what do you use currently now (film camera)? Have you tried a few of the SLR? Do you plan to continue with it (as in how much $$ are willing to spend, all at once or build on it)? Its important to ask these things because 1 you have to try them in your hand. Image quality is almost the same for all beginning slr and even some micro 4/3 are similar. Though I think SLR's sensor are better because its larger, better dynamic range. I suggest picking up a Nikon D7000 or Canon 7d. Then decide for your shelf what's better, based on how fast you learn to use it and if it feels good. Then you can. Go up or down a grade. Try to borrow them or try them in a store. It helps a lot.

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Life is a Game, you have to Play to Win.....
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depends on the camera / lens, day or night and the speed the cars are moving. But generally I'd say the longer the better. It has to draw light trails so it starts at point a and ends at point b. its the car is moving fast, then id say a faster sutter speed like 1", if the car is really far away and moving really slow, it can be like 30 " or more . Hope that helps

--
Life is a Game, you have to Play to Win.....
For tutorials and latest photos of my work, check out
georgexlin.com [link]