Originally Posted by
Tarzanman
People have already chimed it, but I will too. I will make it really simple and avoid talking about exposure.
• Different lenses have different focal lengths. Most lenses are named for their focal lengths. An 100mm lens has a 100mm long focal length which is longer than a 35mm, etc.
• Generally, the longer the focal length the more 'zoomed in' the picture will be. (So, a 35mm lens might give you a photo of someone and people standing around them while 100mm lens might only give you a close-up their face)
• The aperture is the diameter of the circle that lets the light in. Think of a pin-hole camera... some people like to use bigger pins to make the hole bigger and let more light in. More light is sometimes necessary, like for night shots or places where they don't allow a flash.
• The f-number is a shorthand way of saying how big the aperture is compared to the focal length of the lens. It is the focal length divided by the aperture. So, an 80mm lens with a f-number of f/2 has an aperture that is 40mm in diameter. Accordingly, a low f-number (f/1.2) on a lens means a large aperture.
• Modern lenses allow you to change the aperture setting, but they all have a limit as to how big and how small you can go. The more expensive lenses let you have larger apertures (low f-numbers)
• Lenses with large apertures have to be better designed because there is more light to focus and a larger area of the lens is used to focus the light.
• Ever taken a picture that got blurred because the camera moved, or maybe someone in the picture moved? The faster the shutter speed on a camera, the less you get motion blur.
• Since lenses with large apertures (low f-numbers) let more light in, they can take better photos in low light conditions without necessarily having to rely on a flash. Photographers call this ability the 'speed' of the lens since it lets them use faster shutter speeds to avoid motion blur.
• Using a large aperture setting on a lens will give a small area of sharp focus on the subject while making things in front & behind the subject go out of focus. The area that is left in focus is called the depth-of-field (DOF).
• Using a small aperture setting on a lens keeps a large area around the subject in focus. The drawback to doing this is that more light is necessary to get a good photo.
Last... lenses with extremely large apertures are overpriced! (Some people will argue and say that the price difference is worth it). The most ridiculous example of this is the price of the Canon 50mm f/1.8 lens vs the price of the Canon 50mm f/1.2. The f/1.8 has an aperture of 27.78mm and costs $75, while the 50mm f/1.2 has an aperture of 41.67mm and costs $1,329.
Since the 50mm f/1.2 doesn't come with a magic wand that turns you into an awesome photographer, some people (myself included) don't think that the difference is worth the extra cost (unless you have a specific need which requires that particular lens).
Of course, if you're rich.... you probably have $1000's of dollars just lying around the house in your leather couch or in the back pocket of some worn-out Gucci jeans. Spend away!
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