It depends on the camera and the method of auto focus it uses.
Cheaper cameras use contrast detection, that alters the focus until the edges of an object in the focus zone are as contrasty as possible. The smudge makes it harder to find clean edges on the subject.
Old school dSLRs and modern mirrorless cameras often use a different, faster method of auto focus called phase detection. It’s more expensive, because it requires specific hardware to work. It uses two different sensors (or a single hybrid sensor) to split the light coming though the lens and overlay the resulting images. When the images are identical, the image is in focus. But because of the way this system works, smudges don’t really stop the auto focus from working. The smudge will still reduce the contrast and crispness of your final photo, but you’ll still get good auto focus.
So what makes it try to focus on the near smudge in the glass window instead of the larger scene outside?
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, 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. Some lenses also have switches that let you control the minimal focal point distance. This is more common on telephoto lenses, but if your lens has that kind of switch, you can use it to ensure that the smudge is too close to focus on.
You can also change the active autofocus points on your camera so the smudge is 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
Nothing, because that’s not what happens.
Edit - Oops, see my other reply. I didn’t realise you were talking about a glass window, and assumed you were talking about a smudge on the lens itself
I assume because it’s not possible to get a sharp photo due to the smudge?
Yes, because the camera focuses on the smudge.
You’ve solved it!
Yes, because the camera focuses on the smudge.
Auto Focus is working fine, your vision isn’t.
Because it is smudged.
I know what community we’re in but like… There has to be a line right.
Because it can’t tell when it’s focused if the lens is smudgey and smearing the image. It sees what you see for the most part.
