfinder by the little red/green box (I use center focus point most of the time).

    I also often like to shoot wide open and when using fast glass (FA 31, 43, 77) I often find the AF not to be as sharp (accurate) as it should be.. and I do understand that shooting wide open usually results in a "soft" image due to critical focus issues... and that is where I am bothered by the AF of the K5. Sometime it really is spot on but often it just does not happen.

    Yes, I know I could stop down a bit but we really don't buy fast glass only to stop it down, do we?

    Just want to pick your minds regarding the AF issues.. and yes, I have tried making micro-AF adjustments... it's still a mixed bag.

    Your thoughts, please.
    Jim Radcliffe
    www.boxedlight.com

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    Re: K5 Auto Focus - Some questions

    Isn't live view using contrast detect auto focus (CDAF) vs. the regular phase detect AF.

    This is why the AF of the mirrorless cameras is so accurate and why the higher end dslrs offer lens fine tuning.

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    Senior Member Jim Radcliffe's Avatar
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    Re: K5 Auto Focus - Some questions

    Well, I have fine tuning on the K5 but sometimes it just seems not to matter.. I have fine tuned using charts and correct procedure and nailed the focus on the charts but then the real world has to enter the picture (pardon the pun) and things still seem to be a bit off at times.
    Jim Radcliffe
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    Re: K5 Auto Focus - Some questions

    I've owned two K5s, the first one I was never able to fine tune the lenses I have (35mm Macro, FA77 and FA43) but strangely enough I traded it for a silver Limited body and have never needed to fine tune any of those lenses. It' always spot on. On the other hand, LV is rarely successful and I can usually tell before I even click the shutter that the focus is off even though its confirmed.

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    Re: K5 Auto Focus - Some questions

    Quote Originally Posted by Terry View Post
    Isn't live view using contrast detect auto focus (CDAF) vs. the regular phase detect AF.
    The K-5 manual offers a choice of autofocus method for Live View: contrast AF or phase difference AF. (English version, p. 166) So one could compare Live View with phase difference AF versus viewfinder with same. If viewfinder has a focus problem while Live View also on PDAF does not have a problem, Jim has probably found a K-5 design issue. If Live View on contrast AF works better than phase difference, the situation is known.

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    Re: K5 Auto Focus - Some questions

    First my general statement: Autofocus is the K-5's achilles heel.

    Quote Originally Posted by Jim Radcliffe View Post
    I have noticed that many times the focus of my K5 is more accurate when using Live View than it is when I use the normal, through the viewfinder method. This is bothersome to me. I really don't wish to use Live View that much but I do appreciate it on certain occasions.
    Live view is going to be more accurate for being contrast auto focus- unless light is weird.

    Can someone explain the differences between the LV and non-LV AF and possibly explain why one would be more accurate than the other? Shouldn't both modes be equally accurate?
    No. This was seen with micro four thirds. With Phase detection another sensor is used and you need to tell the lens how far to travel/where. I forgot the details now but you can get into back focus situations and other discrepancies for using two sensors. With contrast autofocus you are using the sensor that will capture the image and the lens will be moved back and forth until it locks that focus (hence it was traditionally slower but Panasonic/Olympus broke through that for autofocus of still objects).

    Live View certainly has its place and reasons for use but I really prefer looking through the viewfinder. I know the AF area is larger than what is depcited in the viewfinder by the little red/green box (I use center focus point most of the time).
    The main issue seems to be that the focus/sensor area is bigger than it should be and anything that has details around the point where you want to focus, the AF system will pick it up. I have found that at some point, when shooting people, the camera will start picking up from the background.

    Yes, this is frustrating and is the #1 achilles heel of this camera. It has forced me to learn manual focusing which I was stressed by my first teacher to try to always do- because camera focus systems do fail and it's good to be in control. Of course, with the film camera he uses is really a piece of cake- with dual micro prism images or dual images on a range finder. But with the DSLR's we have, it's not too easy unless you get a nice uber view finder like the one on the Sony A900.

