This is the next installment of the response to a student.

Digital photography and film photography aren’t as different as the publishing industry would have you believe. Much that has been written about film photography is just as applicable to digital photography. After all, a photograph is merely focused light captured on a substrate. Your camera is only a light-tight box. You can make a photo with a shoebox and a piece of film or photo paper (i.e. pinhole photography). In order to expose a photo properly, however, you must control the amount of light that reaches the film or sensor.

Your camera has two ways to do this: the shutter and the aperture. Shutter speed is the most straightforward of these two variables. It opens. It shuts. Depending on the speed, the subject may be frozen in time or blurred in action. As a rule of thumb, it is best to keep the shutter speed at least 1/(focal length [35mm equivalent]). That’s to say, if I have my lens set at 17mm (approx. 28mm in 35mm equivalency), I would want to make sure the shutter speed is at least 1/30 (rounding down to the nearest shutter speed). This helps minimize handheld, shake-induced blur.

The aperture controls the size of the opening through which light passes. While this may seem simple, the aperture (for reasons I won’t get into here) also affects the image in a more direct way. It controls the depth-of-field (the plane of focus parallel to the camera). This is a rather abstract concept that takes practice to fully understand. Simply put, the larger the opening the shallower the DOF, the smaller the opening the larger the DOF. Put another way, the wider the opening the narrower the plane of focus and visa versa. The aperture value is expressed as “f/(x)” and is inversely related to the size of the opening. For example, f/4 is the largest aperture of one of my lenses. This setting would also give the shallowest DOF. Conversely, f/22 is the smallest aperture to which this lens will stop down [nomenclature= to make the aperture opening smaller which concurrently makes the number larger], producing a wider DOF. An exercise that will help illustrate this is to arrange a series of objects approximately equidistant from one another, focus on the center object, and vary the aperture setting. Nearly any introductory photography book will have done this very experiment. It’s also worth noting that the closer to the lens the subject is, the narrower the DOF is at any given aperture, though this may prove to be more relevant later.

A third variable that may affect the image is the sensitivity of the substrate. The sensitivity of film is known as “ISO” and in digital cameras it’s referred to as “ISO equivalent,” or, more commonly, just “ISO.” Each number is either twice as sensitive (200 is twice as sensitive as 100) or half (400 is half as sensitive as 800) as each previous. As an aside, each halving or doubling is known as a stop; 200 is one stop faster than 100. This also applies to shutter speed (1/50 is one stop slower than 1/100) and aperture (f/2.8 is one stop faster than f/4). Your camera’s manual will probably include a list of shutter and aperture values in either whole- or half-stop increments. Generally, the lower the ISO, the less “noise” (or “grain” in film) is present.

One must learn to juggle these three variables in order to properly expose an image. Apart from the particular ways any given lens may affect the image (telephoto lenses compress and wide-angle exaggerate), adding filters, and adjusting white balance, these are the only ways that your camera can affect any given image.

Both film and digital sensors are able to capture only a limited range of light. Possibly the single greatest advantage of digital cameras is the histogram: the graphical representation of all the tones in a photo (from left to right the histogram shows the darkest to lightest tones). With film, the photographer doesn’t know for certain if the photo is properly exposed until it is developed. Digital cameras, whereas, give instant feedback. Though it depends on how the image will be handled in post-production and, more importantly, how the final product is intended to look, it is usually most important to make sure that the highlights don’t clip (which are recorded as pure white).