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07-19-2010, 06:36 AM   #1
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K20D Focus Confirmation Issue

Hi All,

I've searched for relevant discussions and haven't found any, so I thought I'd impose on your collective wisdom.

I've been using a K20D for a little over a year, exclusively with manual focus lenses - mostly screwmount, some k-mount, and a few other odd mounts, via various adapters, bellows, etc.. It's not at all unusual for me to changes lenses several times in a day. I mention that because I'm wondering if it might be relevant to an issue that has manifested itself recently.

Up until a few weeks ago, I had good success using the focus confirmation system as a focusing aid. It wasn't always 100% accurate, of course, but it was usually very helpful. But recently, I've found that it seems to be almost invariably wrong - just "off" the proper focus distance. I've tried a bunch of different lenses, a bunch of different subjects and lighting conditions - indoors, outdoors, on and off a tripod. The result is always the same. I'm in the process of trying to figure out whether the confirmation signal is always front focusing or back focusing; if that's the case, I can at least use it as a rough indicator.

I've used the sensor cleaning routine several times, and I've updated to the latest firmware version.

Maybe I've become a little too dependent on the focus confirmation feature, but I'm having trouble doing without it. I wish my old internal biological focus-confirm system were as good as it used to be (or that the viewfinder image were as bright as that of my old Spotmatic!). I'm using the stock screen, so I may need to invest in a Katzeye or something similar.

Any thoughts on what might be responsible for this issue? Any recommendations by way of diagnosing the problem? (For example, should I attach the kit lens and verify that AF is working properly? I know the K20D allows focus adjustment for AF lenses - would that be useful here?) Do you reckon that the frequent lens changing might have something to do with this? Grasping at straws here, I guess, but I know that my personal shooting habits aren't necessarily typical, so I wonder whether they have something to do with the problem.

Thanks so much for your help!

Cheers,

Jon

07-19-2010, 08:27 AM   #2
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For what its worth, I change lenses and adaptors 5-10 times every day and after 18 months have not noticed any change in the focus indication accuracy. I have calibrated all my main lenses so they are spot on with a focus chart. Some required major help, others were ok out of the box.
07-19-2010, 08:31 AM   #3
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QuoteOriginally posted by Univer Quote
Hi All,

I've searched for relevant discussions and haven't found any, so I thought I'd impose on your collective wisdom.

I've been using a K20D for a little over a year, exclusively with manual focus lenses - mostly screwmount, some k-mount, and a few other odd mounts, via various adapters, bellows, etc.. It's not at all unusual for me to changes lenses several times in a day. I mention that because I'm wondering if it might be relevant to an issue that has manifested itself recently.

Up until a few weeks ago, I had good success using the focus confirmation system as a focusing aid. It wasn't always 100% accurate, of course, but it was usually very helpful. But recently, I've found that it seems to be almost invariably wrong - just "off" the proper focus distance. I've tried a bunch of different lenses, a bunch of different subjects and lighting conditions - indoors, outdoors, on and off a tripod. The result is always the same. I'm in the process of trying to figure out whether the confirmation signal is always front focusing or back focusing; if that's the case, I can at least use it as a rough indicator.

I've used the sensor cleaning routine several times, and I've updated to the latest firmware version.

Maybe I've become a little too dependent on the focus confirmation feature, but I'm having trouble doing without it. I wish my old internal biological focus-confirm system were as good as it used to be (or that the viewfinder image were as bright as that of my old Spotmatic!). I'm using the stock screen, so I may need to invest in a Katzeye or something similar.

Any thoughts on what might be responsible for this issue? Any recommendations by way of diagnosing the problem? (For example, should I attach the kit lens and verify that AF is working properly? I know the K20D allows focus adjustment for AF lenses - would that be useful here?) Do you reckon that the frequent lens changing might have something to do with this? Grasping at straws here, I guess, but I know that my personal shooting habits aren't necessarily typical, so I wonder whether they have something to do with the problem.

Thanks so much for your help!

Cheers,

Jon
I have the K20 also. I don't think changing the lenses has much to do with it, except that one or more of your lenses may have a severe front focus or back focus problem.

You might check your adjustment menu, and confirm that you haven't entered "all" changes that may be affecting your camera.

AF is not perfect, it can be fooled by things like windows or grass that is between you and the subject you are photographing. In addition, the AF search area is nowhere as small as the focus indicator spot on your optical viewfinder. You may think you are telling the camera exactly where to focus, but the camera is actually picking up a contrast difference somewhere nearby to your subject.

