Focusing on Depth of Field and Lens Equivalents

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How much space in front of the lens will be in focus? That question defines Depth of Field – but this simple concept has lead to a staggering amount of confusion in today’s multi-format camera environment. Through some fundamental scientific demonstrations, we will clarify concepts like circle of confusion and lens equivalency.

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ERRATA
The ISO in regards to Lens Equivalents should be multiplied by ISO^2 because we're dealing with two dimensions not just one. Multiplying by just the ISO will result in slightly darker image (as see in the demo)

Some people have jumped to the conclusion that the Circle of Confusion is the Pixel Size. The CoC is LIMITED by the Pixel Size in that it cannot be smaller than the pixel size, but in most cameras the pixel size is much smaller than the CoC (or at least by a standard CoC that calculators use for determining depth of field). This explains why a Full Frame 12MP camera will have the same DoF as a 50MP camera. But if you enlarge the photo enough, eventually you will be able to see more detail in the 50MP image which less of what looked in focus in the 12MP image will actually be in focus.

Some people have commented that the video is a bit soft... when I'm large in the shot, it does indeed look soft - when I'm shrunk down, the video looks sharp. The lens has not changed - only the magnification of the image. This is a very real analog to what's happening in smaller sensors - when a small sensor crops in you can see better detail and it looks soft, when captured by a full frame, the image looks sharp. Therefor smaller sensors (given all other factors equal) have a shallower depth of field.