AF probs, help!

[Roowhite]

Hi all,

I'm have a problem with my auto focus on my D40x and wonder if it is coincidence or if i have caused it.

The diagnosis is auto focus is slightly out on the object i am aiming for. It normally aims just behind. The lens is moving and trying to focus just getting it wrong.

I had dust in the view finder and on sensor recently so i gave them a clean, shortly after this problem happened. How does auto focus work and can i have damaged it?

If not, any magic cures? I've tried different AF setting.

As you might guess i'm a newbie to the DSLR world. I was very careful doing the cleaning.

cheers

[ianpinion]

What are the light levels like? Is it a bit on the darkside, as this can cause your auto focus to struggle to get a proper focus lock. Have you tried making your sensor more sensitive by increasing your ISO to see if this cures the problem before we explore any other possibilities?

[OldBoy]

It could be a lack of contrast on the subject.

[amk1977]

As Oldboy mentions, Nikon's AF system is based on contrast detection. If the object you are pointing at is a uniform colour and lacks any kind of texture (a wall or ceiling for example), then the camera will hunt back and forth trying to lock on to something that does, just as it would in a low light situation that Ian suggests.

When you cleaned the sensor, did you use something like an Arctic Butterfly brush, or did you wet cleaning it, using fluid and swabs? If it was the latter, check to make sure there are no residues left on the sensor or a rogue bit of fluff.

One other thought just occurred to me. As you are new to DSLR, I am wondering if you're aware of the different selectable focus areas on the camera, other than the centre one? When you look through the viewfinder and half depress the button, a series of little boxes should show up red. One of the boxes will have a bigger box around it, letting you know which AF area is currently being used.

If you're aiming at your subject with the centre focus spot and you have accidentally chosen an AF point which is on the side of the frame, then the camera is focusing on that area and not the centre where you are intending it to focus, which would explain your issue. Check this first, to make sure the AF spot is in the middle and test the focus again.