f stops

Started Aug 5, 2010 | Discussions thread
joharis Regular Member • Posts: 419
Re: f stops

dprsok wrote:

'Each f stop lets in half as much light as the previous one, and since the amount of light a lens lets in is proportional to its area'

But you said proportional to its area. This is where I'm confused. The area for radius 12.5 = 491, and for radius 25 = 1963. The area is not proportional to the radius.

More importantly, my confusion is as to how the light passing through an area of 491 square mm is 1/2 of the light passing through an area of 1963 square mm.

It's not that difficult. The difference between 12.5 and 25 is not one stop but two stops. Take a lens with focal length of 100mm. The f number at 12.5 is 100/12.5=8. The f number at 25 is 100/25=4. That is two stops and thus a quarter of the amount of light. And 1963/491=4, so proportional. I hope I made myself clear.

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