Thursday, June 08, 2006

Photography Concept 1

With the ubiquity of digital photography these days, I thought I'd share some of what I've learned about photography over the years with a focus (pardon the pun) on explaining some of the most important concepts in a simple, practical way. My intent is to illustrate some key ideas in each blog post. Hopefully this will help people better understand photography and make better use of their cameras (both digital and film). As always, knowledge is power!

Photography Concept 1: Exposure

Proper exposure is a subjective thing, but it typically means that the photo you've taken is a fairly faithful rendition of what you are seeing with your eyes at the time. Overexposure means that the photo is lighter or brighter than what you saw, and underexposure means that the photo is generally darker than what you saw. And then again some people use over or under exposure on purpose. In any case, it is a very important photographic concept to understand.

Exposure is exactly what it sounds like. In terms of film, it is the amount and duration of light that is allowed to fall onto the film. Digital is no different except for the fact that the light is falling onto a sensor instead of photographic film. Amount and duration are the key things to remember when talking about exposure.

The amount of light is controlled by what is called the aperture. The aperture is the size of the opening through which light is allowed to flow throught the lens, into the camera and onto the film or sensor. The aperture can be automatically adjusted by the camera or can be manually adjusted on virtually all cameras these days with the possible exception of cameraphones.

The duration of time that the sensor or film is exposed to the light coming in through the lens is controlled by the shutter speed. This is the amount of time that the shutter is left open, exposing the sensor or film to light.

Just to make things a little more complex, there is a third variable that plays a part in defining exposure - ISO speed. The ISO speed is the sensitivity of the film or sensor to light. A higher ISO value means higher sensitivity to light. These are the numbers that you would (and still do) see on film canisters (e.g. ISO 400 film). Let's assume for the rest of this discussion that the ISO speed for our camera is constant. So we really only have two variables that determine exposure: aperture and shutter speed.

On a typical camera, shutter speed is denoted in fractions of a second (1/60, 1/200, 1/1000 etc..). Simple enough. However, aperture is denoted by an 'f-stop' value (f/1.8, f/4.0, f/11). This value is actually a fraction of the focal length of the lens (the 'f' is the focal length - usually in mm). It's important to realize that for a given lens, an aperture value of f/1.8 is wider than a f/4.0 value. Sometimes camera makers refer to f1.8 or f4.0 but this is a real misnomer. If you remember that the number is actually the denominator you will remember that the smaller the number, the greater (wider) the aperture value.

Let's imagine for a moment dear reader that we have an empty glass, and that filling the glass to the brim with water will signify the desired exposure. You will note that there are a couple of different ways we can achieve that goal. We can run the tap wide open for a second or so, or open the tap only slightly and wait longer for the glass to fill. Either way we get our full glass of water, but we see that there are many ways we can achieve it. Exposure is the same. Think of aperture as the amount of water we let through the tap (wide open or barely open) and shutter speed as the amount of time we hold the glass under the tap (a long time or a short time).

Now you're probably asking yourself - why does all this matter? It matters because you can achieve different photographic effects depending on the shutter speed and aperture settings. It is important to realize that you have flexibility in achieving the desired exposure. You can use a smaller aperture (to give a greater depth of field - something that will be discussed in a future post) but this will have to be balanced by a slower shutter speed to maintain the required exposure. Conversely, if you wanted to freeze some high speed action, you would set a faster shutter speed, but you would have to balance this with a wider aperture to maintain the proper exposure.

I will discuss things like 'depth of field' and other photographic concepts in upcoming posts, but basics such as exposure are pre-requisites to those discussions.

I hope somebody finds this information useful, and by all means if you have suggestions or comments feel free to post them or email me.