A brief word of explanation on what a "fast lens" is. In order to take a picture, you have to allow light to pass through your lens. Light passes through your lens when you press the button on your camera and open the shutter. The amount of light that passes through the lens depends on two things -- how long the shutter is held open and how wide the shutter opening is. If you can open your shutter very wide, you don't have to keep it open nearly as long to get the same amount of light into the camera.
When someone tells you that a lens is a "50mm f/1.8" or a "100mm f/2.8," they are telling you two things. The first number is how far it is from the front of the lens to the film or digital sensors (50mm, 100mm, etc.). That number is not important for this discussion. The second number tells you how far you can open the lens. The lower the number, the wider the lens opens. So, my f/1.8 lens can open wider than my f/2.8, which opens even wider than my f/5.6.
Shooting in low light presents a problem -- camera shake. Suppose I want to take a shot at night. At night, there might not be much available light to take a shot. If I want the subject to be visible in the final picture, I need to get more light into the camera. But how do I do that? Well, one way is to open the lens as wide as it will go. However, suppose you've got it open as wide as it will go and you *still* don't have enough light. Well, the next solution would be to increase the amount of time that the shutter is open. So, instead of holding the shutter open for 1/50th of a second, I'll hold it open for 1/10th of a second.
The problem with that solution is that my hands shake. If you hand-hold a camera and keep the shutter open for 1/10th of a second (or longer), you are going to notice that your image is blurry. The longer you hold it open, the worse the "camera shake" becomes. The only other option* is to get a lens that will open wider. A f/1.8 lens is pretty wide, so I don't have to keep the lens open nearly as long as I would with my f/2.8 (or f/5.6) to get the same shot. Since I can shoot faster, the lens is called a "fast lens."
Anyway, so I bought the 50mm f/1.8 lens a few months ago. This shot, of the Empire State Building (shot from across the street) is one of the first shots I took with that lens. This shot would have been much more difficult with some of my other lenses.
|From Wolfish Musings Pictures|
Canon XSi, 50mm lens
As always, critiques, comments and criticisms are welcome and encouraged.
Statue of Liberty
Trinity Church, September 11, 2008
* Yes, you can bump up the ISO rating too. However, I don't want to get too technical -- this is a very basic description of a fast lens.