Monday, February 7, 2011

Intro to Exposure Controls

This is a quick post on the three exposure controls on digital cameras and what they do so I can refer people to it when I wish to pontificate on the Interwebz. Feel free to read it: comments invited.

Digital cameras have three exposure controls. Three. That's it. That's all. Learn what they do and you've mastered exposure. At least the technical part.

They are:

1) Shutter speed.

2) Aperture.

3) ISO (Equivalent) Setting.

Those are the ONLY THREE THINGS that control the exposure of your photograph. However, they interact: changing one can make what the other ones do change as well. But fear not: it's entirely logical. There is another setting called "EV" which stands for Exposure Value, and I will talk about what that does, but it does NOT AFFECT the three settings here when you are in manual mode. It only matters when you are in an auto exposure mode such as Program, Tv or Av.

In full manual mode, you have full control over these settings and they will be whatever you set them to. That means if you tell the camera to take a thirty second exposure of the sun at noon on a clear day, it will let you, and you will burn out your image sensor. (If you are looking through the viewfinder you will burn out your personal image sensor, aka the Mark I Eyeball, too.) So don't do that.

Another technical clarification: the word "stop" is used a lot in photography, and not not just by models when Terry Richardson is on location. (Zing!) A "stop," in the generic sense, is a doubling of the exposure of the image. If you go up a stop or increase by one stop, you double the amount of light which contributes to the exposure. If you go down a stop or decrease by one stop, you halve it. Any of the three controls can be used to "stop up" or "stop down" in terms of total exposure. What really makes this maddening is that aperture is measured in "f-stops," which are related but not the same, and usually when a photographer says "stop up" or "stop down," they are specifically referring to changing the aperture value. After discussing each control, I will specify what would constitute a one-stop movement in both directions by means of that control.

Here we go.

1) Shutter speed.

The simplest and most intuitive. This is just "how long do you want me to open the shutter and let light into the camera?" It's measured in fractions of a second. Most digital cameras can shoot at fast as 1/500th of a second and many can go as low as 1/10000th of a second, which is pretty darn fast. To "freeze" a human walking at a normal pace, you want a shutter speed of 1/60th of a second or less. Any slower, and you will get "motion blur," which is just blur caused by the fact that the subject moved while the shutter was open. You should use the fastest shutter speed you can get away with to minimize both "motion blur," caused by the motion of the subject, and "camera shake," caused by moving the camera. (Nobody has perfectly still hands.) Shutter speed affects exposure linearly. Open the shutter twice as long, get twice as much light, get an image with twice as much exposure. Open it half as long, get half as much light, get an image with half as much exposure. Often, when shooting outdoors in bright sunlight, you will have no choice but to use a very fast shutter speed to keep your image from being overexposed, since there are limits to how low you can go with both ISO and aperture. Once you hit them, your only choice is to lower shutter speed. At a very high shutter speed, say 1/1000 or higher, you can shoot under almost any light you can bear to look at without overexposing your image.

Stop adjustment: Doubling your shutter speed (going from 1/30th of a second to 1/15th of a second) will increase exposure by one stop. Halving it (going from 1/30th of a second to 1/60th of a second) will decrease exposure by one stop.

2) ISO Equivalence.

This controls how sensitive your camera's sensor is to light. It usually starts at 100 (200 for most Nikon cameras) and goes up to at least 400, although some high end cameras go into the tens of thousands. It's also completely linear: take a picture at ISO100, and then take the same picture at ISO200 with all other settings the same. The second photograph will be twice as bright. Go from ISO400 to ISO200, and the image will be half as bright. The reason you don't always shoot at the highest ISO you can is that the higher the ISO setting, the "noisier" the image will be. Noise (also sometimes referred to as "grain" because it looks similar to film grain, though it's caused by a different phenomenon) can make images all but unusable at very high ISO settings, so you should always set the ISO as low as you can and still get the exposure you want, or at least at the lowest setting in which your camera has acceptable noise output. I usually have my Canon SLR's set to ISO400 as a default: when working in bright light or in a studio I lower it to ISO100. ISO100 can be pretty dim in anything but direct sunlight or under strobes, so it's safer if you leave it set up a notch or two in case you want to make some quick snapshots in unanticipated or unknown future lighting.

