When should I use a lens hood?
Marcus says - Most lenses come with a detachable lens hood, unless you have a super wide angle lens where the hood may be built in and not removable. Assuming the lens hood is removable, it is usually possible to fit it onto the lens facing backwards to allow for more compact storage. Many people lose their hoods or just keep them in the box along with the silica gel and instructions, but there are three major uses for them.
The first of these is to reduce the amount of peripheral light from entering the lens. Peripheral light is light from areas outside your frame, so you don't want this light interfering with your image. When peripheral light does enter the lens, it will reduce the contrast of your image. Let's say we are shooting a scene in deep shadow (say a portrait of someone standing in a darkened doorway) but above us we have a bright overcast sky. We can't see the sky in our frame, so it is not important to us (except for lighting our subject). If we allow the light from the sky to strike the front of the lens, some of this light will enter the optical system and get bounced around inside the camera. When it finds its way onto the sensor, it will prevent anything that should be black, or close to black being rendered as such. The result is a loss of contrast with shallower blacks and weaker shadow areas. Simply by using your lens hood, you will increase the contrast of your image, which in turn will improve clarity and sharpness. In extreme situations where lots of peripheral light enters the lens, it gets rendered as flare. Flare can either be specular (when you get clear lines of stars or highlights in your shot) or ghost (a loss of contrast as just described). Using a lens hood won't always eliminate flare, but it will always reduce its effects. Hoods on wider angled lenses are less effective than those on telephoto lenses, as that latter can block more light without interfering with the corners of the image.
The second use for a lens hood is to help keep rain or sea spray off the front element. If you are shooting in high winds, facing into the wind, then a lens hood isn't going to be of much use here, but if the rain is falling vertically or across the lens, then a hood could make the difference between a great shot and one which is ruined by blurry blobs all over the frame.
The final use for a lens hood is simply protection of the lens. If you drop a lens without a lens hood attached, the chances are that you will do a lot more damage to the lens than if you had a lens hood attached. The lens hood may crack or break, but this takes the energy out of the fall and helps to protect the lens and its fragile optics.
In short, using a lens hood is only every going to make your images better and help protect your lens. If you have one, use it all the time. There isn't a single example I can think of where using a lens hood will be detrimental to the image.