Re: Leaning tires

[palmer@sfu.ca (Leigh Palmer)]

Larry sacks answers the question:

>        At one time fast photography was accomplished with camera that
> used focal plane shutters.  The slit in the shutter travelled across the
> film; the image at the top was taken earlier than that at the bottom of
> the film (or left-to-right or the reverse, depending on how the camera was
> held).  Much commercial photography was taken with the Speed Graphic,
> which used this type of shutter.  Moving images were distorted, so we
> became accustomed to elongated cars and slanted tires for speeding
> vehicles.

Bill Beaty also got this one (in my opinion this is the right answer).
He sent me his answer last night. Bill pointed out that backward-
rotating spokes and related strobe illusions are other examples of
cultural illusions, these brought on by movies.

The Speed Graphic has a springwound window shade focal plane shutter
which runs a slit over the film plane from top to bottom in about
(guess) 30 ms. There are slits of several widths in the windowshade.
A slit 1/30 the height of the cut film will expose the film for
1/1000 s, but in a way that means the bottom of the image (which is
at the top of the film) is exposed 30 ms before the bottom. Since
the wheel moves during that time it appears oval in the photo. Such
photos were very common on the front pages of newspapers every
Memorial Day.

Some older SLRs also have springwound focal plane shutters of a
different sort. SLR shutters move horizontally when the camera is
held in "landscape" orientation. They consist of two independent
leaves.

Oh well, back to writing my lecture on my springwound Macintosh.

Leigh