> would it be better to get a cheaper image [stabilised] lens like a canon 17-85 IS or is it better to get a constant
aperture like a sigma 18-50 2.8..? <
For any indoor sports photography, at the shorter focal lengths, it is better to have a faster lens than and Image
Stabilized lens: thus, in this regard (and all other issues equal, like focussing speed, for example) the Sigma 18 -
50F2.8 is the better choice of the two lenses you mention.
Adequate Shutter Speed freezes Subject Motion: Image Stabilization does NOT freeze Subject Motion.
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As a general answer, for indoor sport, your lens choice is dependent upon four main factors:
1. How much light you have: goes to determining the Speed of the Lens (how bright the scene is: TV Trans Lighting
might be 6EV more than a School Gym)
2. The speed of the sport: goes to determining the Speed of the Lens (e.g. Chess might requires 1/50 to freeze
motion / camera movement, Ice hockey might require 1/800s
3. The distance you are from the action: goes to determining the FL, or FL range of the lens.
4. Whether (or not) you have the ability to roam: goes to determining the FL or FL range of the lens.
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Whilst F2.8 zoom lenses (e.g. Canon`s 24 to 70 and 70 to 200) are very useful for indoor sport, there are many
times, especially in amateur venues, or professional venues lit for amateur meets, where the lighting and the speed
of the sport will require a lens faster than F2.8. And this means using a Prime Lens.
Each Sport, and the level of that sport (e.g. Schoolboy is usually slower than professional) has minimum shutter
requirements to freeze motion, at different times during the sport and at different camera angles.
Freezing motion is not the be all and end all of everything, but most sports photos do freeze motion.
As an example, I know that at a the proficiency level of National Swimming, 1/640s is right at the limit to freeze a
Backstroke Start, even then I get some blur in the hands and sometimes the feet.
http://photo.net/photodb/photo?photo_id=7303794
For your requirements, I think you could take some test shots with a fast lens you have or borrow one to determine
what light you actually have, and also determine how close you can get to the action.
It would be silly buying an 18 to 50mm Sigma only to find that you really needed a faster lens such as a 50mm,
85mm or 100mm because the F2.8 was too slow or you needed a longer lens like a 200mm prime, to get good, tight
shots.
WW