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04-12-2009, 03:39 PM   #16
axl
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Mike, that's two out of two fast-ish lenses you've recently tested and both with AF issues, I know that 40 had BF and 70 appears to have FF but have you thought this could be camera related?

BR

04-12-2009, 05:42 PM   #17
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QuoteOriginally posted by schmik Quote
Hmmm..... tested again under natural lights. Same results. Shows two things. That AF not liking indoor lighting is probably a myth and that this lens is no good.
The issue with tungsten light isn't a "myth", but it's perhaps a smaller effect than is sometimes claimed.

Anyhow, I know your lenses seemed off in different directions, and you tested on different cameras, but I'm still always skeptical of claims that a *lens* has BF/FF problems. I'm still pretty convinced most BF/FF problems are in the camera, not the lens. It's quite simple to see how a misaligned AF sensor would lead to BF/FF, but it's still never really been explained very well what could possibly cause a lens to exhibit problems. Yes, I know that it might be over- or under- responding to the turn of the screw, but the double check should catch most of those cases. It also seems that if you did the test a bunch of times in row, it would get steadily better each time, and finally get it right and stay right. Does anyone who see problems they blame on their lenses ever actually see that behavior?
04-12-2009, 06:45 PM   #18
Igilligan
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Mike a question

What angle is the camera/lens combo to that focus chart?

I am not saying this is not a dud by any means it just appears like you are at a different angle than I used when I tried testing my lenses. I may have been all wrong.
04-12-2009, 08:19 PM   #19
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QuoteOriginally posted by axl Quote
Mike, that's two out of two fast-ish lenses you've recently tested and both with AF issues, I know that 40 had BF and 70 appears to have FF but have you thought this could be camera related?

BR

I tested the 40 on 3 k100d bodies. It back focused on all of them.

I tested the 70 on my body only but i will borrow another on this afternoon. I really do suspect the lens. Why does pentax allow individual AF adjustments per lens on a k20d?

mike

04-12-2009, 08:25 PM   #20
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QuoteOriginally posted by Marc Sabatella Quote
The issue with tungsten light isn't a "myth", but it's perhaps a smaller effect than is sometimes claimed.

Anyhow, I know your lenses seemed off in different directions, and you tested on different cameras, but I'm still always skeptical of claims that a *lens* has BF/FF problems. I'm still pretty convinced most BF/FF problems are in the camera, not the lens. It's quite simple to see how a misaligned AF sensor would lead to BF/FF, but it's still never really been explained very well what could possibly cause a lens to exhibit problems. Yes, I know that it might be over- or under- responding to the turn of the screw, but the double check should catch most of those cases. It also seems that if you did the test a bunch of times in row, it would get steadily better each time, and finally get it right and stay right. Does anyone who see problems they blame on their lenses ever actually see that behavior?
I have not seem it get better as the tests continue. Starts off crap and ends off crap.

I understand that it could be the camera but with the 40mm it happened on 3 bodies.

And the replacement lens operates perfectly!

Also it is a pentax camera and a pentax body. As a consumer the only option i have is to get the lens checked because the camera is no longer under warranty. Two bits of their equipment don't work together..... and i need to trouble shoot it?

At the end of the day i have a lens / body combo that doesn't work correctly at large apertures.

I know it seems weird that i have baught two lenses and both have not behaved for me and that i seem like some complaining twat but this gear is expensive and i want what i paid for..... i paid for 70mm at f/2.4 and i want to use it.

mike
04-12-2009, 10:42 PM   #21
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ok...... I tested again. I borrowed another body and all was fine. WTF?

So i started from fresh, 40mm and 70mm on both bodies.
Both lenses front focused on my camera only. AT this stage i am very confused. WTF!?!?!

Doubled checked everything. Came across 3 BIG and importan discoveries.

1. I am a knob that shoots his mouth off way too fast without checking everything first.
2. My camera body was set to +150 in the AF adjustment. Damn.... I had forgot to reset it since having the bad 40.
3. The new (good) 40 was front focusing but the focus point was just within the DOF. THe 70 was just out of DOF (at max F-stops). This explains why the 40 was OK wide open and the 70 not, in terms of sharpness of viewed images.

Any, to all I apologise. To pentax, I apologise. In this case it was user error. However, these were just tests, i need to test this in real life shooting, so i reserve teh right to be a knob again

Thanks for putting up with my rants.

Geez.... I feel like a right twat!

mike
04-12-2009, 10:46 PM   #22
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QuoteOriginally posted by schmik Quote
ok...... I tested again. I borrowed another body and all was fine. WTF?

