Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Pictures of fast cars

Learn more physics!

Most recent answer: 10/22/2007
Dear Sir, In photographs of a fast moving object (say a car), why is the background of the object vague while the object itself is very clear? Thank you
- soumya (age 19)
st thomas college,kozhencherri, india
Well, you have to work pretty hard to get a nice picture like this -- professional photographers who hang around race tracks a lot get good at it and the results can be quite striking.

Of course now that cameras are largely digital now, the arguments are a little different, but here's an argument that comes from old film cameras.

Film is exposed in a camera when a shutter opens up and lets light through. The film takes some time to expose (usually a few hundreths of a second to a few thousandths of a second). In this time a car can travel far enough so that the light from the parts of the car at all the intermediate locations ends up smeared out on the picture -- the usual motion blur. Now who wants to look at a blurry car against a clear background? Normally we want to focus our attention on the object in the foreground of a picture and the more indistinct, fuzzy, or bland the background is, the better the foreground looks.

So the idea is for the photographer to swing the camera around so that the car's image is always in the middle of the frame during the exposure. Sound hard? It is. For every picture you see like that there are probably many where the car is blurry and so is the background.

There might be machines that can do a better job than a person in doing this, and perhaps digital techniques can also help out here a lot.

Incidentally, some film cameras have what's called a "focal plane shutter" in which a very short exposure is accomplished with a slow shutter by having only a thin slit of the shutter open at a time, and the slit travels across the film. A piece of film is only exposed during the time the slit is actually over it, so the exposure time can be made very very short by narrowing the slit. Because the total time the slit is in motion is very long, a racing car can move quite a lot in this time span. If the sit is narrow and the exposure time short, all the pieces of the picture can be in sharp focus, but the car may appear stretched out or compressed, depending on the relative direction of travel of the car and the shutter slit.

Digital cameras may require some "integration time" to collect enough light to make a good image and have many of the same issues of motion blur. They don't make them with focal-plane shutters, though.


(published on 10/22/2007)

Follow-up on this answer.