From: "larry.towers" <larry.towers@yahoo.com>

Date: April 12, 2008 1:42:02 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: IR filter with a DMK color .AS camera?

The IR filter isn't just for color correction. The focal length of

lenses depends on the wavelength. Lens manufacturers compensate for

this with achromatic lenses but these only work within the visible

light spectrum. A lens focused in visible light will be somewhat

off-focus in infrared, If your ccd picks up infrared it will be out of

focus with respect to visible light. This will be especially true if

focus on the object is very critical. There will seem to be a

"glowing" aspect to the images you capture. The only reason Not to use

an IR blocking filter is when you specifically want IR capture, or you

want that subtle softening effect.