Questions · Cameras
What lens millimetres do I need?
The ones that make the scene you want to see fill the field of view at the distance the camera is at, no more and no less. Few millimetres open a wide angle and take in a lot of scene with little detail per metre; many millimetres close the angle and concentrate all the pixels into a narrow strip.
The way to get it right is not choosing the lens: it is choosing the width of scene. Decide what width in metres has to fit into the image at the distance the point is at —«the four metres of the doorway at twelve metres from the camera»—, and the lens comes out of that. Every manufacturer publishes a calculator that does that conversion, and anybody who designs seriously will show you that sum point by point.
The usual mistake is the opposite: putting a wide angle on everything because «you see more». You see more and you can make nothing out, because the width of scene shoots up and the pixels per metre collapse. And the mirror-image mistake is a very long lens on a pole that vibrates, because the tighter the angle, the more any movement of the mount is amplified.
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