Nikon SLRs have built-in filters
1. Nikon 39mm has a built-in rear plug-in GELATIN filter holder, which can be used with Nikon AF300 F4 D lens
2. Nikon announced a patented lens technology that has an optical low-pass filter (OLPF) built into the lens. The purpose of having a low-pass filter built into the lens is to eliminate lens aberrations.
3. Some low-end entry-level SLR cameras have some built-in special effects filters in software mode