f numbers - THE TRUTH

david1701

david1701

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What do f numbers stand for???????????????

I can't see a way that doubling f4 to give f5.6 correlates to doubling f1.2 to give f1.8 mathmatically speaking.

The amount of light should be proportional to the area right so is the f number the radius or diameter? I tried modelling that one and couldn't get an explanation.

Someone suggested to me they were a coefficient of cost which made me laugh but can't be true.

I tried running the numbers and got f as a constant(ish) that varies between 0.7 and 0.8

Has anyone else looked at this before and gotten an answer?
 
focal length/value = diameter of aperture

50mm/4 = 12.5mm, area = (12.5^2)*pi= ~490
50mm/5.6 = 8.92mm, area (8.92^2)*pi= ~249

So f/5.6 is roughly half the light of f/4
 
aaaaah that makes so much sense I knew I was missing something and it was focal length.
Cheers guys that had been bugging me for a while
 
david1701 said:
aaaaah that makes so much sense I knew I was missing something and it was focal length.
Cheers guys that had been bugging me for a while
Click to expand...

If you get a zoom lens, set an aperture other than wide open, hold the DoF preview button and twist the zoom. You'll heard the blades moving, even though the aperture is the 'same'.
 
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