More photography terms explained, courtesy of The Glossary of Digital Photography by John G. Blair (Rocky Nook, 2008, $40):
Diffraction Filter: Also called a diffraction grating. A filter made up of fine lines etched into its surface. This filter is used for special effects in photography. When light passes through the filter, it diffracts into different wavelengths of light, causing a rainbow effect of the spectrum of colors of light in different areas of the image.
Low-Resolution: Also called low res or low rez, A subjective description of images in which the number of pixels is less than would be normally expected. Thus an image used on a website that is 72 ppi (pixels per inch) would not necessarily be considered low-resolution, but the some image printed on a magazine page would be.
Sunny-16 Rule: A rule of thumb used in photography to estimate the proper exposure under sunny conditions when there is no light meter available. The rule is that under midday direct sun, the camera aperture should be set at f/16 and the shutter speed should be set be set at 1/(ISO speed).