What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 55,704
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. 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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Thanks for that, yes I had left that hanging, and just to add, the 'stops' value is the log2 of that ratio, in the case of  this example, log2 1.65 = 0.7 stops (usually thought of a 'two thirds of a stop' or sometimes 'three quarters of a stop' depending on the f-number sequence being used).

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