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11-03-2007, 09:04 PM   #1
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Back Focus problem

I've just noticed a back focus problem on my Sigma 30mm f/1.4 EX DC lens. Am I right in thinking that this is something the techs at the Sigma repair facility in New York can correct? Thanks.

DN

11-04-2007, 10:58 AM   #2
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Yes, they should be able to fix it, and if not then they should replace it for you. Either way they will at least be able to determine if there is a problem, so assuming the lens is still under warranty I recommend sending it in.
11-04-2007, 02:03 PM   #3
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QuoteOriginally posted by Clem Nichols Quote
I've just noticed a back focus problem on my Sigma 30mm f/1.4 EX DC lens. Am I right in thinking that this is something the techs at the Sigma repair facility in New York can correct? Thanks.

DN
Hum, I wonder. Im not an expert on this topic. However....

If the autofocus system of your camera is working with pixel contrast ratios and a focus stepping motor plus feedback system. Wouldn't that mean that the camera decides when a something is in focus? Measuring through the lens, independent of the internal mechanisms of the lens. Maximum pixel contrast = in focus?

If this is how it works, and if you are suffering of BF problems, how could the that problem than lie with your lens?
Isn't it allways the camera that is at fault here?

- Bert
11-04-2007, 04:59 PM   #4
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That certainly makes sense. I've only noticed the problem with the Sigma 30mm, however. It may be due to the very shallow depth of field at f/1.4, but I don't seem to notice the problem when using the Pentax 50mm f/1.4. After experimenting with the lens for an hour or more today I've come to the conclusion that the problem may be more with slow/poor focusing in low light than with back focusing. When I push half way down on the shutter release button I can see and hear the lens focusing. If I let up and push half way down again I can usually hear the focusing motor once again activating. It may do this 3 or 4 times before it finally stops coming on when the shutter button is pressed. Even though I'm unable to see any difference through the viewfinder I assume it must be doing something. If the camera is going from focusing on something 10 feet away to something 12 inches away it may not be bringing the focus in close enough on the first push of the button, thereby giving the impression that it is focusing behind where it's supposed to be focusing. I'll have to try focusing on something up as close as possible and then backing off to 12 inches or so and see if it appears to be front focusing. Does that make sense?

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