Camera Principles

There are two general types of cameras for making a movie. They are Film cameras and Video cameras . Most all of the basic principle of these two types of cameras are the same and can be described with general camera principles. However, these two types of cameras work fundamentally differently in their image capturing. And while some of their functions are similar, like they both have viewfinders, take batteries, and have have lenses, they still tend to be different in how they are operated and in there ergonomic design. Most important, though, is that film and video cameras produce images that are noticeably different. Film is often considered to be more

aesthetically pleasing than video so many video camera users will try to get their video image to look more like film. Film cameras are varied and they tend to be modular. The basic camera comes as a package but custom lenses, matte boxes, video assist, high speed motors and many other things are added on to make the film camera fit the needs of it's user. Individual video cameras, on the other hand, are sort of a one size fits all, especially consumer and even prosumer cameras. This is fine for many folks who choose to shoot video with an of the shelf camera like a XL-2 or a DVX-100A.

Both are great cameras but there is a huge potential for custom video cameras that is not being exploited. Like film camera video cameras can be customized. Simple things like adding an anamorphic attachment to you video camera is a great place to start but moving into a complete modular camera design has potential to be the best video camera setup outperforming even the best prosumer cameras in versatility and quality of image. Whether shooting with film or video all camera require support. Quality camera support is often under rated as an issue of concern, but good camera support can make the difference of creating very professional looking shots and facilitating a good work flow during production.

PRINCIPLES OF CAMERAS

The camera is a device to capture an image on a desired medium. In the case of motion picture cameras a series of individual pictures are captured and then presented in rapid succession to give the illusion of having captured motion. But this aside the basic principles between all cameras are the same. The basic parts are: LENS, IRIS, SHUTTER and the MEDIUM and sometimes FILTERS.

The LENS gathers light from the selected image and focuses the light on to the medium.

The IRIS or diaphragm controls the amount of light that reaches the medium. This device can also be used to control the DEPTH of FIELD.

The SHUTTER is a door or gate that opens to allow selected light to reach the medium and closes after a user determined time. The shutter, like the iris, also controls the amount of light that reaches the medium, however it has no affect of the Depth of Field.

The MEDIUM is the material or device where the light is transferred into a recorded image. With film cameras this is a film of acetate with layers of gelatin impermiated with silver halide particles. In the case of electronic image capturing devices the medium is a silicon based chip usually a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS).

FILTERS typically are placed in front of the lens and are used to:
1.) control the amount of light
2.) control the quality of light
3.) create special effects.
Filters that control the amount of light are used in place of a shutter or iris when the shutter may be at a fixed speed like for a motion picture camera or when the iris is fixed in a position for a specifically desired depth of field of focus. Generally for color mediums these filters are Neutral Density filters. These filters reduce the amount of all colors of light equally.
Filters that control the quality of light may be colored filters to make a monochrome medium or in the case of a black and white medium color filters are used to darken certain parts of an image. For example a red filter is used to darken a blue sky in black and white photo image because the red filter reduces the amount of blue light passing threw the lens to the medium resulting in less light from the sky reaching the medium.
Special Effects filters are used for a variety of reasons but none are fundamental to the basic principles of cameras, so they are outside of the scope of this article.

EXPOSURE CONTROL

Proper exposure is a fundamental key component to good cinematography. In fact it is one of the three main elements of cinematography, those being EXPOSURE, FRAMING, and FOCUS. Things like depth of field, moving shots, frame composition, light quality, etc... would all be sub-categories of the three stated elements.

Anyway, it doesn't matter if you are shooting film or video the principles of exposure control are the same. All cameras, including still cameras can control exposure three ways:
1.) shutter speed
2.) iris (diaphragm)
3.) filters (neutral density(ND) filters)

The IRIS is a mechanical element inside the cameras lens. It is a kind of orifice that opens and closes to let more or less light pass through the lens and onto to the film or CCD chip. However, there is another effect of opening or closing the iris. That effect is a change in depth of field. Depth of Field(DoF) refers to how deep the area of acceptable focus is in front of the camera. Opening up the iris results in more light passing through the lens, it also results in a narrower DoF. Closing the iris down prevents so much light from passing through the lens but it also results in a wider DoF.

When shooting outside on a very bright day the iris may be closed down to achieve a proper exposure, but this also results in just about everything the cameras sees being in focus because the DoF is very deep. Even if the camera operator where to crank the focus ring around back and forth on the lens everything stays pretty well in focus, stuff just wont get real blurry with the iris closed down real far.

When shooting indoors in a dark room the iris may be opened up to allow more light through the lens to achieve proper exposure, but this will result in a very shallow DoF. Depending on the lens, or magnification factor, the DoF may only be a few inches deep. In this case if the subject moves very much they will be in and out of focus and they move in and out of the DoF.

