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Old 03-30-2013, 11:13 PM
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Default WHY THE APERTURE NUMBERS ARE WHAT THEY ARE (1.4, 2.8 etc)

Ever wondered? It's not too difficult really, so I'm going to indulge in a wee bit of tech talk which will hopefully give some understanding.

The aperture numbers that many will be familiar with are:
f1.4 which lets in double the light of
f2 which lets in double the light of
f2.8 which lets in double the light of
f4 which lets in double the light of
f5.6
f8
f11
f16
f22


So, each time we close the aperture down to the next number (or "stop") we are letting in half the light. To compare two different surface areas, where one is twice the area of the other one:
one of the circles (aperture) is 1 square unit (and what the unit is doesn't matter), and it has a radius of 0.564 units
and the other circle is 2 sq.units, and it has a radius of 0.79 units

We want to compare these two radii, as a ratio:
0.79 divided by 0.564 = 1.4 and this is our magic number. This tells us that if we want to double the surface area of a circle, then we multiply the radius by 1.4

1.4 x 1 = 1.4
1.4 x 1.4 = 2
1.4 x 2 = 2.8
1.4 x 2.8 = 4
1.4 x 4 = 5.6
and so on.

"Hang on" I can hear you saying, "if f1.4 is twice as bright as f2, why do the numbers increase with diminishing brightness, instead of the other way around????"

And quite right too. I'll defer to any better explanation that anybody else can give, but as near as I can tell this is just a convention to make the number comparisons more human friendly. After all, who wants to work with apertures (in diminishing brightness) of:
f1, f0.7, f0.5, f0.36, f0.26 etc
which is what they logically should be. So they decided that to express the numbers in a friendly way they would multiply by 1.4 rather than divide by 1.4. After all, it's only a reference comparison.



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