Gray optical filter for lens front — reduces light without color cast. Enables longer shutter speeds or wider apertures even in daylight. Essential on set for controlling depth of field.
Neutral Density Filter
You need longer shutter speeds in glaring daylight but don't want to stop down the aperture — this is where you reach for the ND filter. You simply screw this thing onto the front lens, and it absorbs a defined amount of light without distorting color. While a polarizing filter or a warm filter changes the optical character, the ND filter works neutrally: the image becomes darker, nothing else.
The strength is measured in stops — ND 0.3 blocks one f-stop, ND 0.6 two stops, ND 0.9 three stops. The higher the number, the darker it gets in the viewfinder and the longer you can expose. Practical: at f/2.8 and ISO 100 in bright sunlight, your shutter would otherwise race to 1/4000 sec. — impossible for cinematic motion blur. With an ND 0.9, you drop down to 1/500, getting the motion blur the image needs. This is the core: you control the depth of field, not your exposure automation.
On set, you need ND filters as soon as you want to shoot with wide apertures — portraits, close-ups, anything where depth of field is a dramatic creative decision. A 24mm at f/1.4 on a bright day? Impossible without an ND. Graduated filters also help with variable aperture pulls in the same take: you have two or three different strengths in your bag, changing as needed. Variable ND filters also exist — rotation faders instead of fixed values — practical, but optical quality varies. Good glass costs, but it's worth it: cheap filters cause color casts and vignetting, especially with wide-angle lenses.
Important: ND filters also change focusing on manual focus. The viewfinder gets noticeably darker — with a ground glass or focus assist, you work more clearly. Digital cameras show you the live image with reduced exposure, classic film cameras don't: you sometimes fall into pure black. That's why many DoPs train to focus before the filter or mark the follow-focus distance. ND filters are interchangeable with other optics — polarizing filters, effect filters — combinable, but each additional glass surface costs contrast. Professionals hold back and don't stack four filters on top of each other.