How is it that a camera is able to focus on a set distance, yet blurring things both closer and farther? Whats the difference between a near small image and a far large image?
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Imagine you have a single convex lens. At a certain distance the object is in front of the lens, there will be a perfect image formed somewhere, at a certain distance behind the lens. This distance is dependent on something called the focal length of the lens. There is also a magnification associated with the image. If you move slightly away from that perfect image distance (while keeping the object distance from the lens the same), the image will look blurry.
The reverse is also true; if you keep the image distance the same but change the object distance, the image will look blurry. The depth of field characterizes the approximate small range of the object distance where the quality of the image is still sharp. Let’s say you shift the object distance from the lens slightly. The distance from the lens where a perfect image is located will also change. Thus, if you change any one of the object or image distance from the lens, the other has to also change to get a sharp image. The magnification will change too.
We know this is how lenses work due to some math involving modelling light as rays.
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