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General photography questions and answers: Discuss f stops....OK - prompted by Steve's 'Did You Know' thread, who knows the origin of the f number as in f2.8 ...
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Old 17-07-2005, 14:07   #1 (permalink)
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f stops.

OK - prompted by Steve's 'Did You Know' thread, who knows the origin of the f number as in f2.8 etc, and why are changes of aperture sometimes referred to as 'stops' ?

There's no definitive answer to either question by the way, the origins are lost in time, but there's a generally accepted origin for each.
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Old 18-07-2005, 16:41   #2 (permalink)
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You are right there are a lot of thing the f can stand for, I think it is just a simplified math function sign where you usually had the upper and lower thresholds so f over 32 as max under 3.5 as min to 5.6 as current (does that make sense?).
The word stop I believe I am certain comes from the first cameras where the apeture opening was "yanked" by turning a wheel with holes in the to stop them.
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Old 18-07-2005, 18:07   #3 (permalink)
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There is a definitive answer - The f comes from focal, as in focal length. The true way of writing an f stop is, for example, f/5 which expresses the aperture diameter as a fraction of the focal length. In this case 1/5th the focal length. Hope that helps.
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Old 18-07-2005, 18:44   #4 (permalink)
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Quote:
Originally Posted by Adrian
There is a definitive answer - The f comes from focal, as in focal length. The true way of writing an f stop is, for example, f/5 which expresses the aperture diameter as a fraction of the focal length. In this case 1/5th the focal length. Hope that helps.
You're quite right apart from it being the definitive answer... which it isn't.

A lens with a max wide open aperture of 50mm on a 50mm lens would be an f1 lens. It isn't substantiated in fact but it is generally believed to be the origin of the f number to denote aperture size.

The expression 'stops' derives from very early cameras before a mechanical iris was invented. They basically just inserted metal plates into a light trapped slot between the lens and camera. These plates each had a hole or aperture of different sizes and they thought of them as 'stopping' or admitting light depending on the size. Again it's just one theory, but as good as any for why the expression 'f stop' has survived today.
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Old 18-07-2005, 19:16   #5 (permalink)
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Quote:
Originally Posted by CT
You're quite right apart from it being the definitive answer... which it isn't.

A lens with a max wide open aperture of 50mm on a 50mm lens would be an f1 lens. It isn't substantiated in fact but it is generally believed to be the origin of the f number to denote aperture size.

The expression 'stops' derives from very early cameras before a mechanical iris was invented. They basically just inserted metal plates into a light trapped slot between the lens and camera. These plates each had a hole or aperture of different sizes and they thought of them as 'stopping' or admitting light depending on the size. Again it's just one theory, but as good as any for why the expression 'f stop' has survived today.
Well, my explanation of the origin of the f number was certainly definitive and substantiated as f/x where x is the focal length is the equation that has been used to standardise the measurement of the amount of light passing through an aperture since the variable aperture was invented. This is, however, just a method of comparing lenses and providing a reference scale, as the light transmission properties of individual lenses of equal maximum aperture vary. This is measured by factoring in the transmission factor to the f number and is expressed as the T-Stop number, although I think you'd be hard pressed to find anyone outside a lens manufacturer's labs using it.

What's not substantiated are the various theories on the "stop", but the explanation that it came from a simple light stop is, IMHO, the most plausible.
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Old 18-07-2005, 19:45   #6 (permalink)
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I agree that mathematically it's fairly certain that the aperture number is derived from it's relationship to the focal length. It's easy to make the assumption from that that f refers to focal length, but it's not necessarily so, and people have been arguing about it as long as I can remember. I did research this in depth yonks ago, and there are other theories for why the f was appended to the number, one being that it denoted 'focus' for the varying depth of field at different apertures, another being that it was derived from I think a latin word meaning literally 'window'.

Interesting nonetheless.
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Old 18-07-2005, 22:25   #7 (permalink)
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Originally Posted by CT
I agree that mathematically it's fairly certain that the aperture number is derived from it's relationship to the focal length. It's easy to make the assumption from that that f refers to focal length, but it's not necessarily so, and people have been arguing about it as long as I can remember. I did research this in depth yonks ago, and there are other theories for why the f was appended to the number, one being that it denoted 'focus' for the varying depth of field at different apertures, another being that it was derived from I think a latin word meaning literally 'window'.

Interesting nonetheless.
Ah - I see where you're coming from now.
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Old 19-07-2005, 08:38   #8 (permalink)
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....window is finistra in Latin.

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Old 19-07-2005, 09:20   #9 (permalink)
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....window is finistra in Latin.

regards
Ah yes, hence FenÍtre in French and fenestration, etc etc.
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