Nothing, because that’s not what happens.
Comment on Why does auto focus work so poorly when trying to take a picture through smudged glass?
HobbitFoot@thelemmy.club 4 weeks agoSo what makes it try to focus on the near smudge in the glass window instead of the larger scene outside?
ada@lemmy.blahaj.zone 4 weeks ago
ada@lemmy.blahaj.zone 4 weeks ago
Oops, sorry, I assumed you were talking about a smudge on the glass of the lens, not a glass window.
In the case of a smudged glass window, that’s an issue for both types of autofocus, and it occurs because the glass is closer than the subject you’re trying to focus on. And due to the way the physics of light work, it’s easier to find focus on closer subjects than distant subjects, for the same reasons that your depth of field increases the further your focal point is away from your sensor. And because the smudge itself is something that provides sufficient detail for autofocus to work, if the smudge will often become the target of the autofocus.
There are a few ways of dealing with it though. You can step closer to the glass, so the smudge is within the minimal focal point distance of your camera. If it’s too close for your camera to focus on, it won’t accidentally focus on it. You can also change the active autofocus points on your camera so the smudge under an autofocus point. You can also drop the camera down to one autofocus point, move the camera to focus on the distant target without the smudge in the way, then half hold down the shutter button, and then reframe the shot whilst holding the shutter. That will keep the focal length set to where it was, but it won’t actually take the photo until you press the shutter the whole way in