THE ZOOM LENS

The zoom lens is a complex and sophisticated component of any video camera. lts performance may be described in brochures in impenetrable techno-speak that many, at first glance, may feel has little or nothing to do with the ‘real’ business of filming. To take this view is a mistake, as above all else, the lens characteristics are the most important visual influence on the appearance and impact of an image.

Altering the settings on focal length, lens angle, zoom ratio, aperture or depth of field will have a significant influence on the perspective and the depiction of space and composition when setting up a shot. It is a fallacy to believe that the camera will truthfully record whatever is in shot whenever you put your eye to the viewfinder and press to record. If you are unaware of how the choice of focal length, etc., will affect the chosen image then this important creative decision will be left to chance and accident.

Prime lens or zoom?

Before the introduction of colour television, video cameras usually had four, individually turret-mounted prime lenses (a lens with a fixed focal length) on the front of the camera. By rotating the turret, the correct lens for a specific focal length would be chosen for a shot. Selecting a prime lens to match the needs of a shot is still common film practice. For technical reasons, such as faster working methods and the option to zoom on shot to capture close-ups without adjusting the camera, meant the wide majority of colour video cameras were fitted with a zoom lens. With the ability to change the lens angle quickly and easily, it can be easy to forget what focal length is being used which could compromise the space or perspective of the shot so be wary.

Image size

With CCD (charged couple devices) coming in different sizes (2/3 inch, 1/2 inch, etc.), the variation in flange-back distance the mechanical connection between lens and camera, and the variation in cable connections with the camera, it is often impossible to interchange lenses between different makes or models of cameras. The image formed by the lens on the face of the CCDs is called the image size of the lens. This must match the size of the camera sensor. Lenses designed for different sized formats (pick-up sensor dimension) may not be interchangeable. The image size produced by the lens may be much smaller than the pick-up sensor and probably the back focus (flange _‘back) will not have sufficient adjustment. A common lens on a broadcast video camera is a 14 x 8.5 f1.7 zoom with a minimum object distance of 0.8 m or below.

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VIDEO PRODUCTION TECHNIQUES: FOCUS

As an integral element to film and video, focusing determines the sharpness of an object or image by adjusting the lens, providing an acceptable visual perspective known as the depth of field. The art of focus lies in knowing what part of the frame needs it most, and the avoidance of over-shooting. For example, interviews or ‘talking heads’ within television are an excellent example, with the eyes of the given subject being an important point for the viewer’s attention to be drawn to. However, much of this will lose meaning if you don’t learn the behaviour of your camera itself and the way it captures distance, sharpness, movement and zoom in relation to the focal plane (the area in which the eye or camera draws attention to). Without any of these elements, the image within the frame will appear ‘flat’, with no sense of immersion or depth.

Depth of field: Notice how the subject is sharp, while the background is soft

To avoid this, and to gain an understanding of how integral focus can be to your shot, practice is a crucial element. For instance, even with the most professional equipment, manual focus is a critical skill with any level of video production. Here are a few ideas to experiment with and try out:

Pulling Focus:

Within the frame, attention is drawn towards the subject with the sharpest focus. This attention can be changed through the use of focus pulling. For instance, a foreground subject (such as a person) can move away from the frame, with focus instantly thrown back to the background, containing a second subject.

Differential Focus:

By using a deliberately narrow depth of field, emphasis on a principal subject can be increased. This is down to the contrast between the sharp focus of the principal subject, and the heavily out of focus background (and sometimes a foreground subject is used as well which is also out of focus).

Zoom lens and focus:

Provided the back focus has been correctly adjusted, this is designed to ensure the focal plane is maintained throughout its range. By zooming in without manual focus, images can lose focus and become distorted.

By becoming familiar with these ideas, you can be more artistic and manipulative about what you want the viewer’s attention to be drawn to. As an important but often overlooked aspect to filming, these techniques are most often mastered through practice and experimenting. 

To find out more about the different techniques with focus, check out these links:

www.mediacollege.com/video/camera/focus/

en.wikipedia.org/wiki/Cardinal_point_(optics)

www.techradar.com/news/photography-video-capture/cameras/how-to-use-your-new-digital-camera-1049684/8

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