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What Lens Should I buy?

I want to add a lens to my toolkit

What do I need to know before I buy?

Many cameras these days are sold with a zoom lens as part of the deal. Effectively this means you are getting more than one lens for your money. This is a relatively new phenomenon, as glass for SLRs and other cameras was expensive in the era before digital, so usually a 35mm film SLR was sold with a 50mm lens. This lens is considered the standard for 35mm film. It most closely approximates human vision in terms of angle of view and perspective.

Why is this important? Well knowing what your camera's standard lens is can help you make choices in terms of angle of view and depth of field. Generally speaking, focal lengths longer than your 'standard' compress the scene and give less depth of field at a given aperture than the corresponding aperture on a normal/standard or wide angle lens. Any lens then with focal length shorter than the normal/standard is consider wide angle and has a greater depth of field at any given aperture compared to the normal/standard lens.

How then does one calculate 'normal', given now that 35mm film is hardly a standard value?

The simplest method I feel is to simply calculate the diagonal of the format you are shooting. The formula is then, d =√ of a² + b² where d is the diagonal and a and b are the two edges. So a 35mm film camera has a 44mm diagonal and therefore 50mm is considered the closest to 'normal'.

Now this allows you to consider what lens zoom or prime to consider when moving forward with your equipment acquisitions.

A list of common 'normal' lenses

All measurements are approximate.