Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-03-2009, 12:29 PM   #16
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
Original Poster
Since i accidently refreshed while writing, dont want to rewrite all:
-Noticed EXIF, must be the site, JPEG shows 30 mm.
-Further subjects (2..10m): easier to fail. Closer: fail if circumstances match. Dozen centimeters closer and i'd get perfect lock on tree.
-Ken, Google. They all describe constant FF/BF offset. I get wrong targets perfectly sharp with no FF/BF. But the glass has some issues...
-Bought for low light indoors people (1..5m), not landscapes. Getting focus on walls, curtains ans sofas instead, unless the face is all over the frame, or there is no bacground. Not as usable as Ken says.
-Yes, im actually being dumb, since im not able to clearly describe my problem.
-Im about to sell this lens anyway in a week or so...maybe. But the issue must be resolved, since i would love to use fast-normal prime (35/2 for example, wich also exhibited the same symphtoms).
-Again.. do you really always get such incorrectly focused shots, to think mine are the way they should be?

Looking forward to hear from you.


Last edited by ytterbium; 01-03-2009 at 12:53 PM.
01-03-2009, 12:56 PM   #17
Veteran Member




Join Date: Sep 2007
Location: USA
Posts: 1,812
QuoteOriginally posted by ytterbium Quote
-Yes, im actually being dumb, since im not able to clearly describe my problem.
-Again.. do you really get such unfocussed shots, to consider them okay?
No if I got unfocussed shots it would not be OK.

and no you are not being dumb - you are obviously having problems with the Sigma 30mm f/1.4 on your Pentax K100D Super - it's the first samples which were difficult to analyze - leaving it open to speculation that other focus points may have locked on other than the one you intended.

I too would conduct more tests including f/1.4 in daylight.......

Your most recent set does show problems, again we have to take your word that the camera used the focus point and focussed on what you say it did -
but the subjects were complex and using multi-focus points just begs the question that another unitended focus point was activated causing the focus error.

Again no disrespect intended and I certainly do not intend to impune you are not telling the truth in any way shape or form -
it's just the samples used which may seem clearcut to you -
are still open to speculation by others -
who are actually trying to be helpful in pointing out where errors may have occurred.
01-03-2009, 01:22 PM   #18
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
Original Poster
-Hmm.. i still dont get what are you trying to say about the focus points? Can K100D pick different focus points even if it is set to manually select?
Switch distance measure point: manual, the one with many red dots icon, when you use arrows to select the point while shooting.
After MANUALLY selecting center (or any other point), are you telling the camera can still try to use other points and focus on them, without showing it?
-About the daylight. In low light/incandescent light you could get focus error with any lens, so i tried to rule out the light.
Removed samples from first posts, to not confuse others with them.

Im quite surprised, that many people own that sigma 30/1.4 and there are more than few 35/2 users. Why they don't respond. Do they get correct focus, dont shoot like me (use manual, shoot closer than 1..2m, high F-number...), haven't really notice/looked for errors (although error seems obvious to me) or something else...?

Last edited by ytterbium; 01-03-2009 at 01:33 PM.
01-03-2009, 02:15 PM   #19
Veteran Member




Join Date: Sep 2007
Location: USA
Posts: 1,812
QuoteOriginally posted by ytterbium Quote
-Hmm.. i still dont get what are you trying to say about the focus points? Can K100D pick different focus points even if it is set to manually select?
Not that I know of -

I apologize if I seem slow or dense -

but you have written a LOT and I can see the tremendous effort you have put into this -
but I may have missed it that you set the focus point manually -
for example in your " Inbox Foto (foto.inbox.lv) (pretty straight forward - fail) " has led more than me to speculate that the camera chose some other focussing point. However knowing you set a single focus point and it did lock on the tree - then - there obviously is a problem.

How or why - is probably beyond my capabilities.

01-03-2009, 02:34 PM   #20
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
Original Poster
I dont want you to make you think that the camera is faulty.
But since the problem is quite specific, i wanted to list all circumstances, so someone more experienced could have full view of the problem and help me decide, if i should send the camera for repair, or learn how to shoot. My experience with Pentax and SLR's is too limited to do that (first SLR). And the only lenses i was able to test this problem was my 30/1.4 and 35/2 at shop.
That's why the description became so long. Guess it did the opposite, made people not read it trough, thus made them unable to objective analyze my problem.
Guess i should send it to warranty. Most of all i'm afraid of them not being able to notice the problem and for example make the camera front focus, just to adjust/"repair" something (or just send it back untouched in best case).

Im trying to improve and concentrate my thoughts. . Thank you, for your patience and polite attitude.
Btw, there was that biggest disappointment lens of Year 2k8... i think this fits.

Last edited by ytterbium; 01-03-2009 at 02:59 PM.
01-03-2009, 04:30 PM   #21
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by ytterbium Quote
Ok, then briefly:
Inbox Foto (foto.inbox.lv)
The camera needs something with and edge and/or contrast to focus on. A more or less featureless wall such as what is in the center of this frame is *not* something an AF system is going to like. Instead, it's going to look for something with harder edges and/or more contrast.