I don't use the focus indicator light that much, because i'm never quite sure if i stopped in time when i saw it, etc. If you are going to use it, i would think your success rate would go up if you used a smaller apertuer, i.e. larger DOF with the indicator light. perhaps whats changed for you is a current tendency to use larger apertures than what you did formerly. I'm just brainstorming here, you need to be the judge of it.

I would be somewhat lost without my Katzeye screen. my optician says i have the start of some cataracts, so i got the Katzeye with the "brite" treatment as a help. But to get back to your question - the Katzeye helps me monitor AF performance. And i have done AF lens adjustments in the field, based on what i've seen from the Katzeye. From what some experts have said, lenses can change their focusing performance from a close focus to a distant focus. Also, tungsten light indoors, because of its frequency, can cause the camera focus point to move towards a front focus condition. Now K7 was supposed to have a feature that would help conpensate for that - not sure how successful that effort was because i haven't seen any posts on it.

What i'm trying to say is there could be many reasons for this issue, and the Katzeye screen, i have found, is a great tool for helping to monitor the situation except in extreme low light. I first had this screen on my K10, then shifted it to my K20 when i upgraded. it will cause errors with your spot focus metering, but i have found that to be a small loss compared to the flexibility it gives to my focusing. I normally use AF where i can, but there are situations, such as some of my MF lenses, etc, where the focusing screen is invaluable. Also, not all focusing screens are the same. my brother had an inexpensive ebay screen, and now is very happy with the katzeye. I can't speak to all the different varieties in that market.

hope this helps,
07-19-2010, 09:18 AM   #4
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Biggest question in my mind is if you're aware how much bigger the actual AF sensor is than the red dot that appears in the viewfinder. So you might get confirmation that *something* is in focus, but that something is a little above, below, or to the side of the red dot. That's normal. Next question is whether you are aware the red dot does *not* confirm focus has been achieved; it merely confirms which focus point is being used. It's the green hexagon that confirms focus has been achieved. But realistically, while the red dot tends to jump the gun, the green hexagon tends to lag, so you do have to wait a moment to see if the hexagon stays on before firing.

It's a pretty reliable system *if* you are aware of these issues, but if you're thinking you can just fire as soon as the red dot comes on and that means that the object *directly* under red dot will be in focus, then that's not going to be as reliable.


Last edited by Marc Sabatella; 07-20-2010 at 09:02 AM.
07-19-2010, 09:48 AM   #5
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Marc's advice is good. The AF confirm is pretty reliable. It is not as good as a split image finder or even a good matte field, but is much better than the attempting to focus with the stock screen by itself.

The main issue with the AF system is that the actual point of focus on a receding plane is difficult to predict. As Marc says, the area evaluated for AF is pretty large. The classic example would be the cheek to nose surface of a profile portrait. Ideally you would want focus to be on the eye. Good luck on getting that fine a granularity with the AF sensor. It may give confirmation for the leading edge of the cheek, the nose, or any point in between. Not good when shooting at f/1.4.


Steve
07-19-2010, 11:22 AM   #6
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Hi,

Thanks, everyone, for the very helpful replies!

I always find, when I post a question, that the answers help me to see how I could have phrased it more clearly. I should have been clearer in saying that the focus confirmation I'm talking about is the green hexagon; so far as I can recall, I've never used this camera with an AF lens, so this is purely a problem of the green hexagon seeming to have lost its reliability (and I did find it quite reliable until this issue manifested itself).

Nice to hear that my lens-swapping isn't likely to have contributed to the problem. I didn't really think so, but one never knows.

@philbaum: I'm thinking that the Katzeye screen is almost certainly in my future. I thank you for pointing out benefits beyond the obvious ones.

I'll continue to monitor the situation, to see if any patterns develop: indoors vs. outdoors, wide-open vs. stopped-down.

Thanks again, all.

Cheers,

Jon
07-20-2010, 09:03 AM   #7
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Whether you're talking about the green hexagon or the red dot for *confirmation*, the fact remains that you are probably misjudging the size of the focus sensors. They're pretty large, and as stated, you will get confirmation if *anything* within that region is in focus.
07-22-2010, 06:53 AM   #8
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Thanks again, everyone, for the replies. Based in part on all this helpful information, I've decided to have the nice folks at KatzEye install an OptiBrite-treated screen for me.

