Amongst aspiring bird photographers a common question that arises is 'What focal length lens do I need?'  Of course there is no right answer to this question as different lenses are needed in diffrent circumstances.  However, the underlying anxiety that drives this question is that birds are small relative to the photographic targets of many photographers - they are also very often a long way off!  This leads to the conclusion that we should have as long a lens as possible.  Problems that arise with very long lenses are then:

• They can be very heavy eg the Sigma 300-800mm is about 13 pounds (6kg) in weight
• They need to be long and to have very big objective lenses (the ones nearest the target) which adds to their bulk.  The Canon 600mm f4 is 18" long and 6.6" across the objective.
• They are very expensive - eg the Canon 800mm f5.6 is about £9,800 at present
• As they are very heavy they need to be well supported which means a big tripod, a suitable tripod head and that all adds up to more weight bulk and expense

For an idea of what the extra length gets you in terms of getting close to the bird, look at the illustration below.  This shows a gannet that was fairly close to start with and what proportion of the frame would be captured by lenses of different focal lengths.

Vital Statistics

The table below shows details for assorted long lenses to hlep give an idea of what you might be taking on with different models.  I have listed the Canon and Sigma lenses that I understand to be the best performers and most relevant to bird photography.  There are many others in use and in particular the Sigma 150-500mm with optical stabilisation (OS) appears attractive although it doesn't seem to be as sharp as the older 50-500mm model listed below.  Nikon has a similar range of high quality lenses to Canon.

Recommendations

When starting out, buy the best lens you can afford - the camera body is far less important.  If you can afford it, try the Canon 400mm f5.6 which is a super sharp and fast prime lens and fantastic for birds in flight.  Equally popular is the Canon 100-400mm which is image stabilised, more compact for travel and often just as sharp.  It is however more expensive and a lot of people don't like the push-pull zoom.  If you're on more of a budget, the Sigma 50-500mm is well thought of and many fine photographers get great results with it.  If you can't afford a lens this long, the Canon 300mm f4 is more compact and beneftis from IS.  Later you can add a 1.4x teleconverter to get up to 420mm.

I started with a 2x crop system - Olympus - with the 50-200mm lens.  I then moved to Canon with the 100-400mm which I still have and I now use the Canon 500mm f4 often with a 1.4x teleconverter.  I don't regret having moved slowly to this level of kit as I think the weight and bulk of my current equipment might have dampened my enthusiasm before I had got far enough into bird photography.

Postscript

My comments are based mostly on my experience with Canon equipment.  Nikon produces similarly high quality lenses with which I am less familiar and other good systems are made by Pentax, Olympus, Sony and others.  In addition there are cheaper lenses made by Canon and other third party manufacturers which I haven't touched on but which have provided an excellent starting point for many photographers.  I have discussed on the so-called 'bridge cameras' which have some of the qualities of SLR cameras but with permanently arttached powerful zoom lenses.  Digiscoping has also been outside the remit of this article as it is a specialised area that warrants discussion on its own.