Fire Investigation and Camera Lenses

I was on a scene in Moreno Valley off CA-60 and there were three fire investigators and me. (Of course, being the only forensic engineer on the scene I got ribbed for everything. )

However, I did notice that everyone was shooting their cameras while the investigators were shoveling and sweeping debris around. It was probably the worst environment for a camera excluding dipping in liquid acid. The lenses were getting coated with ash and dust.

The usual answer I hear from camera users is: "Oh, I just wipe it off later." Wiping the lens when it is coated with ash and dust is like running fine sandpaper over the lens. It will degrade more and more with each cleaning.

Camera lenses are meant to bend light. But not all colors of light bend the same amount. Early lens designers saw this as creating unwanted rainbow effects around light sources in their images. Modern lenses compensate for this by applying lens coatings. These are very thin coatings on the lens that affect diferent colors of light to bend different in the coating. The result is that the lens coatings compensate for the fact that light colors all bend diferently. If these coatings get damaged, the lens is ruined. But it is not an easy thing to observe.

I use a UV filter or Sky Filter over my lens to protect it from the dust. If that gets ruined, it is only a few dollars to replace.

But if I am shooting in a clean, fairly dust and droplet free environment, then I take off the filter as it degrades image quality slightly (the more glass between your subject and the film, the more distortion and diffusion that can take place).

I will cover lens cleaning procedure in another post--that's a whole story in itself!

Derek Geer
Forensic Engineer
San Diego, California
www.geers.com