QuoteQuote:
-Perfectly flat, opaque object, having perfect detail/contrast for focusing.
Are we talking about the same gray concrete wall? That is *not* perfect detail / contrast for focus - quite the opposite! It's a mass of closely-related colorless values with no hard edges whatsoever. It's probably not *impossible* to focus on, but you'd have to tried very hard to pick a more difficult target.

Here's a good test: walk right up to that wall so it fills the frame (but be sure to be far enough away to be outside the minimum focus distance). See if the camera will focus at all. My guess is it will hunt badly, as it always will when trying to focus on featureless subjects like that.

Most of your subjects were similar - too featureless for the camera to focus on, or else small enough that the camera might legitimately have focused on something else instead.

QuoteQuote:
To make it clear, i did not quite get what did you mean by large rectangle.. how big the center AF area actually is
It's almost as tall as the frame - something like the "medium" picture you posted. I agree, the wall behind the concrete would have been a stretch. In fact, it isn't at all clear what the camera did focus on here. but when you don't give it something good to focus on, it shouldn't be too surprising you don't get good focu, so I wouldn't worry abut what happened in this shot and start concentrasting on more reasonable test subjects.

Actually, something I notice looking at this picture is that kind of random parts look sharper than other parts, but it seems the lens is not very sharp overall at that aperture. This means you need to a subject with *better* than average contrast/edges to achieve focus.

QuoteQuote:
If talking about distance, all subjects were at least 1 meter or more away, minimum distance for sigma is approx. 40 cm. You're saying that im too close
I said it looked like you might have been the picture of the tree. I didn't know the exact distance you were or the exact mimimum focus distance - I could just tell it was possibly an issue. As it is, I'm guessing it avoided the tree because - again - there was no detail to focus on. Maybe with more light there would have been more contrast in the tree bark, but as it was, it is no surprise it looked elsewhere. Had there not been an alternative, I suspect it would have eventually focused on the tree, but something behind it was just too much easier. I don't know. this is definitely the one image I looked at that in which the explanation wasn't obvious. If there *is* a problem, this is the one image that might possibly demonstrate it. But the rest of the ones I saw look pretty much exactly as I'd expect when you don't give the camera a clear focus target.

QuoteQuote:
Have you actually shot with similar configuration and did you ALWAYS had misfocus in such situations, or you used manual focus all the time?
In such situations? You mean trying to shoot a plain concrete wall? Yes, my camera fails to focus on plain walls all the time. But I don't take pictures of plain walls, so who cares? In the real world, sure, *sometimes* I notice the camera choosing a different target than I want - we all do. It's the nature of AF. In those cases I'll usually give the camera one or two more shots. I might try to find something else the same distance from the camera as the actual subject and focus on that instead. But I'll spend an extra couple of seconds trying to convince AF do what I want before giving up and and switching to MF via quick shift.

QuoteQuote:
I highly appreciate your help because i would be very happy to find a solution, since unless it is solved, basically i cant use any normal low light prime.
It's not just low light primes. I was just playing around with my DA 50-200 trying to verify that I am not crazy, and managed to get it to refuse to focus on pretty much the exact same kinds of subjects that you did. That is, my camera won't focus on a plain concrete wall either. So it's silly to say you "can't use any normal low light prime". You just need to learn to use the AF system better - what kind of subejcts ithandles well and what kinds it doesn't, and how to deal with the latter.

Last edited by Marc Sabatella; 01-03-2009 at 04:41 PM.
01-03-2009, 05:07 PM   #22
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
Original Poster
How about this one?
Inbox Foto (foto.inbox.lv)
I'd say the floor is much more worse subject than the box (similar to the wall you described). I made several (many) tries, all failed.
Even moreso, right after that i took my 18-55 and at the same distance i got the box perfectly in focus. Because of the DOF i had to pixel-peep to see that, but at 100..200% it could be clearly seen: box - in focus, floor - slightly blurry.
Must admit that the box is smallish compared to frame.. but it has distinct contrast, and approximately covers AF brackets.

I feel that im being very subjective and trying to convince you... I understand what your'e saying, but still after some shots with the lens its behaviour seems to be unnatural.
On the opposite, i can take a dozzen shots with sigma 70-300, with similar conditions (cluttered enviroment, small subject, low contrast - although on such subjects as the wall my k100d does very well, it is more picky about low light, dark subjects and solid objects) and most of them are okay.
And the cause of error is obvious in those who are not (its not like a hump covering half frame is being missed).

And yes, the lens has uneven sharpness across the frame. Edges are smeared out, center is tack sharp.

Taught just came to me.. as for covering the focus area. Covering it in frame does not mean the same on AF sensor. SIR (secondary image registration) apertures should be located aside from actual focus sensor (thus not in center)... so it does not cancel out the possibility for AF sensor to physically "see" other things. Like with 500 mm mirror lens.. you get the dounut bokeh, but not a black spot in center.
(Information written above may not be technically correct, but should illustrate my point).
Then, if you have seen the split prism focusing screen, you know.. as you stop down the prism gradually starts to get darker beginning from the edges, leaving only part of the image that is more in center. Maybe F1.4 lens shows too much.. on the second hand, shouldnt AF apertures limit this
? . I should better read something about AF sensors...

