What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
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GeraldW Veteran Member • Posts: 6,922
Re: What the heck is an f-stop?

Paul,

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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Jerry

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