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I understand what F-stop and T-stop mean, but I've also heard the term "stop" used like this:
the background is one and a half to two stops darker than the key light
What does "stop" mean in this context, and, looking at an image, how would I determine how many stops there are between a bright section and a dark section?
Stops are all about how much light you capture with the camera. One stop more is twice as much light. One stop less is half as much light.
What does "stop" mean in this context, and, looking at an image, how would I determine how many stops there are between a bright section and a dark section?
When something such as "... the background is two stops darker than the key light..." is said it is usually in the context of metering the scene, not looking at a completed image. If it is said in reference to a finished image it is being used more as shorthand to say that there appears to have been a certain amount less light on the background than on the area illuminated by the main light. Although it is possible to use software tools with digital images or even colorimeters with a physical print to measure the exact difference, in normal practice the measurement is done of the scene itself, not the result.
When speaking about stops in terms of light sources or different areas of the scene, one is referring to the difference in exposure value needed to render each area of the scene at an exposure level that would properly expose the same object (e.g. an 18% gray card).
Assuming constant ISO and shutter speed, if your reflective light meter pointed at the area directly under the key light says use f/8 and the same light meter pointed at the background says use f/4 then you can say that the background is 2 stops slower/darker than the key light. This is because f/4 is two stops slower than f/8. If you're using an incident light meter the same would be true if you place it under the key light and get a reading two stops brighter than when you place it under the light falling on the background.
If you are using your camera's built in light meter, which measures reflective light, use the smallest metering circle available. It is usually labeled spot metering and will cover anywhere from 1.5% to 3% of the frame on some of the higher end models and about 6% to 9% of the frame on lower models. Be sure to place the spot over the area you wish to meter in your viewfinder.
The “stop” traces to an early mechanical method used to adjust the working aperture diameter. John Waterhouse, in 1856 devised a set of metal slides, each with a different size hole, These “stops” were inserted into a slot in the lens barrel. The set allowed exposure adjustment with a doubling or halving of exposing energy that is allowed to play on the light sensitive material. They were first used in an enlarger lens and then the camera lens. This was the forbearer of the modern f/number system.
Photo scientists adopted a ½ stop system to test film to determine its responses to light. Typically, film is exposed to a series of 21 steps, each a half f/stop. After the film is developed, each step is measured and a paper graph is made. The graph displays the amount of blackening the film has and the scale of the material. A modern film, so tested, typically displays clear film for the first step and density maximum for the last step. In other words, a typical film has a density range of about 10 f/stops. You should look up the Zone System to get a feel for the scale of film; this roughly corresponds to what we talk about as dynamic range in digital photography.
This increment (“stop”) of adjustment can be made in many ways.
Adjust working diameter of the lens (f/stop).
Adjust light transmission of the lens with neutral density filters
Adjust the intensity of the light reaching the subject – we can adjust the intensity of the light differently for different areas such as principle subject, backgrounds foreground, and shadows (via a fill light).
We can adjusts the sensitivity of the digital sensor by changing the ISO setting.
We can push or pull film during the developing process.
We can alter the range of the scale to gain contrast. The result is a loss of some of the tones of the scale.
We can alter the range of the scale to reduce contrast. When we do this we forfeit a strong black or a stark white.
By tradition we call these modification “stop” changes. Sometimes the full stop, a doubling or halving is too coarse, we resort to 1/2 or 1/3 stop increments of adjustment.
It takes lots of experimenting and “doing” to get a handle on how these changes affect film or the digital image. Judging this increment on a print is more difficult. We do have instruments that will measure the amount of blackening or the intensity of colors. Working with film and prints, this is called the science of densitometry. This can all be applied to digital imaging. Hope this helps!
the background is two stops darker than the key light
1) You take a measure of the incident light, for example with an incident light exposimeter. First on your background. It says f/5.6
2) You take a measure near your subject, for example a face. It says f/11
This means: "The light over there, at the background is not that much, use a wider f/stop so you can have more light, and over here at the face close your aperture to f/11 because if you do not do so your face will be blowned".
Conclusion 1, the background light is two stops less than the Key light.
You could measure the exact same thing as the case 1 using not an incident light metter, but using a gray card. Put your card on one spot, and measure it on the camera, and do the same near your subject; main light.
The camera is comfort with this graycard "I will not take a photo of this gray too dark or too light" The camera say.
Cameras are dumb (sorry cameras) so they do not know if what you have in front is a bright sky or a dark soil. All they see is amount of incoming light to the camera, reflected light.
The two previous cases refers to the incoming light and reflected light on a controlled target. But not all objects in the universe are gray... are they? At least not on your picture.
Some you need them darker, like the interior of a farm, or lighter like the skin of a caucasic lady.
If you let the camera decide, it will try to expose whatever has infront to achive a middle gray. The exposure on the face could say f/8 which would be ok for the skin on that situation (a gray card is simmilar to caucasic skin)
But the interior would read f/4, so it is saying, "Whatever dark subject or poorly iluminated place this is, for me to show you as grays you need to expose it at f/4". The camera thinks all objects are gray cards.
When you expose your lady at f/8, your background will be darker... 2 steps darker. Which is not bad, it is simply... darker.
Intrinsec object color.
Imagine the "United colors of Benetton" campain, and imagine that you forgot your incident lightmeter.
If your center model has a darker skin (African model) and you use that measure as middle gray the camera will think, "oh we do not have much light, lets expose more" and your blond model will look like a gohst.
Some skins are one or two stops darker than others.
This is not a racial thing... (probably it was at some time in history) but now days is a coincidence to the middle gray of a graycard.
