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Explain.

A two full F/stop change to either side of the starting F/stop will either double or half the amount of light from the original F/stop.

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Isn't this really just an oddly-worded duplicate of photo.stackexchange.com/questions/49 ? – mattdm Feb 15 '11 at 2:36
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I'd tend to think that at the very least it's a 'subset' question of the two questions you cited... Having the understanding of f-stop that either of those questions and answers would convey would make this question unnecessary... – Jay Lance Photography Feb 15 '11 at 3:03
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I've moved "True or False" to the title, to prevent someone from reading the title (which is false) and accepting it as fact. – Evan Krall Feb 15 '11 at 7:12

2 Answers

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False. Going from f/2 to f/2.8 will halve the light; going from f/2 to f/4 will quarter it.

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The explanation: a difference of one f-stop means that the diameter of the aperture of the lens has been changed by a factor of the square root of two. Since the amount of light passing through the aperture is determined by the area of the aperture, and the area of the aperture is proportional to the square of its diameter, a one-stop change doubles (or halves) the area, letting in either twice or half the amount of light. – Stan Rogers Feb 15 '11 at 1:49
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Quite. We are really interested in the area of the aperture opening, but the f-stop describes the diameter. – Staale S Feb 15 '11 at 9:52
up vote 1 down vote

False.

A one full f-stop change to either side of the starting f-stop will either double or halve the amount of light.

Why? That's just the way f-stops were designed. One f-stop means a doubling or halving of light.

Note that when talking about aperture, that is stated relative to its diameter. Each f-stop increases or decreases the diameter of the aperture by a factor of about 1.4 (which is root 2). However, that causes a change in area of the aperture opening, hence the amount of light being let through, to be by a factor of 2, since the area of a circle is relative to the square of its radius.

So a sequence of full f-stops in f-numbers (for aperture) go 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.2, 16, etc with each stop letting in double the amount light of light as the previous stop, even though the diameter only needs to increase by about 1.4 each time.

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