I think I have AF backfocus issue on D200

[Justin]

On the D200. MF seems to be fine - I get sharp pics with MF, but AF seems to go just beyond my focus target. I'm using central focus area only.

Hopefully nothing to do with beer entering the AF mechanism, I don't think it did. I sort of noticed this prior to the 'Stinky camera' incident...

So I've grabbed a couple of the links to test focus, thanks to the 12-24 thread and will report here over the weekend whether this is user error or if I get to have fun manually adjusting the AF ('cos I'm that sort of guy, like to take risks ) but mainly because I would go mad without my camera for 6 weeks.

[Geoff]

Justin - can u post some examples?

[Justin]

will do. I am at work now fretting about it, hence the post

[Justin]

Ok, have run some tests.

The only one that came out OOF was the 18mm end of the 18-200. This showed some significant backfocus, In Capture NX, it shows that the focus area was spot on in the middle of the two 2mm lines.



18-200 at 50mm - spot on



35-70 at 35 - spot on
35-70 at 70 - spot on
50mm MF only - spot on (but sheesh, sharper than anything else!)



J

[Onyx]

Justin, what I would suggest firstly is to watch your focus distance when testing at the 18mm (wide) end, that it isn't closer than the lens' minimum. This could explain why it produces significant (IMHO) apparent backfocus in your test, while it's neutral at 50mm. Also, I believe it's expected of a zoom lens to have some variances in focus point throughout the zoom range.

I've had my share of focus issues with my D70, and even with 'pro' grade glass like my 80-200, it has focusing errors I can consistently reproduce - backfocus at the 80 end, frontfocus at the 200 end; and me having to adjust the AF calibration on my D70 for a 'best compromise' situation.

While it seems you only have erroneous results at the wider end of the 18-200; now that you know of it, you could perhaps compensate for it with a twist of the lens' focus ring when you're forced to shoot at this range (and otherwise take all practical steps to avoid having to use this range).

[Justin]

Onyx - yes I think you may be onto something.

Most of my use of the 18mm end is wideangle anyway where infinity is where I want to focus so even if it is a problem the effect wouldn't be large...

18mm is the wide end on something that was probably a metre away - I'll have to look it up and check the focussing distances but it's very likely my test was not valid (although I did achieve AF lock-on green light etc. at this end)

[Matt. K]

When wide angle lenses are used then focus becomes more critical and more difficult. In theory the extended depth of field should take care of any minor errors but occasionally this does not happen. I would keep an eye on it Justin but don't get depressed unless it shows onthe majority of your pics.