Re: Colored sparks (photography) (fwd)

---------- Forwarded message ----------
Date: Mon, 15 Dec 1997 08:19:28 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Colored sparks (photography)

> I shot a roll of film on my coil.  While most of the shots were the
> normal time exposures of the sparks, I was especially anxious to see
> photos of the brilliant yellow sparks I had produced with table salt.
> Upon getting the prints back, the normal shots came out super, but I was
> utterly disappointed in the yellow spark shots.  The arcs were all
> properly exposed, but their color showed absolutely nothing different
> from the normal sparks.  It's like I was photographing ghosts!  How could
> visual and photographic perception be so different?

Several problems can potentially crop up here:

1) When the prints were made from the negatives, the automatic printer
tries to adjust the color balance so that the picture evens out to a grey
(if you took your picture and fuzzed it all up, it would be a grey blob).
Check the backs of the photos and you'll usually see some letters/numbers
put there by the printing machine. Compare these with some ordinary photos
taken in daylight of "normal" subjects. If they are different, the machine
tried to "fix" your weird exposure.

Check the negatives and see if the image of the spark is colored. It would
be Blue (or bluish green) in the case of the yellow sparks.

2) The spark itself is horribly overexposed. This is most likely. Check the
negative and see if the image of the spark is BLACK (not grey). You want to
shorten your exposure to the point where the spark doesn't saturate the
film. At this exposure, your equipment will be invisible, because the spark
is really bright. To get your coil in the photo, fire off a strobe during
the exposure to light it up.

3) The high amount of blue/near uv from the spark overexposes that layer of
the color film, ruining the color rendition. A little bit of yellow (minus
blue) filtration might help, say a CC30Y, to knock it back.

As you have found, photographing a high contrast subject like a spark (or
pyrotechnics) is quite the challenge. Experimentation is the key to getting
good pictures. That, and understanding where the problems crop up. 

Try to get good negatives first, then worry about the printing. Good means
not overexposed, proper color balance, etc.