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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A lens system focuses light from an object at a certain distance onto a focal “plane,” a surface of a specific size and shape. Light that emanates from farther away reaches the lens at a shallower angle, and achieves focus before the focal plane; by the time it reaches the focal plane, it is out of focus. The lens bent this light too much.
Light that emanates from closer reaches the lens at a sharper angle, and fails to achieve focus before reaching the focal plane. The lens didn’t bend the light enough.
The range of distances within which an object’s image will appear in focus is called the “field” of focus, quantified by its “depth.”
It each case, it’s important to remember that even when objects are closer or farther than the objective focal distance, some light reaches the lens at the correct angle and crosses the focal plane in focus. However the rest of the light travels outside of this “cone” and is what conteibutes to an image’s blurriness.
Just like a pinhole camera, there is a point within an optical system through which all light that follows an angle needed to arrive at the focal plane travels. If you close an iris around this point, it blocks off-angle light and sharpens the image formed by all objects, no matter the distance. The trade off is that the image dims. This is known as “increasing the depth of field.”
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