Basics of Shutter Speed, Aperture, and ISO
If you search the web for shutter speed, aperture and ISO, you’re bound to find hundreds of resources and articles dedicated to explaining the details of the terms. And while it’s very important to understand these concepts, sometimes these aforementioned resources unnecessarily complicate these topics by using high-level, advanced terms. The purpose of this article is to get you familiar with Aperture, Shutter Speed, and ISO. After reading this article, you should know enough to start shooting and learning by experience. After having some experience in the field, I strongly suggest going back to these aforementioned articles to complete your understanding.
First things first, we need to know how to interpret shutter speeds and apertures. Shutter speeds are expressed as fractions of a second, so 1/50 is a longer period of time than 1/200. However, when people say “higher shutter speeds,” they’re referring to the faster shutter speed, not mathematically higher numbers. So even though 1/50 is greater than 1/200, the “higher” shutter would be 1/200.
Like shutter speeds, aperture is also stated in fractions, so it’s important to know that the smaller the number the bigger the opening. Therefore, an f/1.4 is a very large opening while f/22 is a very small opening.
In it’s most basic sense, you can think of exposure as a bucket with an adjustable hole that opens and closes at the top and a continuous stream of water pouring into the bucket. The smaller the hole at the top, the longer the hole would need to be open in order to fill the bucket with water. Conversly, the larger the hole, the shorter amount of time the hole would need to be open in order to fill the bucket.
So in this analogy, the hole in the bucket is the aperture, the amount of time the hole is open is the shutter speed, the water pouring in is light, and a full bucket of water is a properly exposed image. If there is not much water (light), the hole (aperture) would need to be open wide; and/or the duration of the opening (shutter speed) would need to be longer in order to fill the bucket (properly expose the image). If there is a lot of light (water), the aperture (hole) would not need to be open as wide; and/or the shutter speed (the duration of the opening) could be shorter to properly expose the image (fill the bucket).
To simplify and summarize, a lot of light = faster shutter speeds/higher (smaller) apertures, while a little light = slower shutter speeds/lower (larger) apertures.
Note the following shutter speed and aperture sequences in the table below. First, it’s important to note that these are not all of the possible shutter speeds or apertures, but simply an example of a full stop sequence. A full stop means you either double, or half the amount of light in the exposure. So if the hole (aperture) stays constant, going from 1/100 to 1/50 shutter speed is a full stop increase since it would double the light exposure by doubling the amount of time the water (light) fills the bucket. Likewise, if the duration (shutter speed) remained constant, going from f/2.8 to f/2 aperture is a full stop increase in light as it doubles the amount of water (light) filling the bucket by doubling the size of the hole (aperture).
Second, note that these combinations will not necessarily yield correct exposures, as this will depend on the amount of light in each respective scene. However, each one of these sequences yields the exact same level of light exposure based on our explanation above. As you move from one sequence to the next, the shutter speed decreases by one full stop, while the aperture size inreases by one full stop. Thus, the exact same amount of light is exposing the picture. If this still isn’t making sense, just take a glance at the table below and keep reading!
|Shutter Speed||1/4 second||1/8||1/15||1/30||1/60||1/125||1/250||1/500||1/1000||1/2000||1/4000|
Essentially, as you double the amount of light coming in with your shutter speed (by doubling the amount of time), you need to half the amount of light you let in with your aperture and vice versa. The doubling and halving is easy to see with shutter speed (as they are multiples of 2 or a number close to 2). However, aperture is a bit more complicated, as it involves the focal length and calculating the area of the hole. We’ll save this for another post or for you to search elsewhere, but the main concept is that the area of the hole doubles and halves with each of the f/stops (aperture settings) mentioned above.
Is this starting to make sense? If not, don’t worry it will with a bit of practice. It’s actually more important to know what aperture and shutter speed actually do to the composition of the picture. In general, slower shutters capture more blur, i.e. movement or streaks of light, while faster shutter speeds freeze motion. The speed necessary to freeze the motion depends on the speed of the movement of your subjects. If you would like to know what your minimum shutter speed to avoid hand shake, a good general rule to follow is the reciprocal rule (explained in our article on the reciropcal rule).
The lower the aperture, the shallower your depth of field. What does this mean? Well, if I’m at a low aperture (f/1.4, f/2.8, etc) and I focus on one person in a line of people extending towards and away from my camera, everyone before and after him will be blurry. In contrast, if I wanted more people in focus, I would chose a smaller (higher) aperture (f/8, f/11, etc). This is actually identical to the way our eyes work. Ever notice how you squint to try to see something more clearly? It should be mentioned that for overall sharpness, there are ideal apertures that vary for each lens (we’ll save this for another post or for you to research). Furthermore as you shrink your aperture (f/8, f/11, and up), you may start to run into diffraction (see our article on diffraction).
Occasionally Pye, an Alumni of the University of Utah, still shoots football games from the field when he has a chance. In the following image, he wanted to capture the movement in the swinging foot while keeping the body crisp so he chose a shutter speed of 1/200 which is fast enough to freeze the body, but just slow enough to capture some movement in the fast moving foot. Additionally, we wanted the focus of the image on kicker (and the stadium was also relatively dark)so he chose an aperture of f/2.8 to gain a shallow depth of field and let in enough light.
ISO – As if this wasn’t enough information to process already, there is a third factor called ISO (or film speed). Basically, the higher your ISO, the less light you need (allowing for faster shutter speeds and smaller apertures). You can think of ISO as an artificial way of creating more light, or for instance forcing the water (light) into the bucket by pressurizing it. There is a trade off though to using ISO to increase available light, higher ISO’s will yield grainier, less-than-tack-sharp images. The effect of this depends on your camera and its sensor quality. For example, we cap our ISOs at 800 (ideally 400 or under) on our Canon 40Ds and at 6400 (ideally under 3200) for our Canon 5D Mark IIs. Although you can get acceptable images above these figures, we prefer to limit the grain in our images and use our knowledge of light and flash to compensate.
We hope this helps. Feel free to comment if you have anything to add.