Canon Queries

I am fresher to the field of photography and have never attended any course in the subject. I use a Canon T1i and have the following queries: 1) Are all the Canon lenses compatible with Canon T1i? 2) I have Canon 55-250 mm IS lens. I know it is a slow lens. But can you please define slow and fast lens? 3) Also please explain what is EF lens and EF-S lens?

Prasad Kudalkar, via E-mail

1. In India, the Canon T1i is known as Canon EOS 500D. All Canon lenses designed for D-SLR cameras and those designed for film ( EOS) cameras can be used with the 500D.

2. The ‘speed’ of a lens (slow or fast) refers to the diameter size of its widest (maximum) aperture. Wider the maximum aperture, greater the light gathering power and vice versa. Designating lenses in terms of ‘speed’ is useful when comparing two or more lenses of similar focal length, using the same film/sensor format. As an example, let’s consider the following 50 mm ‘normal’ lenses for the full-frame (35 mm) format: 50 mm f/2.8 50 mm f/2 50 mm f/1.8 50 mm f/1.4 50 mm f/1.2

The f/2 lens is 1-stop ‘faster’ than the f/2.8 lens; f/1.8 is 1/3-stop ‘faster’ than f/2; f/1.4 is 1/3-stop ‘faster’ than f/1.8, and f/1.2 is 1/3-stop ‘faster’ than f/1.4. Going the other way, the f/2.8 lens is 1-stop ‘slower’ than the f/2 lens; f/2 is 1/3-stop ‘slower’ than the f/1.8 lens; f/1.8 is 1/3-stop ‘slower’ than f/1.4, and f/1.4 is 1/3-stop slower than f/1.2.

As the focal length increases, the definition of ‘fast’ changes. Here is why: Aperture is described as the focal length divided by the diameter of the front lens element. So if you want f/1.4 on a 500mm lens for example, the front lens element diameter would have to be 357.14mm or 35.71cm (14.28”). This is not practical, if at all possible. It would put the lens out of reach of almost everyone, and would be almost impossible to lug around. Hence, a 500mm f/4 lens is also considered to be ‘fast’.

3. EF is the designation given by Canon for SLR lenses designed for full-frame (35 mm) camera bodies (digital as well as film); EF-S denotes that the lens is designed for the smaller APS-C size sensor camera bodies. Note that EF lenses can be used on APS-C bodies but you cannot use EF-S lenses on fullframe camera bodies.