Saturday, April 19, 2014

Photo Tips for Mom: The exposure triangle

There are three main camera settings that affect the exposure of your photograph. Each of them has side effects, in addition to their effect on exposure.

Exposure is measured in steps or (colloquially) stops, which are factors of 2 in brightness. For example, a difference of 3 stops is a difference of 2×2×2=8 in the amount of light.

Shutter speed:

This controls how long the shutter is open, exposing the sensor to light. The longer the shutter is open, the more light gets to the sensor, and (all else being equal) the brighter the photograph will be. Shutter speed is usually given as a fraction of a second, for example 1/250.

The side effect of shutter speed is that the longer the shutter is open, the more the camera or the subject can move during the exposure. When the camera moves, the result is usually a shot ruined by camera shake. When the subject moves, it will be blurry (sometimes this is intentional, to show motion, but often it’s not).

A rule of thumb from the 35mm film days was that to avoid visible camera shake, you should keep the shutter speed faster than 1/the focal length of the lens. For example, for a 50mm lens, shoot at 1/50 or faster. It’s really the lens field-of-view that matters, though, and because your E-PL5 has a smaller sensor than 35mm film, it has a narrower field of view than 35mm for a given focal length. The corresponding rule of thumb for your camera would be to keep the shutter speed faster than 1/(2 × the focal length). For example, for 150mm focal length, try shoot at 1/300 or faster.

Your E-PL5 actually has an in-body image stabilization system to reduce the effects of camera shake, so you may find that you can shoot at slower speeds than the rule-of-thumb suggests. But the stabilization system can’t prevent blurring if your subject moves!

Aperture:

The aperture is an adjustable opening in the lens. When it is “stopped down” to a small size, less light gets to the sensor, resulting in a darker photograph (all else being equal).

Aperture is measured in f-numbers, which are basically the ratio of the lens’s focal length to the size of the opening. Larger numbers correspond to a smaller aperture. For example, f/1.4 is a large aperture (allowing lots of light) and f/22 is a small aperture.

To change the exposure by one stop, or a factor of two in light, change the aperture by a factor of the square root of 2, or 1.4. For example, f/1.0 and f/1.4 are one stop apart, as are f/4.0 and f/5.6. The sequence of f-numbers (one stop apart) goes: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0, and so on. Your camera can also be set to apertures that are intermediate to these values, for example f/6.7, which is a half-stop between f/5.6 and f/8.0.

The side effect of aperture is that it also controls the depth-of-field, which is how much of the subject is in acceptably sharp focus. A large aperture, like f/1.4, has shallow depth of field, so only a narrow band is in focus. This can be good for isolating a subject from the background, which we often want to do in portraiture. You have to focus accurately when working with a shallow depth-of-field. For people and animals, that usually means focusing on the eyes (specifically, the eye closest to the camera, if the head is turned).

A small aperture, on the other hand, results in a large depth-of-field, allowing subjects to be in acceptable focus at a wide range of distances. This is often important in landscape photography, where we want everything “from here to infinity” to be sharp. At very small apertures, an optical effect called diffraction will actually start to degrade the sharpness of the lens, so we don’t want to go too small. On your E-PL5, diffraction will start to be to have an effect around f/8, and f/11 or f/13 is perhaps the limit of how far you should stop down. When working with a large depth-of-field, remember that subjects both closer and farther than the point of focus will be acceptably sharp, so focus on a part of the subject that’s at an intermediate distance.

There are other factors besides aperture that affect depth-of-field, which we’ll discuss in another installment.

ISO (Sensitivity)

The ISO setting of your camera corresponds to film speed in the days of film. A film with a speed of 64 or 100 was a fairly slow film. That meant you needed a lot of light, or long shutter speeds and wide apertures, to get a good exposure, but you were rewarded with fine grain and sharp details. A fast film, which was 400 or 800 in the film days, was usually grainy.

Digital sensors have an advantage over film in that you can change their “speed” or sensitivity on every shot. When you change the sensitivity, the amount of light hitting the sensor doesn’t change—that’s controlled by the aperture and shutter speed. Instead, the electronics in the camera amplify the signal coming out of the sensor. But they also amplify noise, so when you’re shooting with higher ISO speeds, you’ll probably find more digital noise in your images, which is kind of like film grain.

The good news is that digital sensors perform much better at high ISO speeds than film did. The slowest speed on the E-PL5 is ISO 200. ISO 400 and 800 should still look good most of the time. ISO 1600 will show a noticeable amount of noise. The noise will increase dramatically as you go to ISO 3200, 6400, 12800, and the camera’s maximum value of 25600.

Lightroom and other software can decrease the appearance of noise, to some degree, but the noise reduction tends to smear out detail at the same time. We’ll discuss noise reduction, which is in the Detail panel of Lightroom, in a future installment.

Restoring balance to the Force

So shutter speed, aperture, and ISO are the camera exposure settings you can control. Those three settings must be brought into balance with the fourth element of exposure, which is the brightness of the scene. A brightly-lit scene requires less exposure (faster shutter speed, smaller aperture, lower ISO) than a dim scene. Your artistic intent also comes into play—do you want to photograph to be light or dark in tone?

The camera helps you by metering the scene, and depending on the shooting mode, it can set some or all of the exposure settings automatically. We’ll discuss the different shooting modes in a future installment.

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