[MF3D.FORUM:336] Re: or a ouija board

[Greg Erker <erker@xxxxxxxxxxxxxx>]

># Because light travels very fast, almost instantaneously, it is possible
># to estimate the distance between an observer and a thunderstorm from the
># time difference between the lightning flash and the thunderclap. The
># "rule of thumb" is that every five seconds time difference corresponds
># to a mile distance. Is this correct? In imperial units sound travels at
># approx. 0.2 mile/second (actually 0.214 mile/s) thus in five seconds
># one mile is covered by sound and the "rule of thumb" is about right.
>
>*This* rule is everything the "1/30 rule" isn't.  It's just what it
>seems to be.  There are no special cases that invalidate it.  It's
>not wildly off under common circumstances.  Even a child can get
>within 7% the mathematically correct solution in their head.

  Yes, and maybe no. Does anyone know how much the
speed of sound changes with altitude (air pressure).
If you are at 10,000 ft altitude does the speed of
sound change enough that the 5s/mi or 3s/km "rule"
is significantly off?

>By comparison, the "1/30 rule" is a ouija board.  Witness the many
>misapplications of it in this thread alone.

  Isn't the 1/30 rule (suppossed to be) just for
scenes that include infinity? If so 1 in 30 ain't bad:
http://www.deering.org/curve1.gif

  Mis-applying it to indoor scenes and closeups is
a problem:
http://www.deering.org/curve3.gif

  But at least it gives you viewable slides with
not a lot of depth, instead of eyetwisting ones
with way too much depth.


Greg E.