Stop adjustment: Doubling your ISO setting (going from ISO200 to ISO400) results in a one-stop increase in exposure. Halving it (going from ISO800 to ISO400) results in a one-stop decrease in exposure.

3) Aperture.

This is the one that usually causes confusion 'mongst photography newbies. The aperture is a mechanical device in your camera (in an SLR, it's in the lens: every lens has its own aperture) which opens and closes to control how much light comes in. It's basically the size of the "hole" that the light can come through. Increasing the aperture allows more light in: decreasing it lets less light in. However, because the hole is a two-dimensional circle, it's not linear in effect. Doubling the size of the aperture increases the amount of light that hits the sensor by a factor of four. (Because the area of a circle varies as the square of its radius.) To really make things fun, aperture is measured not in absolute units like square inches or square centimeters, but as a dimensionless ratio called an "f-stop." When you hear somebody say "stopping down" or "stopping up," they are talking about decreasing or increasing the aperture.

"But that's backwards," you may say, and you are right. The ratio is the ratio of the focal length of the lens to the diameter of the aperture. So if you have a 50mm focal length lens, and a 50mm aperture, your f-stop is 1.0. If you have a 50mm focal length and a 25mm aperture, your f-stop is 2.0. So you can see that going from f1.0 to f2.0 doubles the "f-number," but as I noted above, that means that you cut the amount of light to a quarter as much. (Because a circle with a diameter of 50mm has four times the area of a circle with a diameter of 25mm.) So when you increase f-number (go to a larger f-stop) you are decreasing aperture, and therefore decreasing exposure. Because of the physics of all this, the f-numbers are not linear and tend to be very weird numbers, and range dramatically. Your average beginner lens is going to have a maximum aperture of f4 or f5 or so, and a minimum aperture of f32 or thereabouts. A really "fast" lens, so called because it has a large maximum aperture and therefore allows faster shutter speeds, might be down to f1.4, f1.2, or even f1.0 although lenses at f1.0 are both rare and insanely expensive.

Now, just as with shutter speed and ISO equivalence, there is a tradeoff when discussing aperture. The bigger the aperture (the smaller the f-number) the more light gets in. So since you're letting in more light, you can use a lower ISO and/or a faster shutter speed. O frabjous day, right? Well, no. There are two downsides (or one downside and one mixed blessing) to using large apertures.

The downside is that lenses tend to have a "sweet spot" in the middle of their aperture range where their optical performance is maximized. My 50mm f1.4 (lenses are usually described by giving their focal length and their maximum aperture) gives a much clearer, sharper image if I stop it down to f2.0 or f2.2 compared to full-open f1.4. This also applies to the smallest aperture settings, but the difference can be profound at the bottom because of the huge amount of light coming in that the lens has to collate.

The mixed blessing is that the larger the aperture, the smaller the "depth of field." Depth of field indicates how much of the image is in focus relative to the object in clearest focus. If depth of field is large, everything or most everything in the picture will be in focus. If depth of field is small (narrow) then anything any distance from the object of focus will start to blur. At a wide-open aperture, a portrait can have the subject's eyes in focus but their nose may have significant blur. Now, obviously if you want the sharpest picture possible you want to use the smallest aperture you can get away with, but if you are actively trying to blur out extraneous objects (things in the background, etc) this can be a very effective tool. Unless it's a very low-light situation (in which case you open up your aperture to allow the fastest possible shutter speed) selecting aperture is more an artistic decision than a technical one, and the other two settings then become subservient to the aperture desired.

Stop adjustment: Going up one f-stop results in a one-stop increase in exposure. Going down one f-stop results in a one-stop decrease in exposure. However, the f-stop settings on the vast majority of lenses are fractional. (Remember, the aperture is built into the LENS. If your camera can use different lenses, it will have varying aperture ranges.) For a typical SLR lens, here is a one-third F-stop chart.

