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03-16-2009, 10:41 PM   #16
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QuoteOriginally posted by FHPhotographer Quote
Sometimes it just isn't practical to use MF and only AF will do, so, I would like to pin down exactly what the center focus on my K100 is focusing on, and how big that point is.
It might not be "practical", but sometimes it's simply the only viable option - there is no way to pinpoint the focus with the sledgehammers that are the AF sensors.

As for pinning down how big the sensors are, I've descriebd several times how to do that - try focusing on a black dot on a white piece of paper. If it locks, you're in range. Move paper around and try again; repeat until you've figure out the boundaries

Knowing the exact size of the sensor doesn't really help that much, though. If your subject is smaller than the AF sensor, there is simply *no way* to guarantee you will focus on it.

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Personally, I'd like to know that if I have the Pentax focus screen ( o ) centered on an object and the object was filling the ( o ) in the viewfinder, that the focus point is in there
Would be great if true, but as I've explained, it isn't. The AF sensor is much taller, as you can find out for yourself by performing the simple experiment I described. Only if your subejct is large enough to cover the entire AF sensor can you rely on AF to focus on what you want. Otherwise, it's a crapshoot.

03-16-2009, 10:57 PM   #17
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QuoteOriginally posted by wasim_altaf Quote
How do I go about loading the EXIF data? Does it accompany the JPEG?
Normally it does, yes. What you did to post these samples was perfect; the EXIF data is intact. Some image editors (e.g. not PPB) will strip EXIF info by default, as will some hosting services (flickr etc). The EXIF data contains information on the lens used, some focus data, exactly what settings PPL used when processing it, etc., which of course helps when troubleshooting problems.

Anyway, as the others have said the issue is that your subjects aren't in focus, so keep going with their suggestions.
03-16-2009, 11:27 PM   #18
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how did you test the focus point?

QuoteOriginally posted by heliphoto Quote
Also, in the process of testing your focus, pay attention to what point the camera focuses on... I found my K200D center point would actually focused on a point at the bottom of the ( ) or just below, rather than in the center of these marks as you'd expect. Learning exactly where the camera is sensing, can let you be more accurate with focus point placement to get the focus you want.
I agree, but how did you test to find out the center point was not on dead center as represented by the red indicator?
Brian
03-16-2009, 11:46 PM   #19
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I guess I'm in denial...

QuoteOriginally posted by Marc Sabatella Quote
It might not be "practical", but sometimes it's simply the only viable option - there is no way to pinpoint the focus with the sledgehammers that are the AF sensors.

As for pinning down how big the sensors are, I've descriebd several times how to do that - try focusing on a black dot on a white piece of paper. If it locks, you're in range. Move paper around and try again; repeat until you've figure out the boundaries

Knowing the exact size of the sensor doesn't really help that much, though. If your subject is smaller than the AF sensor, there is simply *no way* to guarantee you will focus on it.



Would be great if true, but as I've explained, it isn't. The AF sensor is much taller, as you can find out for yourself by performing the simple experiment I described. Only if your subejct is large enough to cover the entire AF sensor can you rely on AF to focus on what you want. Otherwise, it's a crapshoot.
Well, I've tried it but I guess I'm in sort of denial that AF could be that imprecise. By my camera at 6' the "center" on a center-point focus is really anywhere within about a 10" diameter circle, and obviously it becomes larger the further back I go. I now understand how working at that distance how the object of my attention might be slightly soft while something a few inches off that center object on any side might be sharp.

So, there is nothing to be done with AF, any any camera, other than hope for the best? Maybe it's the old dancing dog analogy again... if you see a dog dancing on its hind legs don't criticize that it dances badly, just marvel that it can dance at all.
Brian

03-17-2009, 12:11 AM   #20
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QuoteOriginally posted by FHPhotographer Quote
I agree, but how did you test to find out the center point was not on dead center as represented by the red indicator?
Brian
Actually, I oversimplified... what I found is what led me to not use the typical focus test chart that many people like.

I was using that chart, and when I used the center "focus here" section, my camera would lock on that center "focus here" line fairly well, within a range of placements - ie, the focus would focus on the horizontal line if it were in the center of the ( ) or at the bottom of the ( ), etc... Then I moved the tropod over, and focused on the scale at the edge of the test chart - this allowed the camera to "see" many lines, and it consistantly focused closer in this situation... Come to think of it I may have drawn the wrong conclusion as to why it did this, maybe the closer lines being "larger" in the field of view bear more weight in the focusing decision, or maybe some other factor is in play that I can't even guess at, but for whatever reason, when many horiz. lines were present, the camera would front focus significantly more than when there was a single isolated line to focus on. Here's an example (note I used a significantly more depressed angle than 45 in order to exaggerate the effect of any misfocus)...



