How To Buy A Lens  - Mike McNamee & Jon Ashton

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In the previous article, Jon Ashton touched on the choice of lens for natural history and garden birds in particular. We decided not to waste the analysis we had conducted when trying to put some order in the price of lenses and this, in turn, morphed into this feature.

More than any other item, except perhaps for camera bags, the choice of lenses on offer is so vast that any decision to buy is a mass of compromises, a situation sometimes only solved by the purchase of more than one lens!

The decision to buy a lens runs something like this:
 

1. You have to start with a need for a lens, rather than a desire to collect neck-jewellery (leave that for the amateurs, but never forget – they buff up the profits so we can have what we need!).
2. What is the image you wish to capture and, crucially, how far away is it and how big? A secondary issue – 'is it dangerous?'.
3. What are your anticipated light levels and shutter speed requirements – this will determine your ISO requirements and may be reflected in your choice of camera body?
4. What are your depth of field requirements? For portraiture they may need to be minimal (buy a really wide-aperture lens!) and for macro they will almost certainly be as much as you can have (so other than assisting with focus, a wide aperture is a bit of a waste).
5. What is your budget? This may influence the decision to buy OEM, independent, second hand or even to hire a lens.
6. Think about secondary things such as weight, physical size, vibration reduction and how you intend to hold the camera. This has assumed greater importance if you have to travel on an aeroplane.

Does your intended use benefit from having a zoom facility? (see next section).

Zoom or Fixed?
Generally fixed focal length lenses (primes lenses) are less expensive than zoom lenses especially outside of the popular mid-range zooms. The aperture of a zoom lens may decrease with increasing focal length; check that your choice has enough light-gathering power at the long end.

It was always held that zoom lenses were not as critically sharp as prime lenses. More glass and more moving parts is never ever going to make a zoom sharper than a prime lens. However, with modern lens design (including the availability of new glass formulations and particularly, moulded aspheric elements), the difference between a zoom and a prime may be swallowed by atmospheric haze or camera shake. The same applies to stabilisation of lenses. Putting a slab of gimbal-mounted glass in an optical train is never going to improve performance on the optical bench, but birds and footballers don't sit on optical benches, they are in the field and moving!

The single greatest danger when using a zoom lens is zooming with the lens rather than your feet! A frequent criticism during qualifications adjudication (in particular for wedding applications) is that the photographer has remained rooted to the spot choosing inappropriate focal lengths instead of moving and then recomposing. A 100mm lens always was, and remains, the best focal length for a natural head and shoulders portrait (to full frame). Staying where you are and zooming wider will leave you with over-large hands and noses as often as not (think post football match interviews, always done in narrow corridors!).

One of the peculiarities of zoom lens use is a tendency to use them only at the extreme ends of their range. If you are one of these users you might ask whether two lenses or a lens and a teleconverter are your better options.

 

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