> > On Fri, 05 Mar 2004 04:35:32 -0800, John Mustarde wrote: > > > Just curious - what shutter speed catches just the right amount > > of motion blur when the subject is traveling 100 mph? > > I generally use 1/125 for cars below about 80 mph and 1/250 for > anything faster. Of course, it also depends on which direction they're > moving in the field of view. Cars moving side-to-side but not getting > closer or farther away can use shorter shutter speeds and still show > motion.
It's not that difficult to figure out, really. A speed of X MPH is as close to 1.5X feet/second as you'll need for photographic purposes. So, with cars going at 100MPH or so, they are travelling at something like 150 feet/second. This means that with a shutter speed of 1/125 the car will travel a little over one foot (and half that distance at 1/250). Either of those will provide a reasonable amount of blur on the tyres or background, and are fast enough to make panning not too difficult. To totally blur the lettering on the sidewall of the tyre, say, you would want the wheel to make an entire revolution during exposure. If you estimate the diameter of the wheel at a little over two feet, then you want the cars to travel maybe seven feet. That dictates a 1/20 second exposure at 150 fps (and a steady hand while panning). It's harder when the cars are coming directly towards you, because you can't pan to keep the image fixed in the viewfinder. I've found that using shutter speeds slower than 1/500 can produce unsatisfactory results in cars with a lot of fine detail on near-horizontal surfaces (such as the logos and lettering on the nose, sidepods and rear wings). Again, that follows from the same back-of-the-envelope calculation; 1/500 at 100MPH means the subject moves three inches during exposure. While much of that travel is towards you, there's enough left over to take the sharpness off edges. Looking back at the ChampCar image I posted, there's only around an inch of blur on the tyre lettering - not enough to really stand out. If my estimate of car speed (50mph) is correct, that means that I was using a shutter speed considerably faster than the 1/250 suggested by Frank. And, in fact, the EXIF data in the original image reports a shutter speed of 1/800 (and an aperture of f7.1); about what I'd expect to be using for a near head-on shot (imagine the cars a bit further away from me, at approximately the apex of the turn). This time I held off making the exposure for perhaps a second (letting Bourdais close up on Tracy, who was running slower than expected). With my PZ-1p (though not with the MZ-S) I *might* have decided to switch to a slower shutter speed during that second or less; there was no way I was going to try that with an unfamiliar camera, even though the controls are in the same place as on the PZ-1p (& *ist-D); Murphy means I would almost certainly have gone the wrong way.
