If an ƒ/2 lens is stopped down to ƒ/8, it will admit more light than an f/4 lens that is also stopped down to ƒ/8.

A lens at ƒ/8 will let in four times the amount of light that is admitted by the same lens at ƒ/4.

An ƒ/2 lens that is set at ƒ/2.8 will admit sixteen times as much light as it does when set at ƒ/11.

If an ƒ/2 lens is set at ƒ/2.8 and an ƒ/1.4 lens is also set at ƒ/2.8, they both will admit the same amount of light.

A 200mm lens produces an image of an object that is twice as large as the image of the same object that is produced by a 100mm lens from the same distance.

The “normal” lens for all cameras is a 50mm lens.

A teleconverter is an attachment that effectively increases the focal length of a lens by magnifying the image that reaches the sensor or film.

A zoom lens is one that allows you to adjust the focal length, from wide angle to telephoto.

If you use a macro lens to focus on a postage stamp that is one-inch square, the image on your sensor or film can be about one inch square.

Flare is the result of light being reflected off the back edge of the sensor.

When shooting a portrait, you’re more likely to produce perspective distortion if you use a 105mm lens than if you use a 28mm lens.

Objects at varying distances from the camera appear closer together when photographed from farther away.

With a 35mm DSLR camera, a 28mm lens would be a good portrait lens.

To avoid linear distortion when photographing a tall structure, position the back of the camera parallel to the face of the structure.