f stop and crop sensors

Started May 11, 2010 | Discussions thread
Crocodile Gena Senior Member • Posts: 1,017
The *crucial* difference between aperture and f-stop

SilverbirdFLG wrote:

Aperture (f/stop) is the size of the opening inside your lens through which light passes. A higher f/stop number equates to a smaller opening inside your lens. As this is completely a function of the lens, the size of your sensor will not affect it in any way.

While I'm fully aware that people use the terms "aperture" and "f-stop" interchangeably, they are, in fact, two very different, but related, measures. The "aperture" is the diameter of the entrance pupil of the lens, and is measures in mm. The "f-stop" is the ratio of the focal length and the aperture diameter: f-stop = focal length / aperture diameter.

The aperture, in combination with shutter speed, determines how much total light that reaches the sensor. This is of central importance because it is the total light, along with sensor efficiency, that determines the total image noise (not ISO or sensor size, as most people think).

The f-stop, on the other hand, determines the intensity of the light falling on the sensor, and, in combination, determines the density of light falling on the sensor. This is important, because the density of the light is the exposure.

So, while neither the focal length nor the aperture diameter are affected by the body the camera is on, the equivalent focal length is affected, and since the equivalent focal length is affected, the equivalent f-stop is also affected. So, if you multiply the focal length by the crop factor to get the equivalent focal length, then you must also multiply the f-stop by the crop factor as well to get the equivalent f-stop.

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