Lenses

Just as a film camera works by using a lens to focus light onto a piece of film, a digital video camera uses a lens to focus light onto the imaging window of a sensor (or group of sensors). Moreover, just as the quality of lens on a film camera can mean the difference between good or bad footage, the quality of the lens on your video camera can mean the difference between sharp images with good color and soft images with muddy color.

At the high-end consumer level, most digital video cameras have fixed, zoom lenses; that is, you can’t change the camera’s lens as you might on your DSLR camera. At the professional level, digital video cameras have interchangeable lenses that let you select from a variety of zoom ranges, wide angles, and telephoto options. Thanks to the popularity of shooting HD video with DSLR cameras, video camera manufacturers are starting to offer less pricey camcorders with interchangeable lenses such as the Panasonic AG-AF100 (see Figure 4.8).

Figure 4.8

The Panasonic AG-AF100 is an HD camcorder under $5,000 that supports interchangeable lenses.

Lens Quality

When evaluating lens quality, look for changes in brightness across the image. Does the lens produce images that are brighter in the middle than at the edges, also known as vignetting? As you zoom the lens in and out, does the image get darker as the lens goes more telephoto?

Look for distortion around the edge of the image, particularly at wide angles. Does the image bow in or out as you zoom the lens back and forth? Similarly, look for changes in sharpness and detail throughout the lens’s zoom range.

70 The Digital Filmmaking Handbook, 4E

Chromatic aberration occurs when the lens does not equally focus all wavelengths of light. This problem is usually worse in single-chip cameras, though three-chip cameras with lowerquality lenses can also suffer from chromatic aberration. You can spot chromatic aberration by looking for fringes of red or green in high-contrast areas or around dark lines. (See Color Plate 7.)

Lens flares are those weird circles that appear when the lens is pointed into a bright light. Although lens flares have their uses—you’ll see them a lot in “hot burning desert” scenes, or around flying titles—you want to be sure that flares only happen when you want them to happen. A lower-quality lens will flare very easily, and most lenses will have trouble with flares when shooting bright light sources at very wide angles. When evaluating a camera, zoom the lens to its widest angle outside during the day and see how easily it flares when you pan it around.

F-stops Versus T-stops

Still cameras and most video cameras rate their lenses in f-stops, but motion picture film cameras and high-end digital cinema cameras rate their lenses in t-stops. T-stops are highly calibrated to account for any absorption of light by the lens itself (for example, light that won’t make it to the image sensor). In practical terms, this means you can swap lenses and keep the same t-stop setting.

Lens Features

Depending on the quality of their controls, some lenses are easier to use than others. To make sure your lens provides controls that let you get the kind of camera movements and effects that you want, consider the following:

nZoom control. Is the zoom control well positioned? How long does it take to zoom from full wide to full telephoto at the slowest speed? Ideally, this should be about 30 seconds. How quickly can it zoom its full length? You’ll want this to be fairly quick to allow for quick framing of shots. How well does the zoom control work? Can you zoom at a constant speed? Can you make smooth zooming accelerations and decelerations? Is there a knob that allows for very fast zooms, or snap zooms?

nManual focus. If your camera has manual focus, where is the control? Whether electronic or mechanical, test the camera’s manual focus for ease of use and reliability. Also be sure it holds focus when set. If the lens in question has a focusing ring (like what you’d find on a 35mm SLR camera), check to see if it has distances marked on it.

nAperture control. As with focus rings, some higher-end lenses have manual rings for controlling the lens aperture. (Apertures are discussed later in this chapter.) Check for f-stop markings, ease-of-use, and accuracy.

nF-stop rating. All lenses have a widest possible f-stop that they can achieve. This determines both low-light capabilities and the ability to shoot shallow depth of field. A lens with a low f-stop rating, such as f1.2, can shoot in very low-light situations and achieve extremely shallow depth of field. A lens with a higher f-stop rating, such as f4, will not do as well in low-light situations. Lower numbers mean better low light/shallow depth of field. A lens with a lower number is considered a “faster” lens.

nMinimum objective distance. How closely can the lens focus? For shooting small objects or getting extreme close-ups for cutaways, this can be important.

Lower-end cameras tend to have lenses built right into the camera’s body, but this doesn’t mean that they’re necessarily bad lenses. In fact, many professional cameras offer highquality, built-in Zeiss lenses.

Interchangeable Lenses

Interchangeable lenses are one of the more exciting trends for digital camcorders. They offer an extra level of control over how your camera records images, and they can give you the flexibility to shoot successfully in all sorts of situations. However, if you are tending toward a camera that features interchangeable lenses, you have some extra research and expenses headed your way. High-quality lenses are not cheap, and having the right selection on hand is a matter of personal taste, affordability, and the requirements of your shooting situation.

nZoom lenses. Lenses with variable focal lengths, or zoom lenses, are often the first purchase for camera owners looking to build a collection of lenses. They offer the flexibility that we’ve all become used to, thanks to camcorders with built-in zoom lenses. If you’re shooting under hectic circumstances where changing lenses isn’t an option, or just hoping to travel light, having a good basic zoom lens is important.

nPrime lenses. Fixed focal length lenses are called prime lenses. Prime lenses typically offer a shaper image than zoom lenses, and they can also have very low f-stop ratings, which means they can be great for low-light photography. A basic set of primes usually includes a wide angle, a “normal” angle, and a telephoto lens (see Figure 4.9).

Figure 4.9

Zeiss Prime lenses for SLRs.

nCinema lenses. High-end motion picture lenses are a little different than still photography lenses. They are designed with the specific needs of motion photography in mind and have very accurate focus marks to facilitate manually pulling focus. They measure light in t-stops rather than f-stops, and cinema zoom lenses try to keep the same t-stop rating across the range of the zoom lens. All this accuracy comes at a price, and a set of cinema primes from Zeiss or Schneider can run upward of $20,000 (see Figure 4.10).

nLens accessories. All lenses are not alike, and you may find that you need an adaptor to fit the lens you want to the camera you prefer. Be sure to look into this before you make any big purchases.