Why do cameras lose focus on certain objects? Can current technology make it so that losing focus is no longer a thing?
In: Physics
When people say “camera”, they usually mean a device that uses some arrangement of lenses to focus the light from a scene onto a (normally) flat detector of some kind (film or a digital sensor) that records the focused light.
Some cameras have automatic focus, and there are several methods they might use to achieve this (often by analysing the light in the scene to look for clues about what part of the image needs to be in focus). There are several ways the camera can get this wrong. For instance, the variation of light in the scene might not be well suited to the method the camera uses, or the camera might struggle to lock its focus on a moving subject.
There are new devices, “light-field cameras” which are not yet very widely used yet. These do not work in the same way as what we normally think of as cameras; they do not record the light onto a flat (or curved) detector. They record light information in a completely different way, that allows the focus to be adjusted to the required part of the image _after_ the image had been recorded.
There’s a number of different methods cameras use to focus, including manual, and you haven’t specified which. The general answer for automatic focus is that the camera doesn’t know what thing in the frame you want to focus on. It has to figure out on its own what is supposed to be the object of focus and what’s in front of and behind that object. If you have one person standing far away and another closer, which one are you trying to take the picture of? The camera doesn’t know, so it tries its best to pick the one most likely.
This stems from the fact that there is a limitation on how much stuff a camera can focus on at once. This is called “depth of field”, which means the range of distance from the lens where objects will be in focus. The size of the depth of field depends on things like how long the focal length of the lens is and the size of the aperture on the camera, which is the little metal iris behind the lens. You can get a larger depth of field by making the aperture smaller, but it lets less light through, which can be a problem in low light environments.
Basically, everything on a camera is a trade-off, and even the smartest chip in the best digital camera can’t always make the same choice about what settings to use as a human might want.
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Can you give us a particular scenario you’re thinking of and what kind of camera you’re referring to?
Camera loses focus because the person operating the camera tell it to focus on something on some parts of the frame. So, when the subject moves out of the part of the focus area then the camera will change its focus to something behind where the subject used to be and the subject goes out of focus.
To avoid that most cameras now have object tracking and it will continuously adjust focus to keep the subject from going out focus.
The new mirrorless cameras now has something called Eye-Focus, i.e. it will track a person’s eye and once the camera locks its focus to his/her eye, then the person can move anywhere in the frame, left-right, come forward, go backwards etc and the camera will continuously adjust focus to keep the subject in focus. Of course, this only works if the subject you’re shooting is a person.
It’s possible to have a lot of things on focus all at once. It’s called having a large depth of field.
But aesthetically, it’s often more desirable to have a “narrower” depth of field so that the subject of a photo is in good focus and other things are blurred.
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