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05-12-2011, 08:00 PM   #16
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Pentax AF is an oxymoron.

05-12-2011, 10:35 PM   #17
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I just had the same thing today with my tamron 18-200mm. when zoomed in. I then switched to center focus, and problem solved! I actually am keeping it on the center focusing as that's where I usually want the subject to be sharp.
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05-13-2011, 01:17 AM   #18
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I think Selar is implying that his camera setup as described is stationary and retains a focus on a specific part of the target, but in spite of this, the camera refocuses. I'm missing the connection between this situation and what you have previously described. The only thing that appears to change from shot to shot in Selar's case is the way the camera chooses to focus, on a target that does not change from shot to shot. Assuming even a small DOF of a few inches, the target should look consistently focused.

I'm not expecting the camera to read my mind, I'm expecting it to retain focus on the same point.

The alternative explanation is that the Pentax focusing system is just not very good.

I followed a tip from a reader on another forum and established which part of the VF covers the single point focus. It is inside the central brackets in the middle of the VF, therefore covering a small circular area just inside the circle defined by the brackets if one had to join the brackets.

In the shots of the child I referred to originally, the focus area covered his eyes and nose. So any part of this area of his face should have been in focus? Please correct my understanding if I am wrong.

And lastly, the child is not hyperactive - he does actually pause for breath now and then.
05-13-2011, 05:38 AM   #19
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QuoteOriginally posted by jlaubza Quote
Pentax focusing system is just not very good.
Bingo! Can't rely on it when DOF is very shallow.

05-13-2011, 07:48 AM   #20
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I'm new to dSLR in general and the K-x in specific. I've had terrible luck (softness) shooting my kids with AF using the 18-55 kit lens wide open indoors without a flash under 5-point, central point, or point select. I've tested quite a bit the last two weeks (I'm new; did I mention that?) I don't think it's camera shake as I use SR and shutter 1/30 or faster.

How deep should I expect DoF at 55mm f/5.6? Wider than a kid's head?

P.S. I've had good luck (sharpness) shooting the kids outdoors with AF.C using 5-point, central, and point select.
05-13-2011, 09:18 AM   #21
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QuoteOriginally posted by jlaubza Quote
I think Selar is implying that his camera setup as described is stationary and retains a focus on a specific part of the target, but in spite of this, the camera refocuses. I'm missing the connection between this situation and what you have previously described.
Nothing about the setup described in any way invalidates my basic observation - the focus "point" is not a point, but a *region*. I don't care how stationary your camera is, or how static your subject is. it's still going to be the case that the focus sensor has a *choice* of subjects to select: anything within range of that focus sensor. And there would be no particular reason to expect it to pick the exact same one every single time.

If you wish to focus once then keep the exact same focus plane on subsequent shots, that why the ability to decouple AF from teh shutter button exists (eg, the AF/OK button on back of camera).


QuoteQuote:
Assuming even a small DOF of a few inches, the target should look consistently focused.
Why would you imagine that? Consider a focus sensor that cover both the eyes and nose in a portrait - as would normally be the case in a head&shoulders shot. If the eye and nose are both within range, then the camera could legitimately choose to focus on either. If it happens to focus on the eye, the nose will be softer; if it happens to focus on the nose, the eye would be softer.

QuoteQuote:
I'm not expecting the camera to read my mind, I'm expecting it to retain focus on the same point.
How is that not reading your mind? The camera can legitimately focus anywhere within the area covered by the sensor. Why should it assume you wanted it to focus the same place as last time? I actually might say that's counterintuitive - the whole reason for focusing again would be to get a *different* result. if you liked where it focused the first time, there's a sure-fire way to keep it - switch (if just temporarily, via the AF/OK button) to MF for subsequent shots.

QuoteQuote:
In the shots of the child I referred to originally, the focus area covered his eyes and nose. So any part of this area of his face should have been in focus? Please correct my understanding if I am wrong.
See above. If the camera the nose, the eye would be soft, and if it choose the eye, the nose would be soft. I don't care what some DOF calculation tells you; they are meant only to tell you how much would be "acceptably" sharp for some typical print size viewed from some typical distance. DOF calcuations grossly overstate how the DOF when it comes to pixel peeping.
05-13-2011, 09:26 AM   #22
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QuoteOriginally posted by Monkey Mash Button Quote
I'm new to dSLR in general and the K-x in specific. I've had terrible luck (softness) shooting my kids with AF using the 18-55 kit lens wide open indoors without a flash under 5-point, central point, or point select. I've tested quite a bit the last two weeks (I'm new; did I mention that?) I don't think it's camera shake as I use SR and shutter 1/30 or faster.