I'm looking forward not only to the enhanced view, but also to the ability to use the split-prism as a diagnostic aid in performing the AF adjustments necessary to correct (or at least reduce) the inaccuracy of the focus-confirm indicator.

I do have one followup question, for folks who have performed those adjustments: what is the appropriate adjustment direction, plus or minus, to correct a front-focusing issue? I haven't been able to find that precise answer anywhere, despite several searches. I know what my own intuition would suggest, but I've learned, over the years, not to assume that interface designers think the way I do.

Thanks for your help!

Cheers,

Jon

07-22-2010, 05:53 PM   #9
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K20D Focus Confirmation Issue

If you do decide to get the KatzEye screen as I did for both a K20D and K-7, you will probably want to send your camera in to them for their installation service where they will calibrate the screen to the camera. This involves replacing the shim between the pentaprism and the focusing screen so that manual focusing is accurate at wide aperatures.
07-23-2010, 07:32 AM   #10
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@cpk: thank you! I thought the very same thing; the camera is on its way there as I write. I was perfectly comfortable with the notion of doing the installation myself, but I really wanted the benefit of the precise calibration.

Now my only concern (apart from struggling to be patient while the camera's out of my hands!) is the AF adjustment necessary to get the focus-confirmation a bit more accurate. Still trying to confirm that "minus" is the right direction for front-focusing...but I can determine that through trial-and-error, if all else fails.

Thanks!

Jon
07-24-2010, 07:49 AM   #11
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I am in Canada where sending in my camera bodies entails extra expenses such as brokerage fees. I ordered the screens and installed them myself hoping that I would luck out in the calibration. I didn't. Each camera had to be sent to KatzEye. I could have had the calibration done here but figured they were the ones with the right experience. I'm glad I did because now I am doing manual focus nearly all the time. I cannot trust the autofocus for a lot of the work I do.

I don't know why Pentax was so sloppy in this area when they could have easily inserted shims of the proper thickness for accurate manual focus.
07-24-2010, 08:24 AM   #12
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I did not know each lens need to be calibrated, individually. I've always thought the camera body was the only thing that needed calibration. Perhaps I've just been very lucky.

There is no substitution for the focusing chart found here: Nikon D70 Focus Chart It's not perfect but it's probably better than anything else we have. Just follow the instructions and you will see if you are back focused or front focused and by how much. Some people love this but I personally think it's a rip off: LensAlign® Focus Calibration System

About the Katzeye, it will make focusing easier but it won't solve your focus confirmation light problem. It's depth of field is much thinner than the stock screen and you will rely on your focus confirmation less but you'll still want to have a correct focus confirmation light sooner or later.

I own a Pentax K-x, which is much newer than the K20D. I've had no problems with the Katzeye, I just put in the original shim that came with my camera and it was perfectly calibrated. Sending it in would have cost additional money and it would not have been worth it for me. Perhaps, it's different for K20D owners.
07-24-2010, 08:28 AM   #13
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QuoteOriginally posted by cpk Quote
I am in Canada where sending in my camera bodies entails extra expenses such as brokerage fees. I ordered the screens and installed them myself hoping that I would luck out in the calibration. I didn't. Each camera had to be sent to KatzEye. I could have had the calibration done here but figured they were the ones with the right experience. I'm glad I did because now I am doing manual focus nearly all the time. I cannot trust the autofocus for a lot of the work I do.

I don't know why Pentax was so sloppy in this area when they could have easily inserted shims of the proper thickness for accurate manual focus.
Why would you think Pentax should include shims for Katzeye screens?
07-24-2010, 08:46 AM   #14
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QuoteOriginally posted by Marc Sabatella Quote
Why would you think Pentax should include shims for Katzeye screens?
The Katzeyes have the dimension as the Pentax stock focusing screens. If the Katzeye is misaligned, that means the original stock screen was misaligned as well. You just would never notice it because the stock screen had such a wide DOF.

I agree though, Pentax probably couldn't even have tested to see if the stock screen was accurate since the margin for error was huge with the wide DOF.
07-24-2010, 08:46 AM   #15
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@Marc. I wouldn't expect that, but I found manually focusing with Pentax's own screens at wide apertures to be inaccurate. That seems also to be the experience of many others from what I have seen on this site and others. I don't know if KatzEye follows the thickness tolerances use by Pentax for their screens; I would assume so since their screens are meant to be a replacement for the Pentax ones. If I am correct in that assumption, then my comment stands; if not, yours does.
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