|What is a "Stop"?|
about one full stop of difference between these two exposures.
I keep hearing you mention "stops," and I keep reading about it in magazines! What is it?
It's a way of talking about light levels and how to adjust for them in photography. A stop isn't an absolute, like horsepower, but a relation to a previous camera setting. Increasing exposure by one stop doubles the amount of light coming to the film or sensor; decreasing by one stop halves it.
Now, this is deceptive. When we think of doubling the amount of light, for instance, making a room twice as bright, we think it might make the room very bright, inducing a squint perhaps. But in reality, we can double the light time and time again before we typically begin to squint. Our eyes can see a fairly large range of light, assisted by both the iris and the sensitivity of the retina.
Film and digital sensors respond to a much narrower range of light than our eyes do, and once you go outside of this narrow range, bright things go to full white, and dark things go to full black. Ever look at a photo of someone taken at midday in full sunlight? The shadows under their hat or neck might go so dark that you can't see anything in there, can you? The difference between the darkest and lightest portions that can be captured in the same image, at the same exposure setting, varies depending on the film type or digital contrast settings, but generally range between five and ten stops. What this means is, with high-contrast film or settings, you often may have a choice of capturing detail in the bright portions of the image (the highlights) or the dark portions (the shadows,) but certainly not both. If you get good detail in the highlights, you will probably lose all detail in the shadows, and even something only moderately shady becomes solid black.
In the image at right, the white flowers are very bright in the direct sunlight, and the shadows beneath the leaves too far outside the range that the image can capture, so they go almost entirely black. It would be very easy for you to see what was under the leaves, and even read a book down there, if you'd been there personally. And I could have set the camera to capture the light down there easily — but then the flowers would have bleached out into a featureless blur. For many photographic situations, you end up choosing one or the other, or getting some extra light in there so the difference between highlights and shadows is much smaller, within the range that the film/sensor can capture.
But what does this have to do with stops? Well, the difference between the exposure you want, like that at right, and one you don't, where the flowers have gone too bright or too dark, is only a stop, maybe two at the most. Cutting the exposure by two stops, reducing the light to 1/4 of what it was, would turn the flowers grey. Increasing by even 1/2 stop would have wiped out the distinction of the petals and the faint evidence of dewdrops.
Your camera settings all revolve around stops. For instance, to increase exposure (brighten) by one stop, you can decrease the shutter speed to half of what it had been set for previously. Say it was set for 1/125 second — you'd double the light, increasing by one stop, by setting it to 1/60 second (some settings are rounded slightly for ease of use.) No, that's not backwards — the longer the shutter is open, the more light is coming in, right? So each time you double or halve the shutter speed, you're changing the exposure by one stop.
The same goes for ISO settings, or changing to a different ISO rated film. ISO 200 is twice as sensitive to light as ISO 100, so it only needs half as much light. So if you have a proper exposure at 1/125 second shutter speed with ISO 100, you will have the same exposure at 1/250 second at ISO 200. You need less light (by one stop) for ISO 200, so you can decrease the light, but have a faster shutter speed, by doubling the shutter speed to 1/250.
So each time you change the shutter speed by halving or doubling it, you change by one stop: 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000 — all of these are one stop apart. The same goes for ISO: 50, 100, 200, 400, 800, 1600.
Aperture, however, is slightly different, as I talk about here. Each of these aperture settings changes the light by one stop: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. There's rounding-for-convenience in there too. You'll notice that the numbers halve or double every two stops, and I explain that on that other page too.
The reason we use stops instead of absolute measurements like lux or lumens is that stops are handier. Those settings listed above all relate easily to one another, and the ability to relate one to another is called "reciprocity." Don't worry, you won't be quizzed on these words. All it means is that, if you change one setting, changing either of the other two by an equal amount in the opposite direction (reciprocal) will give you exactly the same exposure.
