Is it better to rent camera lenses or buy them when you're starting out?

The short answer: It really depends on what you're shooting. If you do infrequent big events then renting can be a great way to shoot with top of the range gear. On the other hand if you mostly shoot spontaneously, or spend your spare time experimenting then you'll probably want to own your lenses. The type of photography you do comes into the equation too, as certain things such as sports/wildlife really benefit from expensive glass. In other areas knowing how to use your existing gear and how to use light is more important.

The obvious advantage to owning is that you have the gear available all the time and you're free to experiment whenever you feel like it, and you know it's reliable and been well looked after (assuming you've looked after it well!)

The obvious advantage to renting is that you get access to extremely expensive glass for a relatively small fee. If you occasionally shoot wildlife you can rent a 600 f/4 without ever being able to buy it.

The great thing about lenses is that the learning curve is extremely shallow - you can just pick up a lens and go, which is great if there's a lens you only need a few times a year. It's a totally different case with camera bodies, as it pays to know you camera inside out. I would only consider renting a body that I had used extensively before. Control systems vary even with manufacturers, a Canon 5D mkII shooter using a 1Ds for the first time would be totally lost! Plus you don't know how the camera has been set up by the person before you.

Another advantage of renting is that you can try a lens to see if it works for you before buying. You can find out if it has problems with flare, is slow to focus or is simply the wrong focal length. This can be especially helpful if you're new to photography and don't have a handle on what a 400mm lens is like to live with.

For rental places, if you're in or near London Fixation are very good. Otherwise lensesforhire.co.uk will post lenses, that are very cheap but I've had problems with them claiming to have lenses in stock but then letting me down at the last minute, so be careful if renting for an assignment!