After finding this, I did find (or think I found - maybe I imagined) that I got better accuracy when I placed the intended focus point lower in the viewfinder (at the point which corresponded with my findings from the chart).
03-17-2009, 01:56 AM   #21
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QuoteOriginally posted by Marc Sabatella Quote
When trying to nail focus on a small object with anything other than a featureless sky behind it, you often will want to use manual focus - that's just how it is.
And with the tiny dark viewfinders MF often won't work either in these situations. Last weekend I also tried to shoot some small birds sitting on trees and in most cases neither camera's AF, nor I with MF could lock focus on the bird. In most cases the AF locked focus on limbs behind/in front of the bird, in the MF attempts the birds looked like sharp in the tiny viewfinder and blurred (out of focus) on the pictures. Honestly, these tiny viewfinders are not usable for MF on small, distant subjects.
03-17-2009, 05:42 AM   #22
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QuoteOriginally posted by simico Quote
And with the tiny dark viewfinders MF often won't work either in these situations. Last weekend I also tried to shoot some small birds sitting on trees and in most cases neither camera's AF, nor I with MF could lock focus on the bird. In most cases the AF locked focus on limbs behind/in front of the bird, in the MF attempts the birds looked like sharp in the tiny viewfinder and blurred (out of focus) on the pictures. Honestly, these tiny viewfinders are not usable for MF on small, distant subjects.
As discussed over the the Accessories forum, the magnifying eyecup helps a lot more than I would expect from the 1.2 magnification.
03-17-2009, 08:18 AM   #23
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QuoteOriginally posted by mattdm Quote
As discussed over the the Accessories forum, the magnifying eyecup helps a lot — more than I would expect from the 1.2 magnification.
Yeah, I know. It is the first item on my "things to buy when next month's salary arrives" list

03-17-2009, 10:34 AM   #24
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QuoteOriginally posted by mattdm Quote
As discussed over the the Accessories forum, the magnifying eyecup helps a lot more than I would expect from the 1.2 magnification.
Does this eyecup provide some nose-relief?
03-17-2009, 10:46 AM   #25
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QuoteOriginally posted by simico Quote
And with the tiny dark viewfinders MF often won't work either in these situations. Last weekend I also tried to shoot some small birds sitting on trees and in most cases neither camera's AF, nor I with MF could lock focus on the bird. In most cases the AF locked focus on limbs behind/in front of the bird, in the MF attempts the birds looked like sharp in the tiny viewfinder and blurred (out of focus) on the pictures. Honestly, these tiny viewfinders are not usable for MF on small, distant subjects.
It's taking me a while to get used to the smaller viewfinder for sure. And with my long nose I also have trouble getting my eye sufficiently close to the eyecup (not a problem with older cameras like xd-5: ~3inches tall and the tip of the nose stays literally bellow the camera). Just about all my pictures are coming out tilted. May be I should get a nose-job ..
03-17-2009, 11:18 AM   #26
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QuoteOriginally posted by heliphoto Quote
I tend to find it useful at about 10 to 15 feet (or less with a shorter focal length). Beyond that, the difference is too minor to really detect, but you could use cars in a parking lot or something similar to test on a large scale. For infinity, I'd shoot a building that's far enough away for the lens to need to be infinity and look for sharpness.

BTW, once again Mark S. has read the details of a post completely before answering, while I miss key points ... Now, rereading that those are 100% crops in the first post (which I missed earlier), I completely agree that it would be extremely easy for the camera to lock onto something other than your intended (very small) target.
Yes, 10-15ft is like infinity for my lens. Tried out a little closer distance. Here is the result, cropped and resized. Sorry about the clumsiness of the test, and the bad WB. I focused on the first CD sleeve, on "jazz in Paris". Looks right on the target, and the ones behind are out of focus.

Last edited by wasim_altaf; 04-04-2009 at 07:00 PM.
03-17-2009, 11:58 AM   #27
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QuoteOriginally posted by Marc Sabatella Quote
It might not be "practical", but sometimes it's simply the only viable option - there is no way to pinpoint the focus with the sledgehammers that are the AF sensors.
Only if your subejct is large enough to cover the entire AF sensor can you rely on AF to focus on what you want. Otherwise, it's a crapshoot.
I've had a few times when 2 or 3 different different sensors would light up simultaneously. I usually switch over to center-focus sensor only if that happens. Selected focus-pint is handy to have, too -- saves recomposing. But I haven't a clue how AF algorithm works or why so many sensors are needed. (Some even have 50+ sensors!) Does any particular sensor take precedence over the rest?
03-17-2009, 02:59 PM   #28
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I agree with Marc on the fact that the camera is probably focusing on the background/somewhere in between. If those are 100% crops then the size of the bird in the actual frame must be fairly small.

In the first set of pics you posted, you'd do better locking focus on the rock the birds are standing on rather than trying to have the focus point light up right on the bird, because of the water/movement in the background possibly throwing everything off. Then stop down a little since it's in full sun to f/8 to give yourself enough depth of field to keep the birds sharp.
03-17-2009, 02:59 PM   #29
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QuoteOriginally posted by wasim_altaf Quote
Does this eyecup provide some nose-relief?
No.

Or at least, not for me. You gain an extra 5mm or so nose room, which wasn't significant for me.
03-17-2009, 03:00 PM   #30
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Here's a great explanation of how DSLR autofocus works: http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
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