Controlling Exposure

I received a few questions regarding my comments on exposure in the HV20 review. I mentioned four different ways of controlling how much light enters the camera, but didn’t describe why total, individual control over each is important. In addition to admitting or limiting light, the iris, shutter speed, filters, and gain also affect the image in other ways.

The Iris
The simplest way to control the amount on light entering the camera is to open or close the iris. Adjusting the iris also affects the depth of field. When the aperture is small, limited light enters the camera, and the whole scene is in focus. When the aperture is large, lots of light is admitted, and the depth of field is very shallow. Only a narrow range of the scene can be in focus, and the wide circle of confusion makes out-of-focus elements very soft.

A wide aperture therefore makes focusing more difficult, but a shallow depth of field is very important to the filmmaker for four reasons. Firstly, it can be very aesthetically pleasing, moreso than leaving everything harshly in focus. Secondly, at allows the director to control what the viewer sees and focuses on.

Thirdly, it can improve the quality of a highly-compressed image; by blurring out background detail, more of the codec’s limited data rate can be used to store the more important foreground detail. Fourthly, it can add production value; since audiences are used to both the shallow depth of field in feature films, and the flat, completely sharp images from cheap camcorders and camera phone, they associate quality and expense with an image that contains a lot of depth.

Shutter Speed
Another thing that audiences subconsciously pick up on is shutter speed. The longer the shutter is open, the more light enters the camera, and the longer the motion blur is. In motion picture cameras that use film, the shutter is a spinning plate that passes between the film and the lens. While the shutter is open, the film is held still, and when it closes the film is advanced to the next frame.

Because of the complexity of the mechanism that drives the film, the shutter can never be open for more than half of the time that each frame is taken. This is known as a 180° shutter, or a 1/48 speed shutter, since the shutter is open for 1/48th of a second (at 24 frames per second).

Video cameras do not have this limitation, and can have a 1/30 shutter speed (at 30 frames per second). This lets in twice as much light as a 180° shutter, since it is open the whole time. However, it results in long, streaking motion blur that audiences associate with cheap video. In addition to shooting at 24fps, videographers attempting to emulate the look of film stick to a 1/48 speed shutter.

However, a faster shutter speed is possible with film cameras, and can be helpful for video cameras as well. A fast shutter speed can shorten or even eliminate motion blur, which adds to the perceived sharpness of HD footage, but it restricts the amount of light recorded.

In still photography, of course, the shutter can be open for any amount of time. Astronomers use hour-long exposures to track stars, and landscape photographers use them to capture dreamy skies and smooth out waterfalls and rivers. Of course, the longer the shutter is open, the more light enters the camera. Long daylight exposures usually let in far more light than the camera can handle, so filters are used to restrict it.

Plastic or glass filters attached to the lens offer great creative control. Diffusion filters can soften an image by lowering contrast and smoothing skin details, star filters can cause sparkly highlights, and polarizing filters can block reflections and cut haze.

The most common are neutral density filters, which are grey and block light without changing color. Most video camera lenses have one or two built-in ND filters for shooting outdoors. The Canon XL H1, used to shoot the images in this post, has both a 1/32 and 1/16 ND filter.

Gradient filters are also handy for daylight photography, since they are darker on the top than on the bottom. This gradient can be a neutral grey, which darkens the sky without affecting the ground, or a warm coral color for enhancing sunsets. Many color and density combinations are available.

Colored filters can also be used to match daylight to tungsten light, but more dramatic results are available. For the above images, I shot through orange and blue gels, but white-balanced the camera for each. The orange filter blocked most the blue light, and the blue blocked the longer red and yellow wavelengths.

Snow is actually a pretty bad example, but you can still see a few differences. The orange filter darkens the sky, and the blue lightens it. The shadow densities are also very different. Underwater photographers use red lenses to block the predominant blue light and get a more natural image.

Of course, all filters work by blocking some light, so they are easiest to use outdoors when there is plenty of light to use. The only way to add light, or at least increase the luminosity of the image, is to boost the electrical signal recorded by the camera.

On still cameras the setting is called ISO, after the different speeds or sensitivities of film, but on video cameras it is called gain. Gain simply amplifies the electrical signal recorded by the camera, which adds noise. As I have said before, I think this should always be avoided.

It’s true that film stock, particularly low-light film stock, has a pronounced and a unique grain, which does add a cinematic quality to footage. However, signal noise doesn’t have the same organic, analog feel. It looks electric and low budget, hogs bandwidth, and compresses badly. If there is no other way to compensate for poor lighting, it is better to boost the levels in post than in camera.

Real World Application
So, how do all these variables translate into real world shooting decisions? It really all boils down to your personal preferences on the final image, and what you can accomplish with what you can afford to buy, rent, and carry.

I use a 72mm linear polarizing filter almost anytime I’m outside, but since I don’t own a matte box, I don’t often use square filters. If I did more set shooting, I would have the time to swap out filters and the room to carry them, but for running and gunning with documentary shooting, it’s not always possible.

I also don’t use manual lenses much anymore (unfortunately). A manual lens usually has an iris ring that allows smooth adjustment of the aperture, while an electric lens restricts you to snapping back and forth between presets.

Because of this, I tend to leave the iris wide open, and ride the shutter speed to adjust exposure (without dropping below 1/60). I, personally, value a shallow depth of field over constant motion blur, but use the ND filters to keep it as even as possible. Others will insist on a solid 180° shutter look, and adjust the iris the maintain exposure.

The important thing is that you have an idea in your mind of what you want to achieve, and that you have put in the time learning to use your camera’s settings so you know how to get it. Shooting well is equal parts technical discipline and artistic vision.

  1. You may find that a slightly lighter or darker version of the scene may give a more pleasing overall exposure. Cameras

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