Filters have always been important to photographers and cinematographers. They can be used both for creative effects and technical control over exposure (Figure 4.13). Filters are available for every conceivable effect, and experimenting with them is a lot of fun.
Figure 4.13 Use infrared filters to handle scenes like this with extensive sunlight.
I believe there are only a few filters that you must have to work with RED: neutral-density filters, an infrared filter (Figure 4.14), and a polarizer. The brand I like best is Tiffen (www.tiffen.com); I use 5 x 5-inch and 6 x 6-inch sizes to cover a variety of lens types. Schneider (www.schneideroptics.com) and Formatt (www.formatt.co.uk) are also excellent filter brands.
Figure 4.14 The Schneider TRU-Cut IR filter gets rid of infrared light.
If you're coming from a DV or HD camera background, you're accustomed to the idea of an f-stop being important mainly for setting the correct exposure. You typically have so much depth of field with a small-sensor camera that it really doesn't matter where the f-stop is, because the depth of field will be similar. But with the RED's 35mm-sized sensor, you can profoundly affect the depth or shallowness of field via ND filters.
An ND filter simply blocks visible wavelengths of light by a very specific amount, forcing you to open up the aperture to another setting and giving a different depth-of-field characteristic. The more you open the aperture, the shallower the depth of field becomes. ND filters are typically notated as .3, .6, and .9, or as 2, 4, 6. With either counting method, you are usually decreasing one stop for each step. So, an ND .3 (or 2) is one stop, a .6 (or 4) is two stops, and so on. You can also get graduated ND filters in which only a portion of the filter halts light. Those can be very useful for balancing landscape shots where you want to reduce the exposure of a bright sky while maintaining the exposure of the foreground and below the horizon. A set of three ND filters will get you through just about any possible lighting situation, because they can be combined for greater strength as needed (Figure 4.15).
Figure 4.15 You can achieve shallow depth of field in bright lighting conditions with ND filters, allowing this shot to be exposed at T2.0.
IR and Polarizing Filters
Because of its CMOS chip, the RED camera has increased sensitivity to infrared light. This can cause unexpected color rendition, especially when shooting outdoors in strong direct sunlight with ND filters. The effect is something like a black light, except in this case colors that should read as black turn magenta. Infrared (IR) and Hot Mirror filters correct for this issue by reflecting away infrared light, while allowing visible light to pass through unaffected. For best results with the RED, use Tiffen or Schneider Hot Mirror IR filters with their stronger dichroic (layer of light-absorbing film) coatings.
Polarizing filters (Figure 4.16) are also very important for dealing with skies and reflections. In general, polarizers work to enhance contrast and reduce reflections, depending on your camera angle. You can rotate a polarizer to dial in the effect's strength. If the polarizer doesn't seem to be doing anything, try changing your camera position slightly.
Figure 4.16 The left side of this frame is with a polarizer and the right side is without.
Diffusion, Softening, and Color-Correction Filters
Diffusion and softening filters are designed to soften harsh edges and enhance beauty. However, I think these are effects better left to post, because they decrease apparent resolution and take away from the advantage of shooting 4K in the first place. The same goes for special effects such as star filters that produce flaring around bright objects. These are easy filters to emulate in postproduction, but once you shoot with them, you're locked into that look.
Color-correction filters are not a 100 percent necessity either, because the RED's RAW sensor has no native color mode. It's technically balanced to daylight, but you can easily alter this in the RAW files. This doesn't mean you won't have to worry about color temperature. For example, if you're shooting indoors with windows looking outside, you'll have mixed color temperatures. To make that scene look natural, you'll need to either gel your lights for daylight balance or gel your windows for tungsten (artificial lighting) balance.