Why are lens apertures usually of only certain sizes? 1.4, 1.8, 2.8 and so on. Why not 1.6, 2.5 or any other sizes?
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For what it's worth, the second half of your question as stated is simply false - for example, here's an f/1.6 lens and here's an f/2.5 lens. – Philip Kendall Nov 22 '14 at 23:15
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Well obviously everyone is going to point out where I can find those mentioned lenses after I mentioned them but I'm talking about the vast majority of lenses. – connersz Nov 22 '14 at 23:17
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What leads you to believe apertures are only certain sizes? An aperture can be any size. Is it because those values are commonly marked on a lens, or marketed as the largest aperture available? Technically, an aperture can be set to 1.4, 1.53, 1.697, or any other value -- they're normally just rounded to a common/simple value. – Dan Wolfgang Nov 22 '14 at 23:51
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Also see What is an easy way to remember the full stop scale? – mattdm Nov 23 '14 at 00:46
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Well, it might sometimes be f/1.6, f/2.5, or other sizes (depending on the lens construction and on the exact f-stop used). Actually, f/1.8 fits in this "unusual" group.
You might have noticed the canonical series goes by powers of √2. {1 ; 1.4 ; 2 ; 2.8 ; 4 ; 5.6 ; 8 ; 11 ; 16 ; 22 ; 32 ...} It's just easier to remember.
What we're all calling "aperture" is relative aperture. It's related to the diameter. f being the focal length, an aperture of, say, f/2 means light travels through a disk of diameter f/2. When you open up one stop, you multiply that diameter by √2, thereby doubling ((√2)² = 2) the area of the disk (ie. doubling the amount of light, all the other parameters - ISO and shutter speed - remaining the same)
Nomaru
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The standard range is based on powers of √2 ≈ 1.4
It is 0.7, 1, 1.4, 2, 2.8, 4, 5.6, 8, 11 etc.
On the low end manufacturesrs tend to be a bit more precise (1.2, 1.8, 2.5 etc are outside the standard range). Consider that a commercial issue, the practical difference between F/1.8 and F/2 is (very) small.
The reason behind the √2 range is that the F number is based on the diameter (F = focal-length / diameter) but the amount of light is proprtional to the square of the diameter.
So the 1, 1.4, 2, 2.8 range is equivalent to and exchangable with the doubling/halving range of shutter speeds 1/250, 1/500, 1/1000, 1/2000.
So at a given light condition and ISO, F/2 and 1/500 will give the same exposure as F/1.4 and 1/1000.
Both are exponentional ranges, going one 'stop' up or down means doubling or halving the amount of light.
Henk Holterman
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At the smaller end, it's typical to use third or half stops. Half series: f/1, f/1.2, f/1.4, f/1.7, f/2, f/2.4, f/2.8..... Third series: f/1, f/1.1, f/1.3, f/1.4, f/1.6, f/1.8, f/2.0, f/2.2... – mattdm Nov 23 '14 at 00:50