What's the best wide angle zoom?

Short Answer:
1. Contax N 17-35mm f2.8
2. Nikon 17-35mm f2.8 AFS
3. Leica 21-35mm f3.5-4 / Canon 16-35mm L f2.8
4. Sigma 15-30mm
5. Sigma 12-24mm
6. Tamron 17-35mm

Long Answer:
It depends. If you can focus properly without AF in the 17-24mm range (the difficulty of which is not to be underestimated), the Contax, Nikon and Leica zooms all offer something more than the Canon L can deliver.

If we consider resolution, the Nikon is better than the Canon from f5.6 to f16, but the Canon L is better from f2.8 to f4. All require stopping down to f8 for critically sharp corners. The Nikon is sharper than the Canon from 17-25mm; the Canon is at least as good from 26-35mm. The Leica is sharper than the Canon and the Nikon from 24mm to 35mm, but not from 21-23mm. The Sigma 15-30mm is amazingly sharp centre frame at almost all apertures, but can't render a sharp corner at any of them. The Tamron has better corners, but isn't as crisp centre frame.

If we consider distortion, the Sigma 12-24mm is impeccable throughout. The Leica is well controlled: neutral in the 21-24mm range and pincushion from 24mm up. The Nikon has fairly strong barrel distortion in the 17-19mm range, is dead neutral from 20-22mm, and pincushion from 23mm up. The Canon and Sigma 15-30mm are similar. The Tamron is spoilt by extreme waveform distortion at a wide range of focal lengths.

If we look at chromatic aberration, the Sigma 12-24mm and Contax excel. The Canon, Nikon and Sigma zooms all suffer to a similar degree at all apertures. The Tamron is very poor in this regard.

Heading into more subjective realms, the Nikon renders blues particularly attractively, with a neutral colour balance and fine microcontrast. The Contax has a vivid drawing style with all the typical Zeiss virtues of 'believable' 3D rendering and punchy, saturated colours. The Leica is also typically on-brand, with open shadows, true colouring and high resolution in the 24mm-35mm range. The Canon also handles colour well, without obvious cast, but the images don't have the intangible magic and sparkle of the more 'exotic' glass. Both Sigmas are what one might might politely call 'warm' (ie, yellow) – the 15-30mm is marginally worse than the 12-24mm.

What's the best mid-range zoom?

Optically:
1. Leica 35-70mm f2.8
2. Zeiss 35-70mm f3.4
3. Zeiss N 24-85mm f3.5-4.5
4. Canon 24-70mm L f2.8 / Tamron 28-75mm f2.8 / Canon 24-105mm f4 L IS
5. Sigma 24-70mm f2.8

Practically:
If speed matters: Tamron 28-75mm f2.8 or Canon 24-70mm L f2.8
If you can live with the loss of a stop (and a bit): Zeiss N 24-85mm
If you have shaky hands: Canon 24-105mm f4 L IS
The Tamron is absolutely as sharp as the Canons: at wide apertures, perhaps even sharper. It's one third the price and half the weight; however, it isn't weather-sealed, or as wide, or as flare-resistant. The Sigma EX is a big, soft, flabby lens with cacophanous AF, nasty handling and jaundice. The manual focus Contax/Yashica Zeiss is a bargain, and a revelation optically, genuinely rivalling any comparable prime, but stop-down metering, manual focus and the narrow range are deal-breakers for many. All the same applies to the Leica, at ten times the price.

What's the best tele zoom?

Good question. I would contend that AF is essential on a tele-zoom. I suspect the next generation of photographers will consider IS to be equally vital. So for the purpose of Canon-body shooting, we'll discard the superb, mountable (but not very practical) Nikon and Leica candidates and concentrate on the usual suspects.

All the Canon L zooms are fabulous, so why look elsewhere? The new f4 L IS is generally considered to be even sharper than the highly regard non-IS version, and all the f2.8s are wonderful, too. None of them are quite as sharp (up to f5.6) as a decent 85mm, 135mm or 200mm prime, but they run them close; stopped down, they're hard to tell apart.

The only other lens seriously deserving your consideration in this range is the Sigma 100-300mm f4 which, in the 100-250mm range is fully the equal of the best Canon makes, though it fades in the last 50mm.

Which wide angles are distortion-free – you know, for architecture?

Sigma 12-24mm | Nikon 15mm | Nikon 17-35mm (from 20-22mm) | Nikon 28mm AIS
The Olympus 18mm and Canon 24mm L are also very well corrected for geometry.

Which wide angle lenses are aberration-free?

Most Zeiss and Leica. Few others, apart from the Sigma 12-24mm.

I'm building a full frame shooting kit on a budget: recommend some options.

