Even the simplest of mods can achieve the most astounding and moving results. Color filters are the staple tool of stage and screen light technicians, they can also play some nice tricks when combined with your Holga.
If you're using color film, the filters can create brilliant, vivid colors and tones as well as add interesting double-exposure possibilities. Using black and white film will brighten or dim contrast depending on the filter used (just like using them in the darkroom).
