Not for the faint of heart - DIY 350D backfocus fix

Started Dec 1, 2007 | Discussions thread
Paul De Bra
Paul De Bra Forum Pro • Posts: 12,499
do *not* try this in sensor-clean mode

The typical suggestion is to put the camera in sensor-clean mode so that the mirror flips up.

Much better is to manually (very gently) lift the mirror up and work with the camera simply powered down. The shutter remains closed and there is no risk of scratching the sensor (or actually the AA filter).

RobertoAvanzi wrote:

Shortly after buying the 70-200 L IS f4 I noticed that the focus was
not spot on, and that the lens had a bit of backfocus. I was not
really noticing it on the 17-85 IS - the lens is not that sharp
anyway, I though, and at those focal lengths and such slow aperture
the subject was always almost completely in the focus range. I
attributed the problems with the SIGMA 30mm f1.4 at apertures below
2.8 to its famous erratical behaviour. And I still had enough
keepers.

But recently, things got worse (AF drift?) and the 70-200 was able to
pinpoint the symptoms of the illness.

Printed some focus charts, all lenses were backfocussing by more or
less the same amount. Went to the shop (not the one where I bought
the camera, but the one where I bought the 70-200), tried my lenses
on a 40D body, also not perfectly calibrated it seems, it was front
focussing by at most 1 mm, nothing to bother about, in fact, except
perhaps for some macro work (that I do not do). But, the 350D was
systematically backfocussing 20 mm with the Sigma at 30 mm, about 16
mm at 70 mm with the 17-85 AND with the 70-200, and still around 12
mm at 200.

Canon in Willich was treating me in an arrogant way on the phone (no
we do not have a price list for fixing AF problems on cameras and
lenses), the shop where I bought the camera refused to help me
(that's one reason more to stay with the new one), and then I
remembered this

http://www.astrosurf.com/buil/autofocus/adjust.htm

Since I already decided to buy a 400D for the imminent travel, and
later to have the 350D fixed and then ebayed, I decided to try it.

Finding two allen keys for 1/20" in germany is not easy, here is all
metric, but I was lucky.

Not for the faint of heart. Waving an allen key in front of the
sensor is not what you always wanted to do. Touch it and you are
boned. And you have to exert some force while turning, and you hear
some clicks.

The result? Now the camera is focussing MUCH better. A bit of front
focus (but the other closest setting had a slightly bigger back
focus), but something like one mm at the fastest setting on the 30mm
sigma, and a couple of mm at 200 mm on the L zoom, nothing noticeable
on the 17-85 IS.

Now, I am exhausted. Tomorrow is (weather permitting) another
shooting day! The purchase of a new camera is delayed. I hope I can
wait for the 40D successor and devote next year's budget to glass. A
wide angle (The canon 10-22 or the new fast tokina 11-16) is due!

Roberto

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Slowly learning to use the DRebel (only around 30.000 shots) and now also the Fuji E900.
Public pictures at http://debra.zenfolio.com/ .

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