Tuesday, May 12, 2009

Using The Exposure Adjustment Triad

Exposure is the total amount of light allowed to reach the image sensor during the process of taking a photograph. We've seen there are 3 things that we can adjusted to control that exposure: the lens aperture, the shutter speed and the ISO sensitivity.

The DSLR Reflective Light Meter

Your DSLR has a built-in reflective light meter. It tells you if your current settings will make a good exposure or an over/under exposure.

In your camera viewfinder is a bar scale with a plus at one end (over-exposure), a zero in the middle (correct exposure) and a minus at the other (under-exposed). Many DSLR's let you select the increment value on the meter to 1 stop, 1/2 stop or 1/3 stop per tick mark as well as reversing which end the +/- is located. The length of the scale shown in the viewfinder is usually 4 stops total (+2 to -2).

Notice the viewfinder also shows your aperture and shutter speed too.

Note: You may have seen a photographer using a hand held light meter. A good hand held light meter can not only measure reflected light it can measure incident and supplemental (flash) lighting too. We will cover hand held light meters in a future installment.

A Starting Point

On a cloudless sunny day you can get a good starting point for estimating a good exposure by using the Sunny 16 Rule: "On a sunny day set aperture to f/16 and shutter speed to the ISO film speed."

If your ISO is set at 100 you would make your aperture f/16 and your shutter speed 1/100 sec. If your camera doesn't have a 1/100 sec setting use 1/125. If your ISO is at 200 make your aperture f/16 and shutter speed 1/200 sec (or 1/250 if 1/200 is not available on your camera).
To fine tune the exposure for the scene you are shooting you look at your in-camera meter and make the fine adjustments to aperture or shutter speed to center the meter indicator to zero on the bar scale.

You can also use the following guidelines as starting points:

Use f/16
When it's sunny,
the shadows will be sharp and distinct.

Use f/11
When it's slightly overcast,
and the shadows will have soft edges.

Use f/8
When the sky is overcast,
and shadows will be barely visible.

Use f/5.6
When the overcast is heavy,
and there will be no shadows.

These are only guidelines, starting points to get you in the ballpark. These settings are generic and don't account for creative exposure settings nor the fact that each scenes has it's own unique blend of light.

Making Creative Exposures

We know from the previous Exposure installments that we can vary our aperture or shutter speed to account for different photographic effects like Depth Of Field (DOF) and stopping motion.

Using the Sunny 16 Rule will give you enormous DOF at f/16 but a relatively slow shutter speed because of the small lens opening so making images of a moving subject may yield a blurry subject but a sharply focused background.

Using the Sunny 16 settings as a starting point we can make adjustments based on what we know about stops.

Leaving ISO unchanged, if I change my aperture by 3 stops from f/16 to f/5.6 so I can decrease my DOF, I also need to change my shutter speed by 3 stops to keep the same exposure. Since I opened the aperture 3 stops letting in more light, I need a faster shutter speed so I adjust it 3 stops from 1/125 to 1/1000. Now my subject will be in focus because of the fast shutter speed and my background will be slightly blurred because of the much larger lens opening visually separating my now sharply focused subject from the slightly blurred background.

Now it becomes clearer why thinking about your exposure settings in terms of stops can be so handy. It also points out the fact that many setting combinations will produce the same exposure of the same scene but the resulting images will differ as to details like depth of field and motion blur or lack thereof.

Another way to express that thought is: there is a narrow range of exposure settings that will result in a correct artistic exposure that pleasingly incorporates depth of field and the control of the sharpness of moving subjects, if any.

High ISO

As seen above changing the aperture or the shutter speed directly affects the artist feel of an image. Changing ISO has much less impact on artistic expression than aperture and shutter speed since it affects neither depth of field and motion blur directly.

ISO is usually changed as a gross adjustment to accommodate the amount of light available: low ISO in bright sunlight and high ISO in dim light.

ISO generally has fewer steps available than aperture and shutter speed. In full stops ISO increments at 100, 200, 400, 800, 1600, 3200. Entry level digital cameras (Canon XS, Nikon D60) generally perform poorly (excessive image noise) above ISO 400. As we move up the camera scale high ISO performance improves with 2009 professional level cameras able to perform at settings as high as ISO 12800 or ISO 25600.

In our next installment we will look closer at the in-camera meter and discover how it works, how it may make decisions you won't like and how you can adjust it to do your bidding.

See you then,

Where everyone learns Photoshop - National Association of Photoshop Professionals