In a message dated 97-07-30 03:34:20 EDT, you write:
> >Anyone know the cause of "starburst" patterns when shooting into
headlights?
> From what I've been told, the effect is caused by the diaphragm. Try
> shooting at different f/stops and see what happens.
>
I've understood that the star pattern is a combination of the iris/diaphragm
and the light itself bouncing around between the various lens elements, and
within each lens itself. And the ratio of exposure between extreme lights
and darks has something to do with how long and saturated the points of those
stars get to be. Notice how the number of points on the stars is the same as
the number of leafs in your f-stop diaphragm, and also how the arms of the
star get longer and finer as the f-stop aperture gets smaller. At f-16 or
22, for example, just the very center (the thickest part) of the lens is
passing light, but to get a proper exposure the shutter must be open longer.
With the lens wide open, a shorter shutter open time is required, stars
seldom appear except under the most extreme light and dark conditions. In
longer night or dusk exposures at smaller apertures, the street and yard
lights will also expose into star patterns, as will traffic and wayside
signals, eventually. To create a star pattern on a headlight, try using the
smallest aperture possible (f-11, or smaller). Remember, the star pattern is
a function of lens and aperture, not ASA, though slower shutter speeds will
add definition to the arms of the star.
Star-causing filters create a similar effect by simply adding fine grooves
(scratches) to a flat filter mounted ahead of the lens. The number of points
on the star is determined by the number of directions of the scratches (the
more directions, the higher the price, too). And the rainbow look is created
since what is actually being photographed is a prism spectrum created by the
angle of the groove (scratch) cut into the filter (notice how shots made with
a star filter also have slightly less contrast, as those scratches fog the
filter slightly). So while Nikon, Canon, Olympus and Leica shooters will
have different points on their stars, star filter users will always have the
same number of points on their stars.
Stars and star-pattern filters do have a place in interpretive rail
photography and videography. But a little bit goes a long way..... A bit of
a star around a single headlight can often be impressive, especially when the
locomotive face is black from shadow, for example. Haven't yet tried it with
ditch lights, but the three-light pattern could either be interesting or
overkill. Will give it a shot.....
....Mike Del Vecchio
--> SPORRS: Serious Photographers of Railroad Related Subjects
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