Identifying and Controlling Exposure Variables

Photography and lighting are both processes with many variables which affect outcome. The challenge for a beginner is learning how the variables relate to each other and affect the final result. The starting point for process control is to first identify all the variables, and then identify which have the most impact on the desired final result. A basic tenet of process control is: You can't control what you can't measure. so another important part of process control is determining the best way to monitor the results. In this tutorial I'll provide an overview of the approach I use for monitoring and controlling exposure for those new to photography.

Exposure Variables

Three variables affect exposure in a digital camera:

ISO Speed

A digital camera sensor is has millions of individual sites which absorb photons and convert them it voltages which are then converted in to binary numerical values. The amount of light hitting the sensor is controlled by the other two variables, shutter speed and aperture. The ISO setting on the camera controls how much the signal voltage produced by each sensor site is amplified. Amplifying the signal by selecting higher ISOs has an undesired consequence; increasing the amount of "noise" which manifests itself as multi-colored artifacts which give the image a grainy appearance. Best image quality results at the lowest ISO settings. Some cameras are better at controlling noise than others.

Shutter Speed

Most single-lens reflex digital cameras (DSLR) have a shutter consisting of two curtains which sit just in front of the sensor. The first curtain opens to start the exposure and the second one closes to end it. The two curtains travel at a constant speed at all shutter speed dial settings, typically taking about 1/300th sec to cross the sensor: the larger the sensor, the more time it takes the first curtain to clear. That clearance interval determines the "x-sync" limit for the flash, the fastest shutter speed during which the sensor is fully exposed which is typically 1/200 to 1/250th second, depending on sensor size. Henceforth I will assume 1/250th as the sync speed. At speed settings above 1/250th the second curtain of the shutter will start closing before the first one has completely cleared the sensor. As the indicated speeds increase the gap between the curtains decreases until at 1/8000th sec. indicated shutter speed there is a very narrow slit traveling vertically across the sensor.

Aperture

Aperture and f/stops are one of the more confusing aspects of exposure control for beginners. The f/stop numbers represent the ratio between the focal length of a lens and the physical diameter of the aperture opening in the middle of the lens. A 50mm lens which has a maximum aperture of 50mm would be (50mm/50mm = f/1.0). If the aperture size was only 25mm the f/value of the lens at its widest setting would be f/2 = 50mm / 25mm. On older manual cameras lenses had mechanical click stops in one-f/stop intervals. Digital cameras control the aperture electronically and each click-stop of the dial is typically 1/3 f/stop.

The sequence of f/stops in 1/3 stop intervals is shown below with the traditional full f/stop intervals shown above the 1/3 stop intermediate values a digital camera will display:

1 _{1.1, 1.3,} 1.4 _{1.6, 1.8,} 2.0 _{2.2, 2.5,} 2.8 _{3.2, 3.6,} 4 _{4.5, 5.0,}

5.6 _{6.3, 7.1,} 8 _{9, 10,} 11 _{13, 14,} 16 _{18, 20,}22 _{25, 29,} 32

Changing the aperture of the lens does three things: 1) changes the amount of light hitting the sensor; 2) changes the range of distances which are in focus, and; 3) alters the ability of the lens to resolve fine detail. The first affects exposure, the second affects composition and how the scene is perceived in the photo, and the third affects image quality. In general lenses are sharpest when set 2-3 stops smaller than the widest aperture (e.g. f/5.6 for an f/2 lens). At f/stops above f/11 the physical size of the aperture become so small that diffraction of the light as it passes through it will degrade the resolving power.

Creative use of exposure variables

Aperture: One of the more fundamental goals in making a photograph is creating the illusion of reality. In person our eyes and brains concentrate on a very narrow two-degree arc around where our eyes focus because that is where the color sensing cells of the eyes are located. When capturing a scene in a photograph the camera aperture can be adjusted to mimic this narrowing of focus by blurring the background. The contrast of the sharp foreground and blurred background sends a subliminal clue to the viewer that the foreground is more important. The "depth-of-field" (DOF) for a given lens is very narrow when wide open, and increases as the aperture is made smaller (higher f/stop number). It is important to understand than DSLR cameras keep the lens aperture at its widest setting until the shutter opens. That means the scene in the viewfinder during composition will usually have less DOF than the images which is captured. Cameras are equipped with a "DOF Preview" button which will close the aperture to the actual setting so DOF can be visualized.

