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Old Apr 6, 2011, 3:06 AM   #1
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Default back focus - lens or camera?

After some tests, as far as I can tell, my new a580 with the 18-55 kit lens has very good focus. Maybe the tiniest fraction of back focus but even at f3.5 my intended subject looks pretty Sharp. However, with the 35/1.8 prime it definitely back focuses by a significant amount. I'm planning on returning/swapping this lens but before I do, I've read a lot that says it's only the body that can be the problem. And if I use the focus check lv it does work better. So, any thoughts on whether I have a lens problem or a body problem that only the shallow dof on the 35/1.8 makes noticeable? Will post some samples this evening if time permits.
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Old Apr 6, 2011, 5:38 AM   #2
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There have been more than a few reports of focus problems with the Sony 35/1.8.

If the camera focuses ok with one lens but not another, the problem is not the body.
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Old Apr 6, 2011, 5:40 AM   #3
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Were the tests you did similar to this: http://www.focustestchart.com/
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Old Apr 6, 2011, 7:25 AM   #4
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Yes, that's one of the tests I did. What I don't understand is why if I use the Focus Check LiveView which uses the main sensor instead of the secondary sensor, it works. That implies the camera is at fault as changing something on the camera 'fixes' it. I woudln't consider that a permanent solution though because of all the delays and mirror-flapping it involves.

But, with the kit lens I don't really have a problem that I can see. I'm starting to think it is a camera problem but not a severe one so it's only when using the wide-open 1.8 that it becomes apparent.

Either way I've been asked to take it back into the shop so they can take a look and advise. Will let you all know how it works out.
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Old Apr 6, 2011, 7:54 AM   #5
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Default Lens micro adjustment

I don't know about the Sony camera's, but my Canon will let you perform a microadjustment with the lens/camera combination and each time you attach that lens, the camera remembers the adjustment. I know that in older Canon models, you had to send the lens & camera into Canon for this to be done, but on the later higher end models, this can be done by the user. The camera can remember the adjustment for up to 20 different lenses. Maybe Sony has some kind of similar capability.
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Old Apr 6, 2011, 8:37 AM   #6
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Not on my one but I'm still unclear why you need to focus each lens differently if it's the camera that determines the focus.
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Old Apr 6, 2011, 9:26 AM   #7
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In 'Focus Check Live View', in autofocus mode, you can use either Contrast AF (the default in 'Focus Check Live View', and the only place it's available) and Phase-detect AF (available in 'Focus Check Live View', and the only option in any other mode.)

Phase-detect AF is faster and more accurate, but Contrast AF doesn't interrupt 'Live View'.

Are you saying that Contrast AF focuses the 35/1.8 correctly, while the Phase-detect AF does not, and Phase-detect AF always focuses the 18-55 correctly?
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Old Apr 6, 2011, 9:33 AM   #8
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Quote:
Originally Posted by MartinSykes View Post
Not on my one but I'm still unclear why you need to focus each lens differently if it's the camera that determines the focus.
First of all, I'm certainly far from an expert on the subject. That being said, what I have discovered is that each lens, even each individual lens of the same make and version, can have minor to major variations in the actual focus point (more front focus, back focus, etc.). The focusing alogrithm/IQ can't make adjustments for the subtle differences in each lens without being told that there is a difference. To my understanding, microadjustment communicates that difference to the camera. Of course, if the lens is perfect, no adjustment needs to be made. What I have found is that even my Canon EF 70-200mm f/2.8L USM was front focusing and needed a micro adjustment to -8. My explanation is probably sorely lacking, and maybe someone a bit more technical can give you a better or more acurate explanation. Again, I'm not an expert, not even close, just learning about this myself. What I can say is that once I made the adjustment, my pictures were sharper in general and I had many more usable choices when I was shooting sports action. I don't know if this is even applicable to the Sony camera, so hopefully I'm not wasting your time as you read this post.
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Old Apr 6, 2011, 12:02 PM   #9
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Quote:
Originally Posted by TCav View Post
In 'Focus Check Live View', in autofocus mode, you can use either Contrast AF (the default in 'Focus Check Live View', and the only place it's available) and Phase-detect AF (available in 'Focus Check Live View', and the only option in any other mode.)

Phase-detect AF is faster and more accurate, but Contrast AF doesn't interrupt 'Live View'.

Are you saying that Contrast AF focuses the 35/1.8 correctly, while the Phase-detect AF does not, and Phase-detect AF always focuses the 18-55 correctly?
I don't have the camera with me at the moment but I'm pretty sure I can use both modes normally (except when using the tamron 70-300 when contrast is the only option) but the only one that works absolutely perfectly in all situations (including the 35/1.8) is contrast detect in focus check lv mode which uses the main sensor so is always perfect. on the 18-55 I think both worked OK.
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Old Apr 6, 2011, 12:26 PM   #10
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I wonder whether it might be because of the axial CA that Tamron lenses often suffer from. Perhaps the phase detect circuitry is thrown by the fact that different frequencies of light are focusing at different depths on the sensor. I don't know that for a fact, but it strikes me as a plausible mechanism for error that would be lens-dependent in an algorithm that resides in the camera body.
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