Now I'm going to make a sweeping statement here that is true for me at least, feel free to disagree vehemently. My 70-200mm f2.8 is often used wide open at maximum aperture, my 16-35mm f2.8 rarely. And hardly ever do I see my long lens stopped down to the minimum aperture of f32, but the wide zoom is frequently down at f22.

There are many exceptions to this generalisation, I spent a whole trip in Laos shooting with a 24mm f1.4 lens wide open, and sometimes I'll stop down the long artillery when all is bolted down firmly and I need a slow exposure to record our swaying world. But clearly these different priorities affect how I choose my lenses.

In a nutshell for wide angles speed is less important to me then corner to corner and foreground to background crispness.

Ultimate depth of field with superlative optical performance; that's the requirement. It's a Big Ask.

Dusk at Sheep's Head from Doneen Head, Co Cork, Ireland. Canon 1Ds mkIII, 16-35mm II L f2.8 lens, 3x ND filter + 0.9ND grad, ISO 50, 8 secs @ f16

Compromise, compromise, compromise; it's been the theme throughout this ramble round the photographic world of optics. Now I should stress that this study is a very personal view of what I need from my equipment; we all think and work differently and that's as it should be. For me optical quality is probably the number one requirement. We've already talked about the zoom v prime debate for medium and long lenses and it is my view that the flexibility of zooms in the medium range makes them indispensable tools with performance virtually inseparable from comparable prime optics. At the wide-angle end I don't think that's true, the wider you go the more difficult the compromises between the convenience of a zoom and optical performance seemingly become.

I remember when I bought my first SLR, an Olympus OM 10. Back then in 1980 THE wide angle was the 28mm, 24mm was risqué and 21mm; well, that was just so extreme. Now we take for granted focal lengths of 16mm or wider, and we expect them to be affordable and optically perfect. Spare a thought for the lens manufacturers; one glance at the front of a bulbous 14mm optic tells us these are not easy lenses to make. Throw in the complexities of variable focal lengths and it stands to reason that compromises have to be made.

Now I don't want to spark off another skirmish in the interminable Nikon v Canon Holy War, and there are many other lens manufacturers to consider, but it is a fact that many professionals have concluded that irrespective of what system you're using at focal lengths wider then 28mm prime lenses have a distinct edge. Cue loads of e-mails along the lines of “my 14-24mm is superb”. That may well be, and getting overly obsessive about lens sharpness is one sure way to leading a very a sad life, ultimately it's the pictures that matter.

My 16-35mm II L lens is a flexible, handy, versatile tool that I wouldn't be without, but it can't be ignored; the 24mm f1.4 prime lens is noticeably crisper then any zoom. But for me it's not just about straight optical performance because with landscapes I'm often stopping down to tiny apertures to get the depth of field I need, and that throws up another problem; diffraction.

Dusk on the Isla del Sol, Lake Titicaca, Bolivia. Canon 1Ds mkIII, 24mm II L f1.4 lens, polarising filter, ISO 160, 1/40 secs @ f7.1

Time for a bit of science. Light is energy that radiates as a waveform. Diffraction refers to the spreading out of such waves past small openings, like the aperture in the lens diaphragm when stopped down to f22. This spreading causes unsharpness, which is why lenses always operate at their best at mid range apertures around f8. There's no doubt the resultant softness caused by diffraction is noticeable at anything smaller than f16. But if you need depth of field from the yellow jobbies just millimetres from the front lens element all the way to Monti Sibillini in the distance you've just got to stop down. Worrying about the lens sweet spot isn't an option, using the aperture that delivers the required depth of field is the only way. Or is it? Well, you can always tilt a little.

Monte Vettore and the wildflowers at Forca Canapine at dawn, Monti Sibillini National Park, Umbria, Italy. Canon 1Ds mkIII, 17mm TS-E lens, ISO 100, 0.6 sec @ f11

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