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Exposure Basics II:
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This ability to control sharpness, and the choices made in aperture settings, is one of the most creative aspects of photography. It has a profound effect on the image and how you depict subjects within the context of the overall scene, and can be applied critically by using the depth of field preview function on your D-SLR. It is a key image effect.

As we explore various exposure scenarios we will continually apply aperture and shutter speed values to attain different image effects.

Shallow depth of field can be used to create visual attention on the foreground subject by having the background go “soft,” or drop off in sharpness. These plants would have been “lost” in the thickets of the background if a deep depth of field were used. Setting the lens at f/4 and focusing on the plants yielded this sharp/soft relationship, an effect enhanced further by using a telephoto lens.

You can play with depth of field even more by using special lenses, such as a PC (Perspective Control) lens or, as shown here, with an optical device such as a Lensbaby. The radical shift in the sharp/unsharp relationship can make for fun and interesting effects.

ISO, Or Light Sensitivity
The ISO number defines just how sensitive the sensor is to light. The degree of sensitivity any given “ISO” delivers is difficult to put your finger on, as it is a standard created by scientists and not something intuitive to the eye or mind. It is stated as a number, with ISO 100 being the lowest in most digital camera systems (ISO 200 in some Nikons). The term “ISO 100,” for example, means nothing onto itself, but in the context of the scene, brightness, aperture and shutter speed values it is a very key element in determining exposure and exposure values. It is part of an elegant, balanced system of exposure.

When the light gets low and the shutter speed gets slow it is a good idea to ensure a steady shot by using a high ISO setting. One of the real advantages of digital is that you can change ISO on every frame. This twilight shot was made on the docks at ISO 1000.

ISO poses part of an exposure solution to a given light level. For example, at ISO 100 on a bright day the correct exposure is usually around f/16 at 1/125 sec, or the so-called “sunny 16” rule. (This says that if your meter is broken and you have to set exposure yourself and it’s sunny out with the sun coming over your shoulder you can set the ISO at 100 and have a great exposure at f/16 at 1/125 sec—and it works!)

The sensitivity of the sensor is calibrated by your setting an ISO number. In round numbers, many cameras offer a range between ISO 100 and 1600, with some going up to ISO 3200 and beyond. Every time you double the speed, or ISO, you are in effect doubling the sensitivity of the sensor, or adding a “stop” of sensitivity to light. But this doubling of sensitivity only makes sense in the context of the aperture and shutter speed settings, which control the amount of light reaching the sensor.

There are times when flash is not allowed or would ruin the character of the shot, and that’s when high ISO comes into play. This photo inside a New Orleans curio shop was made at ISO 2400 handheld.

So, if for any reason you need more or less light to affect how the aperture and shutter speed are set, you simply raise or lower the ISO setting in the camera. Go to a higher number for more light sensitivity (when you need a faster shutter speed or narrower aperture) or a lower ISO for less light sensitivity (when you want a wider aperture or slower shutter speed).

You might think all shots made in low light or after sunset require a high ISO, but that’s only if you shoot handheld. Mount the camera on a tripod and you can shoot at lower ISO settings, which generally yield much less “noise.” That’s the case with these low-light shots. The classic Las Vegas neon cowboy was photographed at ISO 200 and the fireworks at ISO 100, both on tripod, albeit with slow shutter speeds.

In general, you will usually need a higher ISO setting in low light and want a lower ISO setting in bright light. Why not just set the highest ISO for every shot? Another rule to keep in mind is: the lower the ISO setting the better the quality of the image, all else being equal. That’s because to get more light sensitivity a gain, or additional charge is applied across the sensor. As you go higher in ISO this gain adds more noise to the image.

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