Photography 101 - Part Two
How do you make an exposure?
As I said in my previous post on the subject, photography is about light. No matter how good a composition is, or how unique the moment you’re attempting to capture is, the most important aspect is the light in the scene, and how it reflects onto your imaging plane. Light should always be in the forefront of your mind when thinking about making an exposure.
An exposure is the process of making an image. It is called this because the process is about exposing a photosensitive material - be it a sensor in a digital camera, or film in an analogue camera; to rays of light - either natural or artificial.
In addition, exposure is also the amount of light that is projected onto your imaging plane (a sensor or film). This has more than a few effects on your resulting image, but most readily apparent is how bright or dark it is.
In photography, there are three main ways to control exposure. This is referred to as the exposure triangle, as each property affects one another. In addition, each property has a side effect on your image.
The first of these is Shutter Speed. This is how long your shutter remains open, allowing light to strike the imaging plane. This is expressed in seconds, either a fraction like 1/125(th of a second) or as an integer, like 4” (for a long exposure). With a longer shutter speed, you introduce motion blur. This is when an object in motion may appear blurry, as opposed to how sharp it might look at a faster speed. The faster the object is moving, the faster the shutter speed needed to minimize motion blur.
Here's an image I shot handheld on film with a slow shutter speed, to make the people walking blur.
The second piece of the exposure triangle is Aperture. Aperture is how wide the opening in your lens is, controlling how much light is projected onto the imaging plane through your lens. Aperture is measured in stops, which are a common and reoccurring unit of photography. In photography, this is expressed in an F-stop, written like this: ƒ/2.8. The lower the number is, the wider the opening is. So ƒ/2 has a wider opening than ƒ/8.
The way aperture works is like this: inside your lens, there are a set of blades that interlock, forming a circle. Generally, there aperture blades inside a lens, the rounder the opening formed by them, which give a nicer optical quality than less blades.
Some aperture blades inside a lens. With aperture, the other effect controlled is depth of field. Depth of field is how much of a scene is in what’s called “acceptable focus”, compared to how much is out of focus. Aperture is not the only property that affects depth of field, but is one of the easiest to control. In general, the wider the aperture (lower f number), the shallower the depth of field.
The out of focus areas of an image (if there are any) are referred to as bokeh, which is pronounced bo-ka. How many aperture blades are in your lens can have an effect on the look of the bokeh, though this is largely a personal preference.
A portrait I took of a friend of mine, Matt. See how everything behind his head is out of focus? BOKEH! Another thing to note here is that this shot was taken on medium format, which means the imaging plane is 6cm x 7cm: That's huge! Bigger imaging plane = more bokeh!
Finally, the last element of the exposure triangle is ISO. ISO is how sensitive the imaging plane, be it a digital sensor or a piece of film, is to light. The lower the number, the less sensitive to light it is. This is usually expressed in integers in the hundreds, generally ranging from 100 - 256,000 on cutting edge digital sensors. So, with all settings otherwise the same, a photograph made at ISO 200 will be less exposed than a photograph made at ISO 1600 - assuming the scene and lighting is the same.
Film comes in a single ISO speed, and cannot be adjusted while shooting. This is set by the manufacturer, and is listed in the name of the film stock, or type. For example, you might purchase Kodak Portra 400, which is rated for ISO 400. This simply means that the manufacturer (in this example, Kodak) has recommended this film be exposed for ISO 400, and processed as such. There are ways to get around this (search for pushing or pulling film), but I won’t cover them here.
With a digital sensor, the ISO can be adjusted with each shot. There is a often a recommended speed for the sensor, especially on digital cinema cameras, which is called the native ISO. In shooting photography, this is far less important, and the sensitivity can be changed to suit the scene. The drawback of shooting a higher ISO is the introduction of noise into an image. This often appears like different color spots on an image, and reduces the amount of detail in a shot. In general, the lower the ISO number, the less noise that’s introduced to an image. So, ISO 6400 would be considered “noisier” than ISO 200.
See how each jump in ISO introduces more noise to the image? Depending on the image you are trying to make, you should prioritize a different aspect of the exposure triangle, and compensate with the remaining two accordingly. For example, if you were shooting something fast moving, like people on the street, you might want to prioritize shooting with a faster shutter speed, to reduce motion blur. Or, if you were shooting a portrait, you might want to prioritize shooting with a wider aperture, so that you can get a shallower depth of field. Or, if you are shooting at night, you might need to prioritize a higher ISO, so that you can make an appropriate exposure.
With an understanding of these three properties, how they affect one another, and their unique side-effects, you are ready to make an image.