Photography: What is "a stop"?

Question

...?

No, clearly, I mean A-stop.

Answer 1

It's an arbitrary measurement of relative exposure.

The difference in sensitivity between ISO 100 film and ISO 200 film, for example, is 1 stop.

Also, the difference in exposure between an aperture of 5.6 and 8(keeping all else constant) is a stop.

The difference in exposure between a shutter speed of 1/125 and 1/250(keeping all else constant) is a stop.

Answer 2

A stop has several meanings in this context, which is why there's some easy confusion.

"A stop" is a measurement of exposure. The exposure itself is determined by 3 factors: 1) the ISO setting 2) the shutter speed and 3) the aperture, which is also called an "f-stop."

Thus, when you see or hear something like "you need to raise your exposure by a stop" or "you need to stop down your lens," the context is very important.

For example, under the phrase, "you need to raise your exposure by a stop," what that means is that you have to take your current settings for shutter speed or aperture and make a change while keeping the ISO constant. Raising the exposure by a stop means letting in more light, so you'd either have to use a slower shutter speed (like going from 1/125 to 1/60, which is 1 stop) OR use a wider aperture (for example, going from f/8 to f/5.6 in f-stops).

Likewise, for "lowering your exposure by a stop", you'd again have to adjust shutter speed or aperture to change the overall exposure (i.e. going from 1/125 to 1/250 OR from f/8 to f/11).

To confuse matters, you sometimes here the phrase "stopping down" or "stop down" or even "open up". This is a specific reference to your aperture setting measured in f-stops. Going in 1 stop increments, biggest hole to smallest hole: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44, 64. There will be settings in between and your lens may not have apertures at the far ends of this scale. The smaller numbers let in more light; the bigger ones let in less light.

The point for you to understand: a stop is a relative measurement of exposure and it reflects a relationship to exposure from the shutter speed and the aperture.

Answer 3

"Stops" can be anything from shutter speed, ISO settings, aperture settings, and exposure compensation settings. It can also mean the difference in the physical distance between a flash and the subject.

Shutter speed -- 1/60 to 1/125 is one stop less light or 1/60 to 1/30 is one stop more light.

ISO (ASA) -- 100 to 200 is one stop less light -- 100 to 50 is one stop more light.

Aperture setting - ƒ8 to ƒ11 is one stop less light -- ƒ8 to ƒ5.6 is one stop more light.

Exposure compensation -- Normal to +1 is one stop more light -- Normal to -1 is one stop less light.

Flash distance - 8 feet away to 11 feet away is one stop less light -- 8 feet away to 5.6 feet away is one stop more light.

By the way, the lens opening can be measured in terms of the focal length of the lens at infinity divided by the ƒ-stop selected will equal the size of the opening. A 50mm ƒ2.0 lens will have an opening of 26mm. The same lens at ƒ8.0 has an opening of 6.25mm. Photography involves the use of one number -- 2 -- either used as a multiple or the square root -- 1.414. In terms of time it is the multiple of two (or half), but when calculating area, such as aperture or prints, the square root of 2 (1.414) is the key.

Examples: Aperture (area) ƒ8 times 1.414 = ƒ11 -- ƒ11 times 1.414 = ƒ16

Prints (area): 8x10 print made to 11x14 print is 1.414 times bigger. 8 times 1.414 = 11 and 10 times 1.414 = 14.

Answer 4

If you mean an f-stop, that is an aperture setting on the lens.

On F-1, the aperture diameter equals the distance between the lens and the film plane. On F-2, the aperture diameter is 1/2 that distance, at F-3, 1/3rd... You get the idea.

The lower the f stop, the shallower the focus depth. So when you see pictures where stuff in the background is out of focus while the subject is clear, it's been show at a low f stop. A high f stop, like f-22, will have almost everything in focus.

Note that more light hits the film at low f stops, so your exposure times are shorter.

Answer 5

Thumbs Up to Ben

Thumbs Down to Jas

Jas, the f-stop is a measurement of the size of the opening in the diaphragm in the lens. At f1.2 the lens is "wide open", admitting the maximum amount of light; at f22 its "stopped down" and admits very little light.