I just had one crazy taught. Is there a chance that, camera tries to focus further with wide angles and further and closer with longer lenses. Anyway this isn't getting me any closer to sharp pictures.


Thanks.

If the AF is so unreliable as you're saying could someone take a bright, wide-normal prime with k100d or any other cam...
Center af, manual select, put a box on floor, step back, to make it a little larger than spot metering area, like in posted image and get some 6 out of 8 incorrect shots (after throwing the camera OOF before each shot).

I'm unwilling to believe that all subjects are that bad and camera very picky, but technically fine. To be completely pessimistic, even if the focus was random, theoretically there should be 50:50 chance to get foreground/background subjects in focus. Ass you can see in my gallery almost all of the shots have background focus.

Last edited by ytterbium; 01-03-2009 at 06:18 PM.
01-03-2009, 11:10 PM   #23
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by ytterbium Quote
How about this one?
Inbox Foto (foto.inbox.lv)
Yeah, that one looks problematic. Camera should have been able to focus on the stripes.

At this point, hopefully I've at least made my point as to what you need to do to test. If you keep getting results like the above, you've definitely got a problem of some sort.

QuoteQuote:
As you can see in my gallery almost all of the shots have background focus.
Perhaps, but you really need to stop counting the ones where it shouldn't be surprising - which was really quite a lot of them.


Last edited by Marc Sabatella; 01-04-2009 at 10:33 AM.
01-04-2009, 04:11 AM   #24
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
Original Poster
Big thanks to you Marc, and UnknownVT for being so helpfull.
Since it still stays quite uncertain, i think the best thing i can do is sell the Sigma, and use my camera at wide angles with darker glass until it falls apart (and i need to get a new camera).
Obviously other lens is not a option, and the outcome of sending the camera to repair is also unknown.
Then ill just wait until someone does similar test, and maybe try this Sigma, while i still have it with a different camera at shop, to see how it should behave. That might shed some more light.

To illustrate, that the AF system isn't so erroneous, i welcome you to have a look at this picture:
http://foto.inbox.lv/tamrac/sigma/IMGP5355.jpg
As you see, even with the cluttered environment, relatively small target, AF was able to precisely lock on it only because it was closer than some critical distance (in my many trials i was not able to determine exact distance, i doubt there is one).

Anyways its weird.. i have never heard or red anyone complaining about such focus behavior. This confuses me the most.

Last edited by ytterbium; 01-04-2009 at 04:27 AM.
01-04-2009, 10:33 AM   #25
Veteran Member




Join Date: Sep 2007
Location: USA
Posts: 1,812
QuoteOriginally posted by ytterbium Quote
Anyways its weird.. i have never heard or red anyone complaining about such focus behavior. This confuses me the most.
Well, just doing a simple google on "sigma 30mm f/1.4 focus error" returned several pages of hits, of course not all are relevant - but there were a number complaining of front focus problems - like my previous reference from Ken Rockwell - these were mainly on Nikon bodies.

So one can refine the google search to "sigma 30mm f/1.4 front focus" (results),
and even "sigma 30mm f/1.4 back focus" (results)

So perhaps you are not alone.
04-02-2009, 09:15 AM   #26
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
Original Poster
Some news. After 6 weeks of waiting i got my camera back from service.
And it was well worth the wait. Got my camera replaced with a brand new k200d. After doing few tests the AF is absolutely correct. It's like day and night. Now that i could compare i'd rate my k100d's AF as random and there clearly was a problem.

Even in situations i previously taught it would be 100% miss with any lens, i get the focus exactly where i hoped it to be. And the pictures now seem to have more clarity in general than previously (even after turning sharpening down a bit).

I was also very pleasantly surprised by the new additions included in k200d, much better AWB in yellow light, mirror slap is a quiet click now (on k100d i could feel the slap with my hand), no more hot pixels, much less hunting.
It has all the functions i lacked in k100d (Like automatically switching WB to flash when its fired even if WB is on incandescent and not AWB). Simply Awesome. Now i am completely happy with my Pentax.

Last edited by ytterbium; 04-03-2009 at 08:53 AM.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
angle, camera, dof, dslr, focus, lens, notice, photography, shots, sigma, size, subject
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
AF Assist Lamp and Tungsten Focus Inaccuracy GrinMode Pentax DSLR Discussion 11 01-28-2011 09:45 PM
wide angle auto focus prime mnseawa Pentax SLR Lens Discussion 7 06-29-2010 07:22 PM
Wide-Angle Manual Focus Lens hangu Pentax SLR Lens Discussion 41 04-22-2010 04:11 PM
For Sale - Sold: Vivitar 28mm 1:2.8 MC Close Focus Wide Angle Lens amaurosis Sold Items 3 04-25-2009 12:57 AM
PREORDER DA 17-70 f/4 AL (IF) SDM Super Wide Angle Auto Focus Zoom Lens architorture Pentax News and Rumors 14 07-22-2008 02:07 AM



All times are GMT -7. The time now is 09:56 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top