How deep should I expect DoF at 55mm f/5.6? Wider than a kid's head?
There are online sites that will calculate DOF for you, such as this one:

Online Depth of Field Calculator

As I mentioned above, these calculations are grossly overstated when it comes to pixel peeping; they are only meant to tell you what will look acceptably sharp in a typical smallish print. It's still the case that when pixel peeping, you're usually dealing with a *much* smaller DOF in terms of what looks acceptably sharp at those hugely unnatural sizes.

But if you're not using a tripod and your kids is moving in even the slightest, that throws things off further. Tripods aren't just about removing "shake"; they also keep you from inadvertently leaning in or out between the time focus is achieved and when the shutter is snapped. when dealing with a DOF that's already just a matter of inches as it would be in the above case, the inch or so you yourself might move can be quite significant. Of course, you don't *need* a tripod to avoid that, but it is a variable people often don't account for.
05-13-2011, 11:18 AM   #23
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Thanks for the link, Marc.

I'm not pixel-peeping; these are look-at-the-LCD-and-hit-delete soft. My success rate is 20% or so.

At 55mm f/5.6 with the subject at 6ft, the DoF is 9.6in. That means if the AF locks onto the eyes or the nose or the mouth or the ear or the hairline etc, the kid's face should be acceptably sharp, no? Additionally, my test subject is often restrained by his highchair.

Upon reflection, the sharp outdoor pics may not have been with the kit lens. I'll have to test more this weekend. Is it possible for the DA L 18-55 to be the source of such inconsistancy?

EDIT:
Just found this on dpreview forums RE: DA L kit 18-55 being soft wide-open on the tele end:
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=38415631


Last edited by Monkey Mash Button; 05-13-2011 at 11:35 AM.
05-13-2011, 01:26 PM   #24
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QuoteOriginally posted by Monkey Mash Button Quote
Thanks for the link, Marc.

I'm not pixel-peeping; these are look-at-the-LCD-and-hit-delete soft. My success rate is 20% or so.
I'd suggest not deleting one of these, then, and instead posting it here. Otherwise, we're just guessing.

QuoteQuote:
At 55mm f/5.6 with the subject at 6ft, the DoF is 9.6in. That means if the AF locks onto the eyes or the nose or the mouth or the ear or the hairline etc, the kid's face should be acceptably sharp, no?
Viewing on 2-3" screen without zooming in, yes. Zooming in to maximum magnification, not necessarily.

QuoteQuote:
Is it possible for the DA L 18-55 to be the source of such inconsistancy?
Not really. Whatever might be going on in a lens to cause misfocus, it would likely be pretty consistent.

QuoteQuote:
Just found this on dpreview forums RE: DA L kit 18-55 being soft wide-open on the tele end:
DAL 18-55 - built-in soft filter at 55mm?: Pentax SLR Talk Forum: Digital Photography Review
Yes, but again, we're talking "not-as-sharp-as-much-more-expensive-lenses-when-pixel-peeping soft", not "immediately-obvious-on-2-inch-screen" soft.

But also note that even if a subject is sitting in a highchair, the head is normally still moving unless you are specifically waiting for him/her to hold a pose. 1/30" is not generally fast enough to stop even the ordinary head movement of conversation.
05-13-2011, 06:58 PM   #25
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Pentax af is a joke. They should either fix it or go back to making manual focus cameras.
05-13-2011, 07:16 PM   #26
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QuoteOriginally posted by Monkey Mash Button Quote
Thanks for the link, Marc.

I'm not pixel-peeping; these are look-at-the-LCD-and-hit-delete soft. My success rate is 20% or so.


EDIT:
Just found this on dpreview forums RE: DA L kit 18-55 being soft wide-open on the tele end:
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=38415631
The af sensor on my kx was out by 170 microns. Yours might need adjustment as well.
05-13-2011, 07:33 PM   #27
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QuoteOriginally posted by selar Quote
Pentax af is a joke. They should either fix it or go back to making manual focus cameras.
A bad carpenter always blames his tools.
05-14-2011, 07:40 AM   #28
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QuoteOriginally posted by FrumPilot Quote
I just had the same thing today with my tamron 18-200mm. when zoomed in. I then switched to center focus, and problem solved! I actually am keeping it on the center focusing as that's where I usually want the subject to be sharp.
Jake
I had the same issue with my Tamron and solved it the same way.
05-14-2011, 08:01 AM   #29
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QuoteOriginally posted by selar Quote
Pentax af is a joke. They should either fix it or go back to making manual focus cameras.
Have to agree. Auto focus in Pentax cameras is very unreliable. As a Pentax shooter I hate to say that Canikons are way ahead.
05-14-2011, 03:31 PM   #30
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The more I think about it the more I believe liveview focusing is the future. I think I'll wait for a good EVIL body now for my next purchase.
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