What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
D Cox Forum Pro • Posts: 18,680
Re: What the heck is an f-stop?

GeraldW wrote:


Bob has done a great job explaining things. The one item left hanging is the relationship of lens speeds. Since the f number is a linear ratio of aperture to focal length, and the light transmitted by the lens is proportional to the area of the aperture; you need to square the f/# to aseess relative light gathering.

The light transmitted by the lens is proportional to the area of the aperture, but it is then spread over an area of image that is proportional to the square of the focal length (because a longer length lens is further from the image plane).

So the illuminance is directly proportional to the f number. That is the reason f numbers were introduced -- so that you can match up exposures with different focal lengths.

So, 1.8^2 = 3.24, and 1.4^2 = 1.96; then the relative light gathering is 3.24/1.96 = 1.65. The significance of 1.65 is that it is the ratio of shutter speeds needed to gather the same amount of light a those apertures. So, if the f/1.8 lens is shooting at 1/100, then the f/1.4 lens can use 1/165. In digital, it might also be the ratio of ISO settings needed to maintain the same shutter speed.

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