Film gate
The film gate is the rectangular opening in the front of a motion picture camera where the film is exposed to light. The film gate can be seen by removing the lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...

 and rotating the shutter out of the way. The film is held on a uniform plane at a calibrated distance in the gate by a pressure plate behind the film.

As the film passes through the gate occasionally friction can cause small slivers of celluloid to break off and stick in the side of the opening. This debris is called hairs. A "hair in the gate" will remain in front of the film and create a dark line that sticks into the edge of the film frame as the camera is filming a shot. A hair can ruin the shot and is almost impossible to fix in post production without being painted out digitally.

Because of the intractability of this problem the 1st Assistant Camera (in the UK this position is known as focus puller
Focus puller
A focus puller, or 1st assistant cameraman, is a member of a film crew’s camera department whose primary responsibility is to maintain image sharpness on whatever subject or action is being filmed....

) will open the camera and examine the gate for hairs at the end of each shot. Normally the assistant director
Assistant director
The role of an Assistant director include tracking daily progress against the filming production schedule, arranging logistics, preparing daily call sheets, checking cast and crew, maintaining order on the set. They also have to take care of health and safety of the crew...

 will call out "check the gate" when the director is ready to move on to the next shot. The crew will wait until the focus puller calls out "gate is clean." If the gate is not clean, it will be cleaned with orangewood sticks and canned air
Canned air
Gas duster, also known as canned air, or compressed air, is a product used to clean electronic equipment and other sensitive devices that cannot be cleaned using water. The can comes with a straw to direct the forceful wind that it can produce. However the straw should be taken out of the nozzle if...

 and the crew will take the shot again. A good camera crew usually checks all cameras and magazines
Camera magazine
A camera magazine is a light-tight chamber or pair of chambers designed to hold and move motion picture film stock before and after it has been exposed in the camera...

 during equipment checkout with scratch tests using fogged film rolls — this will catch out any possible scratch or hair problems caused by faulty equipment. However, a variety of other factors including environment, humidity, type of film stock, camera position, film ridging, and lacing can each be responsible for a "hair in the gate". Generally, skilled crew and regularly inspected tested equipment make this a rare occurrence, and it is not unheard of for assistants to go for months without seeing one, even on large-scale shoots. The cinematographer
A cinematographer is one photographing with a motion picture camera . The title is generally equivalent to director of photography , used to designate a chief over the camera and lighting crews working on a film, responsible for achieving artistic and technical decisions related to the image...

 Oliver Stapleton
Oliver Stapleton
Oliver Stapleton, B.S.C. is an English cinematographer.-Life and career:He graduated from the University of Cape Town in 1970 with a degree in psychology and from the National Film and Television School UK while working in South Africa and England.One of his first efforts was the student film...

 praised his assisting team on Casanova for not having a single hair or scratch for the whole shoot, even though large portions were shot on dusty exteriors.

Video cameras do not have this problem, as any malfunction to the sensor will render the entire system useless. The Arri
-History:Arri was founded in Munich, Germany as Arnold & Richter Cine Technik in 1917, named after founders August Arnold and Robert Richter. They produce professional motion picture equipment, digital and film cameras and cinematic lighting equipment...

 D-20 and D-21 system, however, does have removable lenses and a rotating optical shutter, which means that the CMOS sensor can be exposed in much the same way as a film gate and thus needs to be kept assiduously clean.

Methods of checking the gate

There are three different commonly accepted ways to check a film gate for hairs. Some may be better for certain logistical reasons than others on particular shots, and different assistants have different preferences for which to use first. It should also be noted that if a hair is in the gate but outside of the aspect ratio being filmed (if for instance, there was a tiny speck at the bottom of a full 1.37 ratio gate but it was framed for 1.85 with common center), the hair may not always be considered enough of a problem, though usually any evidence of one is enough not to risk a lost shot.

Pulling the lens

In this method, the lens is taken off of the camera and the shutter is moved either with an automatic button (usually labeled "phase" or "dust check") or by manually inching it to an exposed position. The focus puller examines the gate with a flashlight
A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

, sometimes attached to a magnifying glass.

Through the lens

This method tends to be most useful on long lenses and zoom lenses, especially when larger lenses may be difficult to take on and off again quickly. The aperture is fully opened, with the shutter opened, focus set to infinity, and if a zoom, zoomed in as tight as possible. The lens itself acts as a large magnifying glass for the focus puller, who only needs to position the flashlight and his line of sight properly to scan the gate.

Pulling the gate

Many cameras, though not all, allow for the gate itself to be removed from the camera and examined under a light by the focus puller.
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