Cameras can be a rather confusing mess of technology for those just starting off in Photography. It's easy to just leave the camera in auto mode and let the camera do all the work. But then you start looking at other people's shots and start asking why your photos don't look like theirs. Some will argue that the metering systems in cameras are so advanced now days that you really don't need to venture out of the automatic modes. Well, that's true to an extent, if you're simply wanting a well exposed photograph of the subject you're shooting. If you want to venture into the more creative side of photography, manual mode is the way to go. This is where you really start to manipulate light to portray things in a way that the human eye cannot see on its own.
Say for example you want to paint light trails from a car's headlights zooming through your photo. A camera's automatic mode really only knows how to adjust the exposure settings to provide a well exposed photograph. In auto mode you might get light trails, you might not. It'll depend on the metering setting you are using, spot vs evaluative, which determine how your camera interprets the lighting conditions. To really take advantage of the creative opportunities your camera can provide, you need to take control of the camera and tell it what to do.
The purpose of this post is to focus on shutter speed. I will cover aperture in a later post. Before we venture off into shutter speeds, lets first talk about ISO. Take a break from reading this post for a second and try what I am about to say.
Go into a dark room, close your eyes for a moment and leave them closed. Open your eyes quickly and then shut them again. What did you see? Absolutely nothing. This is also what your camera sees when you use a fast shutter speed at night.
Now, turn on a flash light and close your eyes again. Open them once more and then quickly shut them. What did you see? Most likely a dimly lit view of the room you are in. Think of this like changing your ISO setting from from 100 to 200.
Now, turn the flashlight off and turn the light switch on for the room you're in, then repeat the exercise of opening and closing your eyes quckly. What did you see? A brief view of a very well lit room. You likely saw a lot more detail and could make out certain objects around you. Think of this like changing your ISO setting from 200 to 800. The higher the ISO, the more sensitive to light your camera will be.
By now you're thinking great, I'll just crank the iso up to 12,500 to make it super sensitive to light when I'm doing night photography. Well hold on there tiger, the more ISO you use, the more noise you introduce into your photograph. When shooting photos at night or in dimly lit situations, you only want to use more ISO if you want to use a fast shutter speed to freeze your subject. A good example of this is if you are shooting portraits past sunset and you're using available light. You want to use a fast shutter speed so your subject will be sharp and available light is diminishing quickly, you can bump up the ISO to make the camera more sensitive to the light that is still available. Just note that doing so will introduce more noise to your photograph.
Now, lets talk about shutter speed for a bit. Shutter speed is exactly what it says, it is the speed in which the shutter opens and closes. Think back to the exercise we just did where you were opening and closing your eyes. That's shutter speed. Leaving the shutter open longer means more light being captured by the camera. The difference between ISO and Shutter Speed is that ISO makes your camera more sensitive to light independent of the shutter speed you use. A longer shutter speed physically leaves the shutter open for a greater length of time. If your subject isn't completely stationary, it'll appear blurred in your photograph because the subject is moving while you are exposing the scene.
A good example of this is if you are photographing a long exposure of a tree during a windy day. The tree will appear blurry because its branches and leaves are swaying in the wind while you are still exposing the photo. If you wanted to freeze the tree, you would have to use a fast shutter speed and possibly increase the ISO if there isn't a lot of available light.
I will now use three photographs of the exact same scene to help demonstrate what the different shutter speeds will do for the scene you are photographing. All three were shot in succession using ISO 100.
The first photo here was captured at 3.2 seconds. The photo is clearly underexposed due to the little amount of available light. There were moving objects in the scene, which were the cars on the freeway. At 3.2 seconds the headlights started to blur. If I were to have shot the scene faster than 3.2 seconds, the photo would have been darker but the headlights would be frozen. The buildings appear dark because the shutter speed was too fast to draw in more light. The signs at the top of the building appear perfectly exposed though because they emit a tremendous amount of light and the fast shutter speed limited the amount of light entering the camera.
The second photo here was captured at 13 seconds. The overall exposure is better but still not perfect. The headlights are fully streaked since the shutter was open for a very long time compared to the blotchy looking headlights in the faster 3.2 second exposure above. The buildings aren't as dark since the longer shutter speed allowed more light to enter the camera. The signs at the top of the buildings are starting to appear over exposed and that's because they emit so much light that the camera physically captured too much light to properly expose the signs on the buildings. If you zoom in, the signs look too bright now.
The third photo here was captured at 30 seconds. The overall exposure is really nice. Most of the areas within the photo are exposed very well, meaning there aren't too many dark spots. The buildings appear bright, the headlights from the cars are streaked perfectly through the photo, and the sky now has a ton more color in it compared to the 3.2 second exposure. The only problem with this photography is that the signs on the building are now completely over exposed due to how much light they emit and how long the shutter was open for. This photo will require significant highlight recovery in post processing but the good thing is the rest of the photo is exposed well and won't require a lot of processing.
As you can see from the three examples above, faster shutter speeds will freeze moving objects and let less light in. Longer shutter speeds will blur moving objects and let more light in for a brighter exposure. Stationary objects, such as buildings and signs, will remain stationary during a long exposure, so long as your camera is mounted on a tripod. When you simply throw your camera into auto mode and let the camera take its best guess at how to expose the photo, (as my daughter would say) you get what you get and you don't throw a fit. If you want to really begin experimenting with the creative possibilities that your camera allows, turn that dial on top of your camera over to "M" and begin experimenting. I've said it before and I'll say it again, digital film is cheap these days. Don't be afraid to shoot hundreds of photos, it's the only way you'll truly learn what your camera can do and start to develop your own personal style.