Shutter Speed: One basic process control requirements for a sharp photograph is keeping the camera steady during the exposure. In the techniques section I have a tutorial explaining the technique I use to hold a camera steady you might want to try, but however you choose to do it you should do some testing with each lens you own to determine how slow you can go with the shutter before movement degrades the image unacceptably. One of the perceptual differences between real life and a photo is movement and to create the illusion of motion in a still photo it is often desirable to allow controlled amount of blur in foreground and/or background via shutter speed and techniques or panning the camera to follow a moving object.

ISO Speed: Sensitivity adjustments facilitate the two creative variables of shutter and aperture. As noted image quality is best as the lower ISO settings, but in low-light it is necessary to raise ISO to obtain correct exposure or achieve the desired stopping of action or depth of field.

A Baseline Workflow and Creative Decision Tree

Photographic scenarios are so situational there is never a single correct method for exposure. That is why it is important to do controlled testing in non-critical situations to know what speed you can hand hold and how much DOF each of your lenses have at various distances. I approach each new scene as a problem to solve based on the goal for the shot. I ask myself "What is most important in the scene?" and "How can I make it contrast with the background?". The first creative decision affecting exposure is determining how much depth-of-field is desired to isolate foreground from background in the same way our eyes and brain selectively focus our attention. The simplest way to pre-visualize the results is to press the DOF preview button and try different apertures until the desired balance of foreground detail and background context is found.

Determining the desired f/stop eliminates one of the three exposure variables. The next step in the decision trees is to determine the shutter speed based not just on exposure, but also on the creative aspect of making the photo tack-sharp or intentionally introducing so blur to create the illusion of motion. The shutter speed desired for creative purposes, combined with the desired aperture for DOF may not produce a correctly exposed file. For example shooting indoors in available lighting an aperture of 5.6 may be desired for sufficient DOF and a shutter speed of 1/125th second to prevent blur due to camera movement. If those settings do not produce a correctly exposed file, then it would be necessary to change the third variable, ISO, shoot at a wider aperture with less DOF, or find a way to support the camera such as a tripod so a slower shutter speed can be used.

When is Exposure Correct?

The answer to that question is that a photo is correctly exposed when what is most important is reproduced realistically and things which the camera is not able to expose correctly are not distracting to the point of the photo looking fake. In other words it a subjective judgement based entirely on perception of the final result on screen or on a print.

For a beginner a good starting baseline for exposure is getting the highlights exposed correctly, below clipping. Depending on the contrast of the scene the camera may or may not be able to record the mid-tones and shadows as seen in person, but there is no way of knowing the extent of the losses unless the highlights are first exposed correctly in the technical sense.

As mentioned at the outset a basic tenet of process control is that you can't control what you can't measure so a non subjective benchmark is needed for determining when highlights are correctly exposed. The two best measurement tools available to a photographer are the clipping warning in the camera and the eye-dropper took in the image editing application. But for either to be useful for objective measurement a benchmark highlight target is needed. Some scenes have areas of white textured highlights but others don't. When shooting portraits and other situations where it is practical I use a white terry towel as my highlight benchmark because they are easy to carry in my camera bag and easily cleaned when soiled. The looped texture of the fabric makes it very easy to see visually when overexposure is occurring. By comparing the camera playback with the visual appearance of the towel and its eye-dropper reading in the Photoshop RAW editor I've come to understand how to anticipate when over-exposure is occurring the the RAW file based on the on-camera "blackout" clipping warning.Raising exposure with a combination of the three control variables to the point where the towel starts clipping in the overexposure warning of the playback tells me I've reached the limit of the sensor.

Once I get the highlights exposed correctly I evaluate the mid-tones and shadows in the playback visually and via the histogram to determine whether or not flash is needed to match the range to the sensor. I explain that process in more detail in this tutorial .