Lumicon Minus Violet and Infrared Blocking Filters are used primarily to "bracket" the color-corrected band of refracting telescopes. The Minus Violet and Infrared Blocking Filters can also be used as "visible" filters in RGBV (red-green-blue-visible) astrophotography with reflecting telescopes. The larger Minus Violet filters are for telephoto lenses.
Most digital/CCD cameras have built-in infrared filters, in which case either the Minus Violet or Infrared filter may be used. The choice would depend on how far into the blue your primary lens is color-corrected. The Minus Violet filter blocks wavelengths shorter than about 420nm; the Infrared filter blocks wavelengths shorter than about 400nm. If your camera does not have a built-in infrared filter, the Infrared Blocking Filter is recommended, which blocks all wavelengths longer than 750nm.
The LF3120, 1.25" Minus Violet Filter, threads into the nose-piece of industry-standard 1.25" Digital Imagers.