1.0 1.1 1.2 1.4 1.6 1.8 2 2.2 2.5 2.8 3.2 3.5 4 4.5 5.0 5.6 6.3 7.1 8 9 10 11 13 14 16 18 20 22

Every three increments on that chart (say, going from f1.0 to f1.4) is a full stop. Note that it is extremely nonlinear (it's based on powers of two.) So f1.0 to f1.4 is one stop, but so is f16 to f22! This is why aperture is so confusing to new photographers, especially those without a background in science or mathematics. To a mathematically oriented individual this chart is laughably simple and makes perfect sense. Everybody else basically needs to memorize it, or else just count clicks on their aperture control. (That latter is much easier. :) )

Bonus: EV Control.

EV control, also called exposure compensation or EV compensation, is a way of using auto-exposure calculation on your camera but still having some manual input. When you are in full automatic mode, the camera will select both a shutter speed and an aperture (and maybe an ISO, although most cameras use the manual ISO setting even in most auto modes.) You can set the EV to tell the camera to fudge a bit. If you increase the EV, or set it higher, you are telling the camera, "Figure out what you think is the best exposure, and then shoot a little bit brighter." The camera will decrease shutter speed (holding the shutter open longer) or increase aperture (letting in more light) or both, depending on its programming. If you decrease EV, it will do the opposite. This can be very useful both for artistic effect and because cameras, while they're pretty smart these days, don't actually know what you're shooting or what you care about in the image. If the main subject is well lit but the rest is dark, it may overexpose the subject trying to make the whole picture even, or vice versa. In that case you'd lower the EV so it would expose for the main subject properly. If the whole image area is very bright, you might actually want to increase EV so the camera doesn't make it dingy trying to even it out. (Cameras are notorious, for instance, for underexposing snowscapes because they think they're brighter than they actually are. When shooting outside in winter, always see if upping your EV doesn't brighten your snow.)

If you're in Program Mode or full-autoexposure mode, the camera will take your EV setting and do as it deems best. Might open the aperture, might lower shutter speed, whatever. You have no say in the matter. Where EV is useful to the fairly serious photographer is in the two modes known as Shutter Priority (abbreviated Tv, for Time Value) and Aperture Priority (abbreviated Av.) These are semi-automatic modes where you manually set one value (shutter speed or aperture, respectively) and ask the camera to figure out the other value according to its programming. Say I am shooting a basketball game, but the light's not even so I can't set to full manual. I could set to Tv, and tell the camera I want 1/200th of a second shutter speed (which will freeze even a fairly fast-moving athlete with no motion blur.) The camera will adjust the aperture according to the light it measures every time I take a shot. Further suppose that I like my pictures dark and broody: I could tell the camera to use an EV of -2, which translates to: "Calculate the exposure according to my set shutter speed of 1/200, and when you pick an aperture, then stop it down two more EV values so the picture is darker than you think it should be." Contrarily, suppose I am shooting a portrait and I want a soft, dreamy picture with a very narrow depth of field. I can set to maximum aperture in Av mode, and then up my EV so my picture is bright and cheerful compared to the neutral exposure the camera would otherwise try to achieve.

EV is usually in fractional stops, but varies, you'll have to read your manual to know how your camera does it. Mine does it, as do most SLR's, as a third of a stop per EV point. On full-manual, it does nothing, because there is nothing for the EV to "adjust." The camera is going to shoot at the shutter speed and aperture you programmed. But it can be very useful in the semi-auto modes.

Stop adjustment: It depends on your camera's settings. Most cameras allow a one-stop increase or decrease in EV, with both directions having three fractional stop adjustments. If this is the case, every click you move the EV up from zero increases exposure by a third of a stop, with movement to the end of the range (usually three clicks) resulting in a one-stop increase in exposure from the camera's calculated target exposure. Note that this is a relative change, unlike the other three stop adjustments: it changes it relative to what the camera thinks it should be. If the lighting changes, so will what the camera thinks the exposure should be, but you will still always get the same relative adjustment to that calculation.
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