A Quick Guide to Digital Nature Photography

By Neil Losin

Section 1: Getting the Right Exposure

This is the most difficult – and the most important – section of the guide. Controlling exposure is the most fundamental skill in photography. Once you master exposure, the rest is easy.

1.1 – Sensitivity: When photographs were captured on film, instead of digitally on a CCD or CMOS sensor, the sensitivity of the medium was fixed. A film’s sensitivity is often referred to as its speed or ISO (the latter is an acronym for International Standards Organization, a group that defines industrial standards in many fields). Film cameras could read the ISO speed from a film canister, and would “know” whether the film was ISO 100, 200, etc.

Higher ISO ratings indicate greater sensitivity to light, and ISO ratings have a simple linear relationship with sensitivity. For example, an ISO 400 film is four times as sensitive as an ISO 100 film, and therefore requires only 1/4 as much light to form an image.

In the digital age, we can adjust the light-sensitivity of our camera from one shot to the next. A typical range of ISO sensitivity is ISO 100 – ISO 1600, but some professional cameras go much higher (the Nikon D3s, for example, can be set as high as ISO 25600). Your camera’s ISO adjustment might be controlled with a single easy-to-access button on the camera body (this is the case for most SLR cameras), or it might be buried deep in some sub-menu. Consult your camera’s manual to determine how to adjust its ISO sensitivity.

1.2 – Aperture: Two factors determine how much light reaches a camera’s sensor: aperture and shutter speed. Aperture refers to the size of the opening through which light passes inside the lens. Somewhat counter-intuitively, large apertures are indicated by small numbers, like f/2.0 or f/2.8. This is because aperture is approximately defined as the lens’ focal length divided by the diameter of its front (“objective”) lens. For example, a 300mm f/4 lens must have an objective lens at least 75mm in diameter. Canon’s 300mm f/4L IS lens actually has an objective element that is 77mm in diameter, so it satisfies this requirement.

Aperture can be discussed in terms of stops. Each stop is equal to a doubling or halving of the amount of light hitting the film (or sensor). Here is a list of the “full” stops:

f/1.0, f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

Notice that the number designating each stop is about 1.4x the previous stop. It’s no coincidence that (1.4)^2 is approximately 2. Each time you increase the diameter of the aperture by a factor of 1.4 (e.g. from f/2.8 to f/2.0), you double the amount of light entering the camera.

You might be thinking: the size of the objective lens is constant, so how can you adjust the aperture without changing lenses? Inside each lens is an iris diaphragm that dilates and constricts like the human iris to regulate the amount of light entering the camera. On older SLR cameras, the iris diaphragm can be adjusted by an aperture ring at the base of the lens, and in more modern cameras it's controlled through the camera body itself. Many cameras allow the photographer to adjust the aperture in 1/3-stop increments, so you have finer control. For example, instead of jumping straight from f/5.6 to f/8, you can choose f/5.6, f/6.3, f/7.1, or f/8.

1.3 – Shutter speed: Shutter speed, or exposure duration, is the amount of time the camera’s shutter remains open, allowing light from the lens to reach the film (or sensor). The numbers your camera uses to display shutter speed are straightforward: a shutter speed of 800 or 1/800 means that the shutter will remain open for 1/800th of a second; a shutter speed of 60 or 1/60, 1/60th of a second. If the shutter speed is longer than 1 second? Many cameras display numbers followed by the second symbol ("). For example, 15" means 15 seconds, not 1/15th of a second.

The relationship between shutter speed and the amount of light striking the film (or sensor) is simple – the amount of light is exactly proportional to the amount of time the shutter is open. Like aperture, we can talk about shutter speed in terms of “stops.” For example, a shutter speed of 1/400th allows one stop more light than a shutter speed of 1/800th. Faster shutter speeds are often desirable in wildlife photography; birds and other animals are active, and faster shutter speeds enable the photographer to freeze fast action. Fast shutter speeds are less crucial for landscape and macro work, where the subject (usually) isn’t in much of a hurry (see Fig. 1).

Shutter speeds can usually be adjusted in 1/3 stops, just like aperture. For example, you aren’t just limited to shutter speeds like 1/125th and 1/250th of a second. If you want to use something in between, you can set your shutter speed at 1/125th, 1/160th, 1/200th, or 1/250th.

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To read the rest of A Quick Guide to Digital Nature Photography: Section 1, which covers creative exposure control, depth of field, metering, outsmarting your light meter, and more, click here to download the full PDF!