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08-14-2009, 09:11 PM   #1
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Lens problem? update on FF of K-7

Hi All,

Today I got a new lens 55-300. It nearly doesn't have any problem with my two K-7s. I use -5 for AF tuning any it is just right on 300 end. On 55 end it is impossible to adjust since the DOF is too thick.

For my 50/1.4, there is some update. The problem changes with aperture! For AF adjust -10, I found the lens FF at f1.4-2.5 and almost right on at 2.8 and BF on 4.0. Does that mean my lens is wrong or this is totally normal? I gonna buy another FA 50 to have a try out.

Thanks for your comments! It seems I see some shed light to keep the K-7!

Tony

08-14-2009, 11:10 PM   #2
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On the FA50 f/1.4 @ f1.4 to f/2.5 if you have a "busy" focus field i.e. there are a number of objects near and far which the AF may pick on, it is very unpredictable what you end up with.
If you take several shots of the same scene, you might end up focusing on different distances each time.
That's not an FF or BF issue. Not much you can do about it other than switching to manual focus. Personally, I stay away from f/1.4 unless the light is so dim, its the only way I can get a decent shutter speed hand-held.

However, if its acccurate at f/2.8, the focus shouldn't get worse at f/4.0 because the DOF increases slightly going from f/2.8 to 4.0

The DA55-300mm has very few complaints on FF or BF. Mine is exactly spot on - OK, its a toss up between -1 or 0, I just leave it at 0.
08-15-2009, 01:03 AM   #3
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I used a chart that the focus is on a black-wight contrasty line, so I think the focus is on where it should be. But I observe the phenomenon that focus will get further back on smaller apertures on FA 50, DA 16-45, and Nikon AF50/1.4d. At wider apertures, the focus distance changes fast and slower ans slower when small apertures were taken. Mostlikely, smaller than 2.8 the item would be in focus. However, the focus error will enlarge when the subject is far away. For example, my 16-55 won't focus in infinity at f4. I got to do more test on smaller apertures to make a conclusion.

If you are interested, would you like to take a similar test? Just simply change aperture and you may find the focus center move backwards. It sould be interesting if you have spot on focus in most apertures, which still means I got a lemon lens or body.

Many thanks,
Tony
08-15-2009, 06:37 AM   #4
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QuoteOriginally posted by fulcrumx29 Quote
I used a chart that the focus is on a black-wight contrasty line, so I think the focus is on where it should be. But I observe the phenomenon that focus will get further back on smaller apertures on FA 50, DA 16-45, and Nikon AF50/1.4d. At wider apertures, the focus distance changes fast and slower ans slower when small apertures were taken. Mostlikely, smaller than 2.8 the item would be in focus. However, the focus error will enlarge when the subject is far away. For example, my 16-55 won't focus in infinity at f4. I got to do more test on smaller apertures to make a conclusion.

If you are interested, would you like to take a similar test? Just simply change aperture and you may find the focus center move backwards. It sould be interesting if you have spot on focus in most apertures, which still means I got a lemon lens or body.

Many thanks,
Tony
Somewhat unlikely it's the body since it focuses wide open, then
stops down the lens. Sounds like focus plain shifting with the aperture.
diglloyd: A Brief Introduction to Focus Shift
Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.

Quite literally, the optimal plane of focus moves, depending on aperture! With every lens Iíve tested to date, the focus moves farther away.


08-15-2009, 11:58 AM   #5
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Thanks for the article. But my problem is my body+lens front focus wide open. This includes FA50/1.4 and DA 16-45. Maybe it is a compensation by Pentax on purpose for smaller apertures to be accurate?
08-16-2009, 06:50 PM   #6
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QuoteOriginally posted by fulcrumx29 Quote
Thanks for the article. But my problem is my body+lens front focus wide open. This includes FA50/1.4 and DA 16-45. Maybe it is a compensation by Pentax on purpose for smaller apertures to be accurate?
I didn't get that from your other posts.......
08-16-2009, 06:55 PM   #7
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Then what do you think? Am I not getting front focus at wide apertures? I compared with live view focus and it is really front, maybe the sample I showed is not clear enough.
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