In Search Of The Perfect Exposure

By Chip Wilder – January 12, 2013

For every light situation, there is a perfect exposure. That is, the correct amount of light is transmitted through a camera's lens to the film or light sensor to properly capture the subject image. All we need to do is adjust the three exposure elements of the camera (ISO, Aperture & Shutter Speed) to achieve this goal. To accomplish this, photographers must understand the relationships between these elements and choose the proper settings. Let’s take a closer look at each exposure element.

ISO

- This is the measurement of the light sensor’s sensitivity to light. The film equivalent is ASA rating or "film speed". With digital camera equipment, we have the ability to change the ISO setting from shot to shot while film users must use the same ISO (ASA) for the entire roll of film. The lower the ISO value, the lower the sensitivity. ISO 50 requires twice as much light than ISO 100 to record the same image. If you double the ISO number, you adjust the sensitivity of the sensor by 1 f/stop.

 Lower ISO - better for well lighted & still images – less noise (fine).

 Higher ISO - low light or action shots – more noise (grain).

Aperture

– The aperture setting determines the amount of light that is allowed to pass through the lens to the light sensor. These settings are expressed in f/stop values. A typical prime lens may have a range of values of f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. These are the standard values for full f/stop increments. Each increase in value (i.e. f/4 to f5.6) allows ½ the amount of light as the previous setting while each full f/stop decrease allows twice as much light through the lens. Digital cameras allow you to adjust the f/stop values in less than full f/stop increments.

Lenses that have the ability to be set to f/2.8 or lower settings are considered to be "fast" lenses" and are very desirable for low-light photography. The "depth of field" of any lens increases as you increase the f/stop value.

 Lower Aperture – Less depth of field, focus more critical.

 Higher Aperture – Greater depth of field, focus less critical.

Shutter Speed

– This setting controls the amount of time, the light passing through the lens, is exposed to the light sensor. If the shutter speed is slower than 1/60 of-a-second, a tripod may be required. If fast moving subjects are to be photographed, a shutter speed of 1/250 of-a-second or faster may be needed. If you double or halve the shutter speed, you have adjusted the exposure setting by 1 f/stop.

So, how do we use these different adjustments to capture images with our digital cameras? If we set it to "automatic mode", the on-board processor will select all of the exposure element settings and the results we get, may, or may not, be achieved.

Let’s use the "Sunny f/16" rule to help us determine the exposure settings. It recommends "On a sunny day, set the aperture to

f/16 and shutter speed to the reciprocal of the ISO setting for a subject in direct sunlight". Let’s assume our subject is in bright sunlight and we set the ISO value to 125. We would set the aperture to f/16, the shutter speed to 1/125 and we have our perfect exposure. Or

do we? In this shot, we want our background to be out of focus and f/16 is going to make everything in the image nice and sharp. To get the image we want, the aperture needs to be more wide open. We can increase the aperture in 1 f/stop increments and shorten the shutter speed in equal f/stop increments to get additional exposure options. Assuming an f/2, 50mm prime lens is being used, the range of options are:

f/16 at 1/125 f/11 at 1/250 f/8 at 1/500

f/5.6 at 1/1000 f/4 at 1/2000 f/2.8 at 1/4000

f/2 at 1/8000 f/22 at 1/60 (increase f/stop – reduce shutter speed)

Each of these settings allows the same amount of total light to be captured by the light sensor. So each setting is "the perfect exposure" but the images captured, will all be different. At the slower shutter speeds, moving images may be blurred or a handheld camera shot may not be possible. At the wider apertures, foreground and/or the subject may be out-of-focus. At smaller apertures, undesired elements in the scene may be in-focus and destroy the desired effect. In our example, the f/2.8 or f/4 setting would probably capture the image as planned.

In the above example, if we used a higher ISO setting, the number of exposure options would be fewer, because the camera may not have a fast enough shutter speed to open the lens wide enough to get a shallow depth of field. In like manner, if you were at Colonial Downs photographing a horse race and wanted everything in focus, the 125 ISO would not allow you to use a small aperture and a fast shutter speed to stop motion. An ISO 1000 will allow you to shoot at a range of f/16 at 1/1000 to f/5.6 at 1/8000. In a low light environment, such as a bright home interior, a 3200 ISO would give you a shooting range of f/2 at 1/1000 to f/16 at 1/15 second.

Exposure guides, hand held light meters or the camera’s internal metering system can be used to determine a starting exposure setting. Those with Smart phone devices may find exposure apps to be helpful as well. Once the final setting has been determined, a test shot will confirm it. If one feels a little uncomfortable using the manual mode but understands the concepts of the camera’s exposure elements, program mode (P)(set ISO), aperture-preferred mode (Av)(set ISO & aperture) and shutter-preferred modes (Tv) (set ISO & shutter speed) will allow them to exercise more control over the exposure process.

Photographers get the results they want, not by just the perfect exposure, but by making perfect choices of ISO, Aperture and Shutter Speed to create the images they envision.