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the 100mm f2 is a useful lens. the focal length and fast apperture make is perfect for portraits, and compared to the 70-200 it's a lot less tiring to use.
being a telephoto with an f2 apperture makes it appropriate for things such as sports, where an f2 apperture can let in twice the light of a 2.8 lens, and give you shutter speeds that are more suitable for action sports (IS doesn't help much when your subject is moving)
For the speed you do give up some flexibility, if you're not in the right place when you frame your shot then too bad, because you can only zoom with your feet. Shooting with 2 bodies is good practice for professionals using prime lenses.
The af speed on this lens is fast, it can keep up with people running. The lens is also very well made.
Optically it's a fine lens, sharp a f2 and with great bokeh, the only complaint is the purple fringing at f2, but it's not a deal breaker- the 135mm L has better IQ but is considerably more expensive
the 100mm f2 is a great lens, and pocket and wallet friendly
If you're choosing a fixed focal length lens for portrait photography, there are several options from Canon. There are 50mm/1.8, 50mm/1.4, 85mm/1.8, 100mm/2.0, 135mm/2.8 - I only name affordable ones. There are also some alternatives from Sigma: there are 50mm, 70mm and 105mm, all f/2.8. Tamron has 90mm/2.8, and Tokina has 100mm/2.8.
First of all, several words about portrait lenses in general. Why do you need a fixed focal length lens for portraits? The answer is simple: a zoom lens will never beat a fixed-focal in quality. Also, it's nice to have a fast lens (with wide maximum aperture), and fixed-focals are usually faster then zooms. The wider you open the aperture the shallower depth of field you get, which means more creativity. You should always look at the background blur, which is called bokeh. Different lenses have different bokeh, and it's a matter of taste which one you prefer. And keep in mind that 50mm is a bit too wide for portraits, and you'll see some wide-angle distortion when shooting head shots. Portrait focal lengths start from 75mm.
I was up to a Canon lens, and I was initially going to buy the cheapest one, 50/1.8, but it wasn't in stock at that moment, so I decided to go for 100/2.0. I was never disappointed.
This lens is quite fast: 2.0 at 100mm is a pretty wide aperture. The bokeh is very nice as well. In terms of quality, it's one of the best lenses from Canon. It produces pin-sharp images starting from f/2.0. It captures the skin tones very accurately, and the images look very natural. The contrast is great, and it deals with contra-lighting very well.
The lens is quite compact and light. The autofocus is enabled with USM (Ultra Sonic Motor) and works quickly and quietly. However, I would suggest that you try several lenses before you buy, because sometimes the autofocus is adjusted badly. Mine is almost perfect, but sometimes it needs further adjusting in the service centre.
The only drawback of this lens is that I can't use it all the time (and I'd love to) because the focal length is a bit too long for shooting indoors or in a small studio. If I need to shoot a half-length portrait I have to use a wider lens.
Another thing is that the lens is a bit overpriced in UK. I got it for around $500 in USA, which is £250, but the official Canon website says it's £390! I would recommend shipping it from USA, but that means you won't be able to try it beforehand...
You can find the lens specifications on the Canon website: www.canon.co.uk. Here is some additional info:
Diagonal angle of view: 24°
Lens construction: 8 elements in 6 groups
Number of diaphragm blades: 8
Closest focusing distance: 0.9m (3ft)
Filter diameter: 58mm
Size: 75mm x 73.5mm
Overall, this is a great lens and I personally find it better than both of the 50mm lenses from Canon. Another one which is worth considering is 85/1.8, as it's a bit faster, and the bokeh is very nice as well. But I am very happy to own the 100mm/2.0 and I don't feel like changing it to anything.