How does letting more light into a camera with a wider aperture give a shallower depth of focus?

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How does letting more light into a camera with a wider aperture give a shallower depth of focus?

In: Technology

Well, this is much easier with a diagram than with words, but I’ll give it a shot. Let’s say you’re looking at a candle. The light from the candle heads out in all directions – 360 degrees. The lens captures some of that light, let’s say 10 degrees of that light… think of this as a cone of light. The lens (including the aperture) then brings the light into focus and brings it to a point, so you have a cone on the opposite side of the lens also. Now, if you increase the size of the aperture, you’re looking at a wider cone on both sides of the lens.

Now, ideally you want to place your film (or digital sensor) at the point of that 2nd cone… but realistically you can move it forward or backwards slightly… this is known as your depth of focus. If your cone is more shallow, you can move it forward and back more, so you have a large depth of focus. If you have a wide aperture, then your cone will be steeper, which means you can’t move the image plane forward and back very much before you get really bad focus. This is what leads to a shallower depth of focus.