the meaning of "stop"

[jli]

I'm just going to say what I believe to be true, and let you guys correct me .

The term "stop" means a change in ISO, shutter, or aperture that results in a doubling or halving of the amount of light that reaches the film/sensor (with n stops resulting in 2^nx more/less light).

To get one more/less stop: double/halve the ISO speed; or double/halve the shutter speed; or make a square root of 2 change in aperture (so doubling/halving would be 2 stops).

Stops are equivalent to EV stops. -2, -1, 0, +1, +2 gives a range from 2 stops down to 2 stops up from what the camera meters as 0.

I feel like sometimes people use the term "stop" to mean a single step up or down in aperture or shutter. For example, they'll call a shutter change of 1/200 to 1/250 a stop down, even though a stop down would actually be 1/400. Is this common? Perhaps "stop" was just a typo for "step"?

Thanks!
John

[FrankLamont]

'Stop' actually is directive to aperture - originating from optics thus. Generally it is f-number or relative aperture, focal ratio or f-ratio that are more applicable.

In aperture, obviously the diagraphm of the lens would open a certain amount, which is the f-stop: a f-number is the focal length divided by the effective aperture diameter.

There of course you have the famous calculation, which is f-number = focal length/diameter.

Stops (f-numbers) tie into aperture stops: but they are really just basic units applied to title or quantify exposures. So, a stop means a factor of two. f-stops, in this general application, as in; f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, etc, run on root 2, but rounded off - not inclusive of the middle bits in between, such as f/6.3 and f3.5, etc. So shutter speeds also contribute to this, as does ISO - ISO directly goes in halving the amount of light (unless you manually adjust it to go off the course of 100, 200, 400, 800, 1600, etc.), while shutter speeds run on 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, etc - of course with fractional stops once again (like the aforementioned f/6.3 and f/3.5) like 1/20, 1/80 and so forth.