Focal Length Comparisons
With the advent of digital SLR with APS-C sensor, 10 and 12mm have become common focal lengths for wide-angles. 10mm lenses (e.g. Canon 10-22, Sigma 10-20) offers a pretty wider angle than 12mm (e.g. Tokina 12-24, Sigma 12-24, Nikkor 12-24).
The Sigma 12-24mm is the only 12mm that works on fullframe (24x36mm) cameras - it offers an amazing and unparalleled angle of view. A 14mm lens - the widest rectilinear wide-angle in Nikon and Canon lineup - has a pretty narrower angle of view than Sigma 12-24, and 16mm (e.g. Canon 16-35 f/2.8) does not come even close to the amazing angle of view of a 12mm.
The Canon 17-40mm is the widest wide-angle for fullframe cameras in the same price range of the Sigma 12-24. The Sigma lens has an huge advantage!
17-55, 17-85 and similar zooms, created for APS-C cameras, offers a pretty wider angle of view than standard zooms for FF cameras, as the Canon 24-105 L IS. Even though I prefer the latter, it is necessary to combine it with a good wide-angle lens if you want a good wide-angle coverage.
24 and 28mm are common focal lengths for mid-range zooms. Whenever possible, I'd recommend a lens that starts at 24mm, unless you already have a lens that covers the widest focal lengths.
In wildlife photography, the 300 f/4 is often compared with the 100-400 and 400 5.6. Unless you photograph very collaborative subjects, you would often use the 300 f/4 with 1.4x, to compensate for the much shorter focal length.
The difference between a 280mm (70-200 + 1.4x), a 300mm (as the 70-300 VR or a 300 f/4 without TCs) and 420mm (300 f/4 +1.4x). If you have Nikon, in my opinion the 300 f4 AFS + 1.4x TC is the best choice for wildlife in its price range - the 70-200 VR is too short, and the only other lens that offers a comparable focal length (the 80-400mm VR) has very slow AF and average image quality.
The f/4 supertele comparision - 420mm f/4 (300 2.8 + 1.4x TC) in comparison with 500 f/4 and 600 f/4.
One of the most common doubts of wildlife photographer that are trying to choose a big lens: 500 or 600? The 600 offers a considerably higher magnification, that is great for small or shy subjects.
The 600 f/4 supertele with teleconverters - 840mm (600+1.4x) and 1200mm (600+2.0x). Of course, this comparison holds true for every lens with 1.4x and 2.0x teleconverters.