I hate crop factors!
Crop factors create more confusion than almost any other subject. Why? Because they get applied to a lenses focal length which is completely incorrect. The focal length of a lens does not change, no matter how big or small the sensor is.
What changes is the field of view. A bigger sensor gives a wider field of view, a smaller sensor gives a narrower field of view. The focal length stays the same and it’s important to know the true focal length of the lens as this will govern your Depth of Field and any DoF calculations you make must be based on the lenses actual focal length, not some converted equivalent focal length. Use an equivalent focal length and all your calculations will be wrong as may be any estimations of the expected DoF.
All you need to know is how the sensor size in the camera your using affects the Field of View. Compared to a full frame 35mm camera, for any given focal length, a super35mm film or video camera will give a 34% narrower field of view for any given focal length, so 1/3rd narrower.
Cinematographers and DoP’s with film based backgrounds don’t use crop factors. For them a 20mm lens is a 20mm lens, a 50mm lens is a 50mm lens. They know the field of view that lens will give them on their camera. If you start talking about “50mm equivalent” etc then it just gets confusing….. equivalent to what? The only thing a 50mm lens is the equivalent to is a 50mm lens. What changes is the camera or more specifically the cameras sensor size and as a result different cameras result in different field of views, but the lens itself does not somehow change focal length.
Sensor size does not change focal length, only field of view.
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