Understanding Exposure
Let’s start with the extreme basics. Photography is about exposing light to a light sensitive material (film or a digital sensor) in a controlled manner. (OK – you’ve got a program mode camera, and you’ve always had good exposures….the kick is, good exposures don’t make good photographs. The key to expression photographically is to control how an image is captured to convey the desired feeling inherent in the image itself – it makes the difference between a “snapshot” and a “photograph”. So, how do we achieve that?
The first step is to understand the mechanical process that creates of a photograph – and it’s a lot easier than it sounds. It can be broken down into three categories dictated by a single deciding factor: Shutter Speed, Aperture, Film Speed or Sensor Sensitivity dictated by a metered level of light – and even that sounds more complicated than it is. Whether working with a digital camera or a film camera, the same basic principals apply (correct exposure is correct exposure!). To create an exposure, you expose a specific amount, or intensity, of light to a light sensitive media for a specific duration of time – the result is a photograph.Regardless of the camera you have – SLR or compact, digital or film – all cameras create an exposure dependent on three settings: shutter speed (the duration of light), aperture (the amount of light), and ISO (the sensitivity to light) – So let’s look at these individually:
Shutter Speed
Let’s start with a full understanding of the shutter speed. The shutter speed refers to the amount of time that the shutter is open during an exposure, the duration of light allowed to strike the film plane or sensor (from this point on, “film plane” or “sensor” will be referred to as “media”). Shutter speeds are typically displayed by most cameras as whole numbers, however these numbers refer to fractions of a second. Displayed numbers such as 1000, 500, 250, 125, 60, 30, 15, 8, etc. – refer to shutters speeds of 1/1000th of a second, 1/500th of a second, and so on until you get down to 1, which is a full second. Shutter speeds longer than a full second are typically displayed either in another color such as red, or have the term “sec” following them, so a “4” in red or a display of “4 sec” refers to a shutter speed of 4 seconds. Many cameras offer a “B”, or “Bulb”, setting. This is a shutter speed setting that actually keeps the shutter open for as long as the shutter release is pressed (or a locked shutter release cable is attached), allowing for extremely long exposures (even hours long!).
In order to use shutter speeds to create the look or feel that we want in an image, we need to understand how the shutter speed affects the image, particularly when we’re dealing with a moving subject. When shooting a moving subject, we usually have the option of freezing that motion in our photograph, or allowing that motion to blur to give the feeling of movement or speed. This option is controlled by the shutter speed chosen. Obviously, faster shutter speeds (such as 1000 or 500) will likely freeze any motion during your exposure, and slower speeds (such as 15 or 8) will render any motion as a blur – the choice is ours to make! A couple of things to consider:
1: With any given lens: the faster the shutter speed, the less motion (blur) will be visible in the final image.
2: With any given shutter speed: the closer you are to your subject, the more motion (or blur) will be visible in the final image.
3: With any given shutter speed at any focused distance: the longer the lens, the more chance that motion (blur) will be visible in the final image.
4: At any given magnification, regardless of lens: The higher the magnification, the more motion (or blur) will be visible in the final image.
Aperture
As we have learned, the shutter speed controls the duration of light that strikes the media, but “light” is not exactly a constant. We need a way to control the amount of light allowed to enter the lens – not the duration, but the amount, which is exactly what the aperture setting does. The aperture itself is the opening inside of the lens that allows light to pass through. Think of the aperture as operating in the same fashion as the iris of your eye – when it’s bright, the eye’s iris gets smaller, and when it’s dark, the iris opens up wider. That’s exactly what the aperture inside of a lens does – but it’s a setting that you can control. Aperture setting numbers are a bit archaic at first, consisting of numbers such as 1.4, 1.8, 2.0, 2.8, 3.5, 4.0, 5.6, 8, 11, 16, 22, etc., and are usually referred to a “f-stops” and are displayed with an “f” in front of them, f1.8, f2.0, f2.8, f3.5, f4.0, f5.6, f8, f11, f16, etc.These numbers refer to the physical size of the opening inside of the lens that is allowing light to pass through. (For those of you who just have to know, the aperture number is actually a ratio of the size of the physical lens opening divided by the focal length of the lens – a 50mm lens with a 25mm aperture opening would be set at an aperture of f2, a 100mm lens with a 12.5mm opening would be set at an aperture of f8). The mathematical solutions aren’t real important, what is important is to understand that larger apertures allow more light through than smaller apertures. What may be confusing to some people is that larger apertures are represented by smaller numbers – f2 is a larger opening than f4, and as a result allows more light through – the smaller the number, the larger the aperture
Besides controlling the amount of light allowed to pass through the lens, aperture settings provide a huge
creative control over the final image. Aperture settings control how much will appear sharp in you final image – not the sharpness itself, but the depth of the sharpness – referred to as depth of field. Whenever you take a photograph, you focus on your subject, this focus is referred to as the plane of focus. There is also an area in front of and behind this plane of focus which also appears in focus – and this range of “in focus” is the depth of field. For example, you may be taking a photograph of a vast scenic, and you want everything from the fence post in the foreground to the mountains in the backgrounds in focus – you would need a deep depth of field – on the other hand, if you were taking a portrait of someone, and wanted the background to be completely blurred, you would want a shallow depth of field. This “depth of focus” is controlled by the aperture – larger apertures (smaller aperture setting numbers) yield shallower depth of field, whereas smaller aperture setting numbers (larger apterture setting numbers yield deeper depth of field. This does not mean that smaller apertures make your lenses sharper, they only increase the depth of sharpness for that image.
Combine Them:
Shutter speeds control the duration of light and affect the action-stopping quality of an image. Apertures control the amount of light and affect how much will appear in focus in the final image. These two setting have to work together in order to generate an exposure – more accurately, a correct exposure, so, how do we combine the two? Let’s look at the problem this way: we have a bucket that we need to fill with water – once we fill that bucket up, we have a correct exposure. Now, we can turn the water on very slowly and let it run for a long time to fill the bucket, or we can turn the water on high and fill the bucket up quickly – our choice. We can allow a lot of light through the lens (large aperture that will give us a shallow depth of field) for a short duration of time (a fast shutter speed that will freeze action), or we can allow a small amount of light through the lens (small aperture that will give us a large depth of field) for a longer period of time (longer shutter speed that will, most likely, blur any motion during the exposure) – or anywhere in between – our choice. But how do we determine what shutter speed combined with what aperture will produce a correct exposure to begin with? It would help to know what we’re aiming for!
ISO
Here is your target – the ISO (International Standards Organization) preset rating of the light sensitivity of that media (whether it be film or digital). ISO ratings for media range from 12 (not very sensitive to light) to 1600 or higher (very sensitive to light). Typically, higher rated ISO settings (film or digital) will render more grain or noise in the final image.