Let's say you want a photo of a fast moving horse and don't want it blurry. You know you need a fast shutter speed to stop this action, and you know that you will have a proper exposure in these conditions with ISO 100, f16 aperture, 1/125 second shutter. But you want 1/500 second shutter speed to stop the horse in midair. 1/500 is two stops less light than 1/125 second (1/125, 1/250, 1/500 — that's two steps between the settings,) so you need two more stops of light to come through some other way. You can change the aperture by making it two stops bigger — f11, f8, f5.6 — or you can change the digital ISO (or switch films) to get two stops that way — ISO 100, 200, 400, so it needs less light to get a proper exposure. Just choose one or the other, but not both.
Or, you know from reading the other page, that you want a much higher depth-of-field with an aperture setting of f32. Since that's two stops smaller/darker, you'll need to let in more light another way. Shutter speed goes from 1/125 to 1/60 to 1/30 to gain those two stops, or the ISO can once again go from 100 to 200 to 400.
Your camera may have settings in between those that I've listed. This is good, actually, in that it gives you more control. You may not want a full stop of difference in light, but perhaps only 1/2 stop or 1/3 stop — just a slight tweak, like you might want to do between the two examples at top. So that's what these extra numbers in between allow you to do.
The key thing to remember is, halve or double. You might use stops in exposure compensation, where you second-guess the camera's exposure reading because you know the scene better than the simple camera logic does. You can also now get exposure readings for the highlights and the shadows and know if your film/digital settings might capture the full range, or lose something. For instance, I know that Fuji Velvia 50 slide film has only about five stops of latitude, or the range of light levels it can capture at once. Sure, I can set the camera for really long exposures, or really short ones. But if the light level from two different parts of the subject exceeds five stops of difference, I know I will lose one or the other.
Okay, here's an example situation. We know in this case that our film has six stops of latitude, meaning we can only capture a range of detail that has six stops of difference. This green anole at left is in shade, backgrounded by bright sunlight on the foliage behind it, and spotted with bright sunlight peeking through the leaves. So we can measure the light off of the anole, and the background, and if it's less than six stops, we're good, right?
Wrong! That's six stops between black and white, the whole range of the film! But we want the anole to fall in the middle, so we can allow for only around three stops of difference.
So we take an exposure reading from the background: f22, 1/500 second. We won't bother with ISO because there's no need to adjust that unless we fall outside the range of camera settings on either shutter speed or aperture.
Then we measure light from the shaded portion of the anole's back. We get f8, 1/250 second. Yeah, I changed both of them, and the camera will often do this, so get used to it now.
So, hmmmm, that's three stops more light through the aperture (f22, to f16 to f11 to f8) and one stop more light through the shutter (1/250 being half of 1/500) — that totals four stops, or more than the image can capture if we keep the anole in the middle of the range. So, does it work better to darken the anole to capture the background, or keep the anole in the middle exposure and let the background bleach out? In this case, I went for the latter (actually, I grabbed this shot on the fly, so conscious choice wasn't part of it, but it serves to illustrate my purpose.) The background would have added little to the scene, and might even have made it distracting, while the anole was the key subject. Even so, the white spots from the bright sunlight, too bright for the film to capture, look a bit odd. Knowing this, I could also use a flash or strobe, or even a reflector, to brighten the anole in shade and bring the light levels closer together.
So now when someone or something refers to brightening up by a stop, or underexposing by a half-stop, you know what it means and what affect it will have on your photos. And when you see high contrast situations, you know that you may not be able to capture everything in the image, and how to check for it.
You can even measure the range of a particular digital setting on your camera (you can do this with film, too, but it wastes a bunch of frames and you can typically find it online anyway.) Get something in a middle tone, like a grey shirt. Take a large number of exposures, with the camera on manual, each exposure being one stop different — be sure this is well above and below the exposure setting the camera meters for. It helps if you put the camera on a tripod and don't move the subject so nothing changes between shots.
You will should end up with a range of images with the shirt running between pure white and pure black. Count the number of frames where you can just barely make out some color inside of pure white and pure black, and this is the exposure latitude of those camera settings. This will let you understand when you have scenes that might be tricky.