The electronic shutter: light under control

Most video cameras have the possibility of reducing the exposure time of each frame by electronic means. This function allows you to obtain an additional level of control over the light that hits the camera lens, but it must be taken into account that it offers different possibilities than the mechanical shutter of a cinematographic camera.

The electronic shutter allows you to modify the time used to capture an image in the photosensitive elements of the camera. Normally this time is expressed in fractions of a second, which vary between 1/100 and 1/4000, depending on the camera models. The image obtained in this short period of time is encoded and delivered to the recording equipment as part of a current video signal.

A non-shutter video camera is equivalent to a pocket camera, with a fixed exposure time. In these cases, the exposure setting depends solely on the lens diaphragm and external light, rather scarce resources for the creative work expected of a competent videographer.

For some manufacturers, shutter adjustment should not be a constant use feature. It is usually necessary to use the camera's configuration system, although some models allow its manipulation through dedicated controls that facilitate its use.

The shutter settings allow, among other things, to obtain the following effects:

1. Vary the fidelity or apparent quality of the moving image. When you want to photograph an object that passes in front of the camera at high speed, a high shutter speed of around 1/500 to 1/1000 can be convenient, this in order to obtain a clear record. If the shutter speed is low, you will have an image composed of a series of blurred and poorly defined frames, as the subject will move a little while capturing the image in the camera. Although this effect may be desirable in some cases, it almost always works best for the "frozen action" appearance that is achieved with a higher shutter speed.

2. The manipulation of the electronic shutter also allows the adjustment of the F number or aperture of the diaphragm, in order to enable the creative use of the depth of field.

3. The shutter setting allows you to put the camera in "tune" with other electronic equipment. An appropriate adjustment allows you to make shots of video monitors, LCD screens and other similar equipment with good results. Some cameras include special functions to match the shutter speed to the most frequent sweep times on personal computer screens.

In the first instance, the shutter allows to obtain more defined images of moving subjects. This offers spectacular results when you want to use the material with slow motion, as it significantly reduces the effect of loss of resolution that normally accompanies the playback of variable speed in video. On the other hand, we must not forget the effect of loss of luminosity that unfailingly accompanies the increase in shutter speed.

You have to take certain precautions and avoid unnecessary use of the shutter under normal circumstances. Sometimes, the blurring or blurred appearance of the moving image is unconsciously accepted by the viewer as one of the factors that make the video image credible.

In certain cases the strobe effect that is obtained with the shutter when making very sudden camera movements can be unpleasant. Moreover, this feature can be harnessed to make more visible a light effect, or raindrops that are seen through a window. The important thing is not to work as if the shutter were a simple replacement for an ND filter; it's just a resource to help reduce the amount of light.

Depth of field is the area of a camera's field of view within which images are in focus, which corresponds to a given range of distance between the lens and objects. It is higher or lower according to two fundamental variables: the adjustment of the diaphragm and the type of lens used.

The longer the focal length, the lower the depth of field you get, and this holds true for both fixed and zoom lenses normally found in video cameras. The camera operator should always keep in mind that when "closing" the zoom it loses luminosity, depth and some detail.

On the other hand, reducing the aperture of the diaphragm increases the depth of field. This is why it is possible to manipulate the depth with the shutter, because by reducing the amount of light that is captured it is necessary to open the diaphragm to obtain an acceptable exposure. It is necessary to experiment in order to determine the behavior of a particular camera in different circumstances, but in any case the rule is still valid: the lower the F number, the greater the depth.

Now, what useful effects can be obtained by manipulating the depth of field? For example, it is possible to obtain diffuse backgrounds, which allow you to highlight the character that is in the first place, or suppress an unpleasant element that reaches the field.

Shots with selective focus can also be achieved, which allow to achieve very interesting dramatic effects by taking the focus "back and forth" within the same frame. By skillfully combining focus changes with relatively high depth it is possible to achieve very interesting perspective change effects.

The creative use of the shutter is another of the technical resources that can offer more creative possibilities to users of video cameras. And like any really powerful tool, it should be used with caution...

In a cinema camera, the shutter works the same as in a camera. It simply reduces the exposure time, with the net effect of limiting the incidence of light on the film. This setting allows you to modify the aperture of the diaphragm to control the depth of field or adapt to extreme light conditions. In fact, the physical shutter of the camera is one of the photographer's most valuable resources for controlling the exposure of the image.

At first glance the visual results of the electronic shutter are similar to those of its mechanical counterparts. However, it must be borne in mind that the character of the image capture mechanisms in each medium is different. While the cinema captures a series of photographs, the video captures images in sequence, by making an electronic sweep of the sensitive elements that react to the light.

This means that the rigorous arithmetic relationship between the diaphragm value scale and exposure times cannot be successfully applied to video cameras. In fact, in most cameras the response does not vary significantly in the first values of the scale, up to 1/1000 of a second, approximately.

This is because the photosensitive elements of video cameras, usually of the CCD type, "record" the image in a very short time, much lower than the recording frequency of 1/25 or 1/30 of a second.

This prevents variation from becoming apparent until drastic changes in sweep time are resorted to. In other words, the values of the shutter adjustment scale do not correspond to changes in the amount of incident light according to conventional photographic scales, although manufacturers have made significant efforts to make the scale of values offered by their cameras allow useful work to be done with shutter settings.

On the other hand, when it comes to manipulating the depth of field there is a small detail that must be taken into account. Most zoom lenses used in video cameras produce a small change in focal length when adjusting the focus; and if combined with the increase in sharpness that is obtained by increasing the shutter speed, it can damage a carefully prepared effect.

This problem does not occur in fixed lenses or internal focus zoom lenses, which fortunately have become standard equipment in the latest model cameras.