    I find the K-5 is just enough to do this to some degree. Like you I find there are situations that are pretty hard to do AF for a camera, and the K-5 nails it. Yet there are some that one would think would be trivial and the K-5 has trouble with it. This unpredictability is horrible. I have found some predictability by making sure the subject I am focusing on is relatively big compared to the AF point area.

    I also often like to shoot wide open and when using fast glass (FA 31, 43, 77) I often find the AF not to be as sharp (accurate) as it should be.. and I do understand that shooting wide open usually results in a "soft" image due to critical focus issues... and that is where I am bothered by the AF of the K5. Sometime it really is spot on but often it just does not happen.
    Shooting wide open as far as I know, will stress the AF of any phase detection camera. You will see there are specs on DSLR's like CAnon and Sony that specify the center point is a "cross sensor and optimized to pick up to F2.8" So if you are using F1.4 good luck (from what I understand). Of course the shallower DOF will make this even more critical.

    Yes, I know I could stop down a bit but we really don't buy fast glass only to stop it down, do we?
    I mostly agree with you but that's pretty much the advice I have heard from two pros as far as wedding shooting goes.

    Just want to pick your minds regarding the AF issues.. and yes, I have tried making micro-AF adjustments... it's still a mixed bag.

    Your thoughts, please.
    My thoughts given, this is a K-5 issue. But some of what you said will be an issue of any phase detection camera. The best phase detection AF as far as I understand, read and seen in the market is Nikon's.

    - Raist

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    Re: K5 Auto Focus - Some questions

    My experience with 2 different K5 bodies was that I could not get consistent AF.
    Others had better experience, but for me I jsut could not get it under control and often got images slightly OOF. (However I had the same experience with a Canon 7d and fast glass some month before).
    My D700 doesnt have this problem and my S2 doesnt have the problem, the A900 also seems to work fine in this regard and also the D300 I had some years ago were fine.
    So my opinion: AF accurancy is the weakest point of the K5, the K5 is not alone in this regard, but it is not a general problem valid for all DSLRs.
    However the larger the viewfinder the smaller the AF sensors get in relation to the full view, plus with the larger viewfinder of full frame cameras you also can rely more on what you see with your eyes and see focus errors during shooting+manually correct.

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    Re: K5 Auto Focus - Some questions

    Jim,

    there is an applet http://graphics.stanford.edu/courses...tofocusPD.html which shows the PD-AF function.
    The outer rays going through the lens are used for PD-AF but Pentax uses about f:4 as the outer border and not that of a faster lens. In my opinion this is why the K-5 is semi-pro. I've read that Canon uses two PD-AF systems in it's top level bodies. One for slow and one for fast lenses. Falk Lumo knows much more about it.

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    Senior Member Jim Radcliffe's Avatar
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    Re: K5 Auto Focus - Some questions

    Quote Originally Posted by Kerusker View Post
    Jim,

    there is an applet http://graphics.stanford.edu/courses...tofocusPD.html which shows the PD-AF function.
    The outer rays going through the lens are used for PD-AF but Pentax uses about f:4 as the outer border and not that of a faster lens. In my opinion this is why the K-5 is semi-pro. I've read that Canon uses two PD-AF systems in it's top level bodies. One for slow and one for fast lenses. Falk Lumo knows much more about it.
    Thanks for that link.. will read when I get home from work today.
    Jim Radcliffe
    www.boxedlight.com

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    K5 Auto Focus - Workaround?

    Quote Originally Posted by Kerusker View Post
    ...Pentax uses about f:4 as the outer border and not that of a faster lens.
    If I understand your point, it implies a workaround: lock focus while stopped down to f/4, then open to desired faster aperture to take the shot. Don't have a K-5 (yet!), so don't know how practical that procedure would be.

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    Senior Member ecsh's Avatar
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    Re: K5 Auto Focus - Some questions

    I Tried this using F4 then focusing, then changed to F3.0 and it does change for you.
    Of course, if you move then refocus, its all for not.
    Joe

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