Figure 1. Lenses create the optical image for presentation to a video or film camera. Their primary role is to shape the attributes of the image that are reproduced from an object scene. Click here to see an enlarged diagram.
Today's intense interest in digital imaging tends to overshadow an appreciation for the role and importance of lenses. There is, unfortunately, the perception that lenses merely prepare optical images before the digital camcorder records them onto digital media. In reality, however, lenses are where image creation first takes place. (See Figure 1.) Not only does the lens create the optical image for presentation to a camera, it also offers a powerful means of manipulating the image to enhance the art of storytelling.
The technically challenging role of the camera, meanwhile, is to transform the optical image faithfully into an electrical signal. Subsequent digital recording must likewise faithfully store the camera's digital reproduction.
The fact that the lens is the primary arbiter of final image quality — both aesthetically and creatively — does not in any way diminish the role of today's digital cameras or other systems for image storage. Major new trends in this area, such as DTV, HDTV and digital cinema, are changing the video acquisition landscape radically. Because of these changes, an increasing number of professionals find themselves deciding which lens to purchase for new digital camera systems.
Choosing the right lens is difficult. First, there are many categories of lenses presently available in both the HDTV and SDTV domains. Secondly, high-end optical design is an extremely refined science with its own peculiar technical terminology and descriptors, which are sometimes difficult to sort out and understand. And competing lens suppliers sometimes offer conflicting messages, further complicating the process.
Figure 2. The angle of view is the limit within which the lens can image the scene in accordance with the lens’ focal length, its image format size and its shape. Click here to see an enlarged diagram.
Optical image attributes
So, to make an intelligent choice, it's necessary to first understand the basics of lenses and optical images. All images have a number of separately quantifiable attributes. It is the subjective aggregate of these attributes that produce the final aesthetic quality of the image. The lens' primary role is to shape most of these attributes. The lens also predetermines important picture quality attributes. These attributes can be further modified by the camera, the recording system and, finally, by the display system.
Let's briefly examine each of the image's attributes from the standpoint of a lens' creative and technical capabilities:
A future look
- Angle of view: This attribute determines the essential framing of the image. Basically, it determines how much of a scene a lens can image. Technically, it is the limit within which the lens can image the scene in accordance with the lens' focal length and its image format size and shape.
- Zooming: This is a strictly optical phenomenon that emerged relatively recently in the history of lens development. It introduced the ability to make dynamic, real-time adjustments of the angle of view without moving the lens relative to the scene. It represents an artificial, yet powerful, augmentation of the imaging process. It presents an image in a way that the human visual system cannot.
- Depth of field: This a fundamental optical constraint on a lens. The lens is focused on a particular subject in the scene, and the other objects in the image appear in focus within a limited distance in front and behind the subject on which the lens is focused. Depth of field varies with the lens' image diameter, its focal length and its aperture settings. Cinematographers creatively exploit depth of field by skillfully adjusting lighting and lens settings to present selected scene objects in sharp focus while simultaneously defocusing other portions of the scene. This helps portray a sense of depth to the two-dimensional optical image.
- Perspective: This is an essential attribute of any image representation (drawing or painting, film, or electronic image). It seeks to portray a 3-D representation in the same manner that the human visual system sees a live scene — with objects linearly diminishing in size as they recede from the position of the viewer.
- Aperture control: The lens' iris allows the camera operator to manage a vast range of light levels. In that sense, the lens emulates the human eye's ability to control the amount of light to a level appropriate to the capabilities of the sensor (human retina, digital camera sensor or the chosen film stock).
- Contrast or tonal reproduction: This is a measure of the lens' ability to reproduce the full dynamic range of the light levels contained within the scene. It is a measure of the faithfulness with which the lens optics can distinguish the many brightness levels in the scene. A high-contrast lens is one that can distinguish the many levels of brightness within the high-brightness portions of the scene while simultaneously and clearly distinguishing the many lower levels of brightness in the shadowed areas of the scene.
- Color reproduction: This is an important attribute of the image that concerns the light transmission characteristics of the lens. The balance of colors of the light coming through a lens is expressed as a color temperature. In the case of the film camera, the lens design must take into account the colorimetric characteristics of the emulsions in the particular film being used. In the case of the digital video camera, the lens' spectral characteristic must be coupled to that of the light-splitting system in the camera and the colorimetric characteristics of the sensors employed.
- Picture sharpness: This is a measure of the response of the lens' optical system to the overall reproduction of contrast over a range of spatial frequencies. This is the most complex attribute of any image. Judging a lens' sharpness involves assessing the lens' resolving power and modulation transfer function (MTF) at a variety of points over the two-dimensional image. A reproduced image combines the sharpness of the lens, the capture media (film or video), and the reproducing medium (print, display or projection system). But it is the lens that primarily determines the final perceived picture sharpness.
This look at optical image attributes sets the stage for a series of articles set to appear in Broadcast Engineering in 2005 that are designed help the reader appreciate the role that lenses play in creating an image. This tutorial will continue in the January issue with an in-depth examination of television studio lenses.
Larry Thorpe is national marketing executive and Gordon Tubbs is assistant director of the Canon Broadcast & Communications Division.