recommendation: 400mm F5.6 or 300mm F4 IS for long lens
I use two camera bodies for car racing. The lenses determined by where I am shooting from on the track. One with a long lens, and a second with a shorter lens.
400mm F5.6 / 135 F2, 400mm F5.6 /70-200 F4, 135 F2 / Sigma 50mm, 135 F2 / 24mm, 300mm F4 / 70-200
I find the focus on the 400mm F5.6 to be very fast. Same for the 135mm F2, or 70-200 F4. Focus is often easy as cars are large. What takes practice is panning with the subject to show motion (wheels turning, blurred background). That involves getting the right shutter speed to match the car speed and panning. I shoot Tv mode (shutter priority).
I like to include some of the track in the shot. To show the turn being negotiated. Not just a close up of the car. Shots with multiple cars are the best.
The difficulty of car racing pics is that it occurs in mid-day light. So a lens hood is mandatory. There is only so much you can do to make mid-day light look good. Here in New Mexico, USA it is "hot and sunny".
A polarizer filter can help with glare off glass and chrome.
I am mostly shooting from the flagstands at the track.
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