The Exposure Triangle

There are three controls that essentially determine what sort of image you're going to get when you take a picture:

  • Aperture
  • Shutter Speed
  • ISO

If you shoot in manual mode, you can control all of these independently. If you shoot in aperture priority, shutter priority, program mode, or one of the auto settings, the camera will set some or all of these on your behalf. But even if you rely on the camera to choose your setting, you need to understand what those settings mean, so you can know when to override them.

Controlling Exposure

Aperture, shutter speed, and ISO can all be used to increase or decrease the exposure of an image.

Aperture refers to the size of the hole through which light is passing. The size of the hole is varied by blades in the lens which come together to form a larger or smaller opening. Larger means more light comes through. Smaller means less light comes through. Aperture is measured in f/stops, which refer to the ratio of the physical aperture to the focal length of the lens: f/2, f/2.8, f/4, f/5.6, f/8, etc. The smaller the f-number, the larger the aperture, and vice versa.
Shutter speed
The shutter covers the sensor. When taking a photograph, the shutter opens to allow light to pass through. The longer the shutter is open, the more light gets in. Shutter speed is measured in fractions of a second and in seconds. (When displaying seconds, the camera uses a ")
ISO refers to the sensitivity of the medium used to capture the image. With film, you choose the sensitivity by choosing your film emulsion. With DSLRs, you can choose the sensitivity for each shot. High ISO means more light is recorded by the camera. (Note: If you have auto ISO turned on, remember that you need to keep in mind what ISO settings you're using.) Using a high ISO will introduce grain (in film) or noise (in digital). These effects can be undesired so in general the lower the ISO the better.

What do you mean by "stops"?

In photography, a "stop" is a unit used to quantify ratios of light or exposure. A difference of one stop corresponds to doubling or halving the exposure. This is also referred to as a unit of EV (exposure value).

With most Nikon lenses which have aperture rings (used to set the aperture of the camera), there are markings for the standard full f-stops. Turning the aperture ring from one stop to the next either doubles the exposure (lets in twice as much light) or halves it (lets in half as much light). New Nikon bodies (such as the D40/x D60) will display an error (-EE) if the user attempts to change the aperture on the lens. This is now all done in body.

Full f-stop values
f/# 1 1.4 2 2.8 4 5.6 8 11 16 22 32

Some lenses, however, have non-standard maximum apertures, and the difference between the first marking on the lens and second marking on the lens may not be precisely one stop. Examples include the 55mm f/3.5 and 105mm f/2.5.

With the D40/x/D60, unless the lens is a non-metering type, the aperture is controlled on the camera, and is adjusted in 1/3-stop increments. Three clicks of the command dial are needed to achieve a difference of one stop.

Moving from f/4 to f/2.8 allows twice the amount of light in keeping the shutter and ISO constant. This works the same for any of the other values in the Exposure Triangle. Keeping, aperture and ISO constant, moving from 1/250s to 1/125s allows twice the light in. Finally, keeping aperture and shutter speed constant, moving from ISO 200 to ISO 400 allows twice the light in. This works for any full f-stop value and works opposite when moving from f/2 to f/2.8, where half the light comes in.

Full ISO values
ISO 50 100 200 400 800 1600 3200 6400 12,800 25,600

When working with the D40/x/D60, ISO — unlike aperture and shutter speed — is adjusted in full stops.

Understanding how to adjust exposure in terms of f-stops is not as important with a modern camera as it once was when working with manual cameras. However, having an intuitive grasp of how much shutter is needed to compensate for how much change in ISO, etc., is important if you want to be able to quickly and accurately control your results.

And these concepts are essential if you are using older non-metering lenses on the D40/x/D60.

Controlling DOF, Motion, and Grain

Aperture, shutter speed, and ISO also each have other effects. Understanding these effects is very important:

Aperture controls depth of field, meaning "depth of field of focus," meaning how much of the image appears to be in focus. The smaller the aperture, the greater the DOF. The biger the aperture, the shallower the DOF. Shallow DOF is usually used to isolate the subject from the background; great DOF is usually used in shots where everything needs to seem in sharp focus. For more on DOF, see the Cambridge in Colour tutorial, which is extremely thorough.
Shutter speed
Shutter controls motion. If your shutter speed is too slow, you may not be able to shoot handheld without camera shake causing blur. The traditional rule for film photography is that you should not go slower than 1/(focal length of lens) seconds. However, that is a very approximate rule; you should do some test shots to determine for yourself how slow you can handhold a shot. Shutter also controls subject motion. If you're shooting a moving object and you need to freeze its motion, you need to use a fast shutter speed. If you're shooting a moving object and you want to exaggerate its motion by blurring it, or blur the background with panning, you need to use a slower shutter speed.
Noise. Higher ISO means more noise. (In film, it means more grain.) If you want to get the least noise, make sure to use the lowest usable ISO. If you want to emphasize noise for effect, increase your ISO. (If you want to get low-noise shots in low light, you should also make sure not to underexpose if you can avoid it. You'll generally get less noise with a correctly exposed ISO 1600 shot than with an underexposed ISO 800 shot.)

Additional considerations

Critical aperture
Most lenses perform better or worse at different apertures. When shot wide open (at the largest aperture) most lenses are not as sharp as they can be, and some also have issues with vignetting or chromatic aberration. Most lenses perform best stopped down 1-2 stops from wide open. (If you have a lens with widest aperture of f/4 or so, try shooting at f/5.6-f/8.) But don't stop down too far (past f/11 or f/16 or so) unless you need to, because diffraction can reduce sharpness at smaller apertures. When in doubt, do some test shots with the actual lens and determine for yourself the sharpest aperture(s) for your lens.
Mirror slap
With SLR cameras, there are some shutter speeds at which the force of the mirror box swinging out of the light path will cause camera shake even when shooting on a tripod. With many cameras, there is the option of locking the mirror up in advance to prevent this problem, but, sadly, not with the D40/x/D60. The only solution is to use a faster or slower shutter speed. This is usually not a problem, but sometimes (if shutter, aperture, and ISO are all critical) it can make it impossible to get a shot. The precise speeds at which mirror slap become an issue will vary; a solid tripod and a moderately hefty lens will definitely help dampen the resulting motion.
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