Because there are other ways to control exposure, the iris may be best suited for DoF control instead of exposure control. The DoF should be used a stylistic tool rather than a side effect of trying to get proper exposure. Often a shallow DoF is used to "cut out" a subject from it's back ground, or to change the point of subject with a given frame like with a rack focus shot. Sometimes a wide DoF is desired to include multiple subjects or to create an establishing shot that shows the subjects and their surrounds all in focus. However, the DoF is decided to be used consistency and intent are points for careful consideration.

Both film cameras and video cameras may have shutter control. Increasing the shutter speed allows less light to reach the cameras film or CCD, and slowing the shutter speed down allows more light onto the cameras medium. Speeding up or slowing down a cameras shutter speed can be a way of controlling exposure.

However, few cinema cameras actually have shutter control because slow shutter speed is almost always preferable to faster shutter speeds for nearly all normal cinematography. The exceptions are high speed cinematography for special effects shots, and sport analysis cinematography or other types of scientific analysis. Typical shutter speeds for cinematography are 1/48th of a second to 1/60 of a second. Faster shutter speeds result in a "strobing" of moving subjects in the frame, or as an entire frame strobing during a pan or tilt, especially during a high speed pan or tilt. This strobing effect is a result of a lack of blur caught by the camera. If the shutter opens and closes fast it captures a crisp clean image with little or no blur. Because of the frame rate of cinema, even television, some blur for moving objects in essential to complete the illusion of "moving" images. Sometimes some cinematographers do choose the strobing effect as a matter of style, but it is used as an unsettling effect because on the big screen strobing images can be very annoying to a general audience.

Since shutter speed needs to be fixed at 1/48th of a second for proper motion imaging, for normal shooting, changing shutter speed to control exposure is not viable option.

Neutral Density Filters have no other effect on an image other than reducing the amount of light passing through a cameras lens. ND filters are basically sunglasses for your camera. The term "neutral density' refers to the filter have no color effect on the light passing through it. That is, and ND filter filters all colors of light equally. Adding ND filters in front of a cameras lens results in a reduction of light passing through the cameras lens which results in a darker exposure. If a shot, out side, on a bright day, is desired and it is also decided, for stylistic reasons, that the subject should be distinct from a very "busy" background, then opening the iris wide, to create a narrow DoF, and adding ND filters, to achieve proper exposure, is the way to go.

OTHER CONSIDERATIONS:

Another way to control exposure is to simply increase or decrease the amount of lighting on the set. Dimmers on lights are often used for this and make for a very easy way to achieve proper exposure with out the use of ND filters. However, dimming quartz, tungsten, or any incandescent bulb will result in a subtle color shift to red. If dimmer settings are changed between shots be sure to re-white balance. Also, light on different dimmers with different setting will result in different colored lights. It is a subtle effect but can be noticeable and spoil an otherwise "perfect" shot. Dimmers can not be used at all for color film stocks because there is no practical way to white balance for individual shots.

Polarizing filters are designed for controlling unwanted reflections, but if a pair of polarizing filters are used in tandem they can be a great way to control exposure. But details on this effect is a whole other post.

Film Speed and Gain: These each perform the same function as the other but film speed indicates the sensitivity to light a given film stock has, and gain indicates how sensitive a CCD is to light. (well not really, a CCD is only as sensitive as it is but the gain actually amplifies the signal coming from the CCD so it is kind of like chip sensitivity) When a faster speed film is used the resultant image is grainier and more contrasty than a slower film speed's image. Like wise increasing the gain on a video camera will result in a grainier and more contrasty image. changing to a film stocks with different speeds or changing the gain on the camera can be ways of controlling exposure, but like DoF consistency and intent should be carefully considered when using these functions.

DEPTH OF FIELD

Motion pictures often uses a very narrow depth of field to cut, the subject, out from the back ground. The narrow focus field will have sharp focus on the subject while everything in front of and behind the field of focus if very blurry. This method forces the viewer's attention on the subject and creates a striking "look." This is very different from how most consumer video cameras work, which are designed to have a wide depth of field in most shooting situations. This makes getting the subject into focus easier but also pulls much of the background into focus and results in a busy image.

Depth Of Field is controlled primarily by three factors:
1.) lens viewing angle (also known as) lens magnification
2.) focal distance
3.) aperture size

CREATING A NARROW DEPTH OF FIELD
A relatively narrow lens viewing angle, or a high magnification lens, often known as a telephoto lens, reduces the DOF. Relatively close focal distance reduces the depth of field. A large aperture setting reduces the depth of field.

CREATING A WIDE DEPTH OF FIELD
A relatively wide-angle lens will increase the depth of field. Relatively long focal distances will increase the depth of field. A very narrow aperture setting will increase the depth of field and distracts the viewer's attention from the subject.

So, for a very narrow Depth Of Field use a high magnification lens, bring your subject as close the lens as the lens will focus, and open the aperture as wide as it goes. For a wide DoF do the reverse on each of the these points.

SELECTABLE FOCUS
Being able to select certain objects to be in focus while other are out of focus give a photographer the ability to direct a viewers attention. But not all cameras have the ability to control the depth of field to a high degree. Consider the chart below: