Choosing a Lens. Part 1

Ok so quite a few people have been asking about the best lenses to buy so I thought I would give a few opinions on the matter.

Whenever I buy a lens I always consider the following variables. Zoom or Prime, Optical Quality, Range, Crop, Speed, Build, Handling and Stabilization. I'll now break each of these down to explain exactly what I mean by each.

Zoom or Prime?

Firstly you need to decide if you want a Prime lens or a Zoom lens. A prime lens is limited to just one focal length (i.e 50mm) so the only way to change your framing to get closer or further away from your subject. The big advantage of primes however is that they use less elements of glass, meaning you get a sharper image (generally speaking) and a faster aperture for low light/ shallower depth of field. Primes are very useful lenses to have but depend upon your workflow. If you are covering a live event or shooting a documentary then you don't want to have to waste time constantly changing lenses as you will almost certainly miss something important. If you are shooting say a short film however, a set of primes is fantastic as you have time to set up every shot.

A zoom lens is of course a lens with a variable focal length (i.e 24-70mm).

Range

Once you have decided the type of lens you want, whether it be a set of several prime lenses or just one or two good zooms you will need to make sure you have the right lenses to cover the focal lengths you desire. 

It is worth mentioning at this point that a very key part of choosing a lens depends on your sensor size. All lenses are measured in full frame terms. That is to say that on a camera with a full frame sensor like the 5D Mark II, a 50mm lens will be the equivalent of a 50mm lens on a full format film camera. Cameras like the 7D or 550D have a smaller sensor which has a crop factor of 1.6. This means that any lens you put on these cameras is magnified by x1.6, so that 50mm would become an 80mm and a 100mm would become a 160mm etc. This is not necessarily a bad thing but is something to consider. With a crop sensor camera a wide angle lens is considerably less wide, however a telephoto lens suddenly has a lot more zoom to it.

Optical Quality

Straight forward enough this covers just the overall performance of the lens in question. So sharpness, vignette, chromatic aberrations, barrel distortion, quality of bokeh etc. I normally judge these by reading a large number of reviews and looking at test stills and video from the lens.

An interesting thing to consider when buying a lens for video is that too much sharpness is currently a bad thing. As current video DSLRs suffer from problems with aliasing, more sharpness results in more aliasing. You should however also consider that these lenses are an investment that will last longer than your current camera body. It's really up to you where you draw the line between cheaper short term buy and more expensive long term investment.

I'll cover the rest of the factors to consider in Part 2.