So i started from fresh, 40mm and 70mm on both bodies.
Both lenses front focused on my camera only. AT this stage i am very confused. WTF!?!?!

Doubled checked everything. Came across 3 BIG and importan discoveries.

1. I am a knob that shoots his mouth off way too fast without checking everything first.
2. My camera body was set to +150 in the AF adjustment. Damn.... I had forgot to reset it since having the bad 40.
3. The new (good) 40 was front focusing but the focus point was just within the DOF. THe 70 was just out of DOF (at max F-stops). This explains why the 40 was OK wide open and the 70 not, in terms of sharpness of viewed images.

Any, to all I apologise. To pentax, I apologise. In this case it was user error. However, these were just tests, i need to test this in real life shooting, so i reserve teh right to be a knob again

Thanks for putting up with my rants.

Geez.... I feel like a right twat!

mike
You know you have one opportunity to change your user name don't you mate. Far be it from me to make any suggestions, you already seem to have a few of your own.
04-13-2009, 10:03 AM   #23
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QuoteOriginally posted by schmik Quote
ok...... I tested again. I borrowed another body and all was fine. WTF?

So i started from fresh, 40mm and 70mm on both bodies.
Both lenses front focused on my camera only. AT this stage i am very confused. WTF!?!?!
I think this demonstrates nicely something I've been saying all along - it is *incredibly* easy to mess up a focus test. It's far easier to get it wrong than to get it right, in fact. If you happen to be lucky enough to get inconsistent results using the same lens and body, you'll *realize* that it must be measurement error. But all too often, what happens is that we see a difference that shows up between two different lenses on one body or between two bodies one one lens, and immediately jump to the conclusion that the problem is with one of the lenses or bodies rather than considering the very real possibility that the problem is with our own measurement.

Took me about a month of obsessive testing, complaining, sending one camera in for repair due to focus problems, buying another, returning it for exchange also due to focus problems, rejecting one of the offered exchanges, and finally culminating in about two hours of controlled testing in a very deserted camera shop with all the employees having nothing better to do than taking part in the testing as well, and all of us getting entirely different results, before the light started coming on for me. At this point, I'd say we're about even :-)

04-13-2009, 03:01 PM   #24
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Marc, it makes me feel a whole lot to hear your story. It made me laugh because i can see myself doing the same thing. And i hope you had a good laugh at my rants here too.

I agree and disagree with your comments about how easy it is to mess up a focus test. As a newb to focus tests it is very easy to mess it up. Once you have spent countless hours performing the tests and, most importantly, learning how the equipment works then it is not really that hard a test to perform. Especially using fast glass.

To put it all in a more positive spin, I have learned so much from this exercise. I understand now how to get the AF system to focus on certain points and i know what the AF system is looking for. This helps to prevent AF hunting in low light. I also learned how to adjust AF and test the results. Once i get a k20d or higher then I will be able to calibrate every lens to get it 'just right'. I have no intention of spending too much time on this rather than taking photos but it is a great feeling to know that your gear is calibrated perfectly and know that you can recalibrate it if needed.


Does anyone have any opinions on why the k20d has individual calibration settings for different lenses? Are pentax admitting that not all lenses are created equal?

cheers
mike
04-13-2009, 06:59 PM   #25
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QuoteOriginally posted by schmik Quote
I agree and disagree with your comments about how easy it is to mess up a focus test. As a newb to focus tests it is very easy to mess it up. Once you have spent countless hours performing the tests and, most importantly, learning how the equipment works then it is not really that hard a test to perform.
Oh, I agree. I think I'm pretty good at it by now too. But my point is, until you've experienced all the ways to screw it up, you're almost guaranteed to fall into each possible trap one at a time, and be convinced you've found a problem as a result, until finally something happens to expose your error, and you learn from that not to make that same mistake. Then you repeat the same process with the next mistake :-)

QuoteQuote:
To put it all in a more positive spin, I have learned so much from this exercise. I understand now how to get the AF system to focus on certain points and i know what the AF system is looking for. This helps to prevent AF hunting in low light
Couldn't have said it better myself. I also have a very good concept of when I'm better off switching to MF.

QuoteQuote:
Does anyone have any opinions on why the k20d has individual calibration settings for different lenses? Are pentax admitting that not all lenses are created equal?
I think it's partially that, but also the fact that a misaligned AF sensor in the camera might manifest itself differently at different focal lengths. I'm still convinced misaligned AF sensors has got to be by far the #1 cause of AF problems, because it's the only explanation that makes physical sense and explains the actual symptoms well.
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