Can digital zoom affect focus

Question

I was always under the impression that digital zoom has little use since the same thing can be achieved on a computer afterwards by cropping.

I recently purchased a point & shoot digital camera for my wife to use in bird photography (Canon PowerShot SX720 HS). When photographing birds in trees, the camera sometimes has trouble focusing on the subject, presumably because it considers other objects such as tree branches and leaves as the primary subject rather than the bird.

My question is this: is it possible that by using digital zoom one can more completely indicate the object of interest to the camera thereby allowing it to focus on it more accurately?

Answer 1

A number of cameras I've seen switch to center focus once you are in digital zoom territory. The focusing itself does not have more material to work with, but the focus mode may be different. Similarly, some cameras switch to center focus once they engage focus assist light due to insufficient light (if your camera does that, it may make sense to disable focus assistence: the camera will then be less reliably focusing on where you think it should rather than on where it thinks it can do better). However, switching to center or spot focus when making bird pictures is prudent anyway since more often than not they are surrounded by foliage and branches that do not form points of interest with regard to focusing.

So the answer is, of course, not to use digital zoom (which has its own problems) merely because it may have the side effect of switching to center focus, but pick center (or spot) focus in the first place when you are birding.

Answer 2

Focus and zoom are independent systems, whether optical or digital. What both do though is increase the likelihood of autofocus locking when focus point selection is automatic. In a lens it is possible that changing the focal-length changes focus but that's just because of construction of the optics and mechanical parts.

Think about it, even if the camera choose where to focus randomly, it has more chances of doing so when the subject occupied a greater portion of the frame.

Now, if you control the point-of-focus and you place it at a particular spot within the frame, then the cameras has to focus there and it has no more or less information to do so.

Answer 3

My understanding is focusing is not related to the digital zoom. The sequence of operations of camera is (approximately): focusing, metering, take photo, editing (sharpness, saturation, white balance, digital zoom).

So digital zoom is long time (dozen and hundred of milliseconds) after the focusing process.

Answer 4

Cropping the image for focus will not give it more pixels to work with.

In fact your camera would need some way to distinguish the pixels from the bird from the pixels from the leaves, to be able to maximize contrast on the bird pixels. This is something that Artificial Intelligence is working on, but I'm afraid that current battery technology doesn't provide enough capacity to support a whole neural network in a camera yet.

Answer 5

It's normal I think. In opto-mechanical zooms like in a DSLR variable focal+aperture lenses the more long length you use, the less light you have on the sensor so you get a worse performance on the camera to acquire points to calculate focus.

But in digital zooms this is not the case although a lack of light can worsen the autofocus procedure. But in this case the problem is managing the amount of data you're handling by cropping the image in your interface. In some cameras it even becomes laggy (or unusable) when zooming too much.