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[OM] Back focusing

Subject: [OM] Back focusing
From: Ian Nichols <ian.a.nichols@xxxxxxxxx>
Date: Mon, 4 Oct 2010 12:43:51 +0100
How does this happen?  surely it's a closed-loop system, i.e. focus
sensor detects that image is out of focus and adjusts lens until it's
in focus, so how can one lens focus perfectly and another lens
consistently bac-focus?  I can see how a misadjustment of the focus
sensor would cause it, but it would cause it whatever lens were
fitted.

I ask because I'm now officially utterly fed-up with my Sigma 30mm
f/1.4.  I tried using it at a dimly lit gig again on Saturday, and the
wretched thing would not focus accurately.  The wall behind was more
focused than the subject in most cases and I got far better results
with the 14-54, despite the shutter having to be 2 stops slower and
quite often being a longer focal length.

I wish I'd realised this was an issue sooner, instead of assuming it
was my poor technique or simply the lack of light.

-- 
Stand firm for what you believe in, until and unless logic and experience
prove you wrong.  Remember: when the emperor looks naked, the emperor *is*
naked, the truth and a lie are not "sort-of the same thing" and there is
no aspect, no facet, no moment of life that can't be improved with pizza.

-Daria Morgendorffer
-- 
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