• Sigma 12-24mm / Tamron 28-75mm / Canon 70-300mm IS ($1600)
• Olympus 24mm f2.8 / Flektogon 35mm f2.4 / Canon 50mm f1.8 / Canon 70-200mm f4 L ($1200)

I'm building an APS sensor shooting kit on a budget: recommend some options.

• Tokina 12-24mm / Tamron 28-75mm / Canon 70-300mm IS ($1600)
• Canon 18-55mm kit lens / Canon 50mm f1.8 / Canon 70-300mm IS ($900)

I'm building a shooting kit. Money is no object: recommend some options.

• Sigma 12-24mm (from 12-14mm only) / Leica 15mm f2.8 Super Elmarit / Contax N 17-35mm (from 17-20mm only) / Contax 21mm Distagon f2.8 / Canon 24mm L (from f1.4 to f4 only) / Zeiss ZF 25mm (from f5.6-16 only) / Nikon 28mm f1.4 D (from f1.4 to f5.6)* / Contax Distagon 28mm f2.8 (from f8-f16)* / Canon 35mm f1.4 L (from f1.4 to f2.8) / Leica Summicron 35mm f2.8 (from f4-f16) / Zeiss Jahre 55mm f1.2** / Contax 60mm 1:1 Makro / Canon 85mm f1.2 Mark II (for AF-critical work only) / Zeiss Jahre 85mm f1.2 / Leica 100mm APO macro f2.8 / Canon 135mm f2 / Leica 180mm APO f2 / Canon 200mm f1.8 L/ Canon 300mm f2.8 L / Canon 400mm f2.8 L / Canon 70-200mm f2.8 L IS ($60,000)

* In good light these two lenses could be replaced by the Leica 28mm f2.8 Elmarit.
** Consider also a Contax 45mm. If autofocus-critical work is required at this focal length, you would need a Canon 50mm f1.4

What's the best upgrade for a Canon EF-S 18-55mm kit lens?

What are you looking to upgrade? If you need IS, get the 17-55mm f2.8. If you can't afford the 17-55, and you really, really need IS – and you're not that unhappy with the resolution of the kit lens – get the Canon 17-85mm IS, but know this: it is considerably softer, and up to two stops slower, than the excellent Tamron 17-50mm f2.8.

At the time of writing, there is no Canon equivalent of the excellent VR (image stabilised) long Nikon zooms. If you need more reach, without sacrificing wide end coverage, Sigma's 18-125mm is excellent optically, but a dog's dinner mechanically.

In the absence of a really convicing single lens upgrade, you might consider a Canon 10-22mm (or Tokina 12-24mm if funds are limited), plus a 24/28-XXX tele zoom like the 24-105mm f4 L IS or even the Tamron 28-200mm XR.

What's the best XXmm prime lens?

Please see the fantasy shopping list for the buyer with an unlimited budget, above.

I like to shift / stitch. Which lenses should I buy?

You need StitchPix. However, in summary, buy Pentax and Hasselblad. It's an invariable truth that the larger the format, the lower the resolution of the lenses; the question you need to answer is: which medium format lenses resolve to the limits of my digital sensor? Corner performance on roll film may not be critical if your camera movements are modest.

Outstanding optics suitable for use on a Canon 1Ds II include:
• Arsat 30mm Fisheye VERY SHARP UNSHIFTED BUT BEWARE DETERIORATION IN OUTER IMAGE CIRCLE
• Pentax 645 FA 35mm f3.5 NOT THE OLDER SMC MANUAL FOCUS VERSION
• Hasselblad 40mm CF-E or CFi MUCH BETTER THAN NON FLOATING ELEMENT VERSIONS
• Hasselblad 50mm ALL VERSIONS SUPERB: LATER MODELS SUPERIOR
• Pentax SMC or FA 55mm f2.8 ALL VERSIONS SUPERB: LATER MODELS SUPERIOR
• Mamiya 55mm Shift
• Arsat 65mm Shift
• Zeiss Jena Biometar 80mm AVOID EARLY 'ZEBRA' STRIPED MODELS
Most lenses over 80mm resolve superbly on any digital sensor, including any APO enlarger lens by Schneider, Rodenstock or Nikon. The Pentax 120mm f4 Macro, 150mm SMC and FA are particularly fine, as are all the longer Hasselblad lenses.

Not as well suited for for use on a Canon 1Ds II (relative to the above) are:
• Mamiya 35mm f3.5
• Hartblei 45mm Super Rotator
• Pentax 45-85mm
• Pentax 75mm

Why haven't you tested any 90-135mm prime lenses?

I have – they're all great. Really.

Why are all your recommendations based on the Canon EF mount?

Because for the last five years, Canon have made by far the most capable digital SLRs. Also, thanks to its relatively large diameter, the EF mount is uniquely versatile, facilitating not only the production of some of the world's finest lenses (by Canon), but also the easy adaptation of many more of the world's finest lenses (by Nikon, Leica, Zeiss, Pentax and Olympus).