Expert: John Oliverio - 6/12/2008

Question
What do the numbers on the lenses actually mean? E.g. Canon EOS Lens EF 70-300mm f/4-5.6 IS USM

Answer
Stuart,

70-300 deals with the focal length (sometimes called power but this is incorrect) of the lens.  To put focal length into perspective, on a digital camera a lens around 35mm is "normal".  So 70-300 is a tele-photo zoom lens more powerful than normal.

f/4-5.6 is the aperture of the lens.  This expresses how much light the lens transmits, and is sometimes called the "speed" of the lens. The reason there are two numbers is that this lens gets darker as you zoom it out.  The larger the number, the darker the lens is.  4-5.6 is average for an amateur lens.  A fast pro lens might be f2.8, for example.

And in case you were wondering, EOS just means it is a Canon AF lens.  IS means it is image stabilized.  And USM means it has an ultra-sonic motor instead of Canon's lower end EF motor.

John