LensThe lens is the single most important component of the camera system, and choosing a good lens can significantly improve the overall image quality of your production.

Interchangeable lenses used in professional camera systems are made of glass, reducing image artifacts and ensuring a sharp image. Although glass is vastly superior in quality to the plastic lenses found in low-end consumer camcorders, it is also very expensive, so manufacturers have begun using lenses made of fluorite—a composite plastic material that while still inexpensive, offers a high picture quality and low-light sensitivity. Refer to the cinematography chapter to learn more about lenses.
Prime vs. Zoom LensesLenses01
Prime Lenses
A prime lens has a fixed focal length, or angle of view. The lower the number, for example 12 mm, the wider the angle, and the higher the number, such as 120 mm, the narrower the view and the closer the lens brings us to the subject. Typically, prime lenses come in a set from which the director of photography will choose the ideal lens for the desired field of view and depth of field for the shot.
Prime lenses have fewer pieces of glass for light to pass through than zoom lenses, which results in a sharper, crisper image, making them the first choice among directors of photography, especially when shooting a movie that will be projected or shot under low light conditions. Although primes are more expensive to rent than zoom lenses and require more time on set to change, the results are well worth the cost and effort.
Many professional camcorders accept either 35mm or ²/³-inch bayonet-mounted interchangeable lenses, giving the director of photography the opportunity to use primes. For cameras that have built-in lenses, 35 mm adapters can be attached to the front of the camera and accept 35 mm lenses. Despite the advantage of obtaining a look closer to that of film, the amount of light lost through the adapter requires up to four times more light on set to obtain the same exposure than if the scene were photographed without the adapter.
Zoom Lenses
Lenses02Zoom lenses feature variable focal lengths because of additional pieces of glass added to the lens. The majority of video cameras feature non-interchangeable zoom lenses and offer greater flexibility and ease of use. Although they are faster to work with when setting up a shot, zoom lenses aren’t ideal for high-quality motion picture usage because the added glass elements required to zoom reduces the amount of light that reaches the imaging plane.

"The biggest secret to making Hollywood-quality movies lies in the lens you choose and how you use it. Many filmmakers spend thousands of dollars on high-format cameras, quality lighting equipment, and camera-support equipment, never realizing that if they use a cheap lens, the potential clarity and sharpness of the recording format will never be fully utilized. The quality of the optics, especially in high-resolution formats like HD or 35 mm film, will make a substantial difference in the quality of the image. Rent the highest quality lens you can find, even at the expense of being able to afford the best recording format."

Fixed vs Interchangable Lenses
The Sony EX-1 features a non-removable lens.  Although you can add wide-angle or telephoto adapters to the front of the lens, these adapters can be expensive andLenses03 can cause lens distortions.

Many consumer cameras feature built in, non-removable lenses. Although not the highest quality optics, fixed lenses are designed for each camera body and generally produce a crisp clean image.  You can purchase a wide angle adapter or a telephoto lens to increase or decrease the focal length, but understand that adding additional pieces of glass will slightly soften the image and require you to open up the iris to compensate for slight light loss.
If the budget is able to accomodate a higher-quality camera package, I'd recommend renting a camera that allows you to interchange lenses.  For example, smaller 1/2" CCD cameras like the Canon EX3 or larger 2/3" cameras like a Sony SDX-900 or Panasonic HPX500 allow you to use other 2/3" lenses.  Keep in mind however, that if you want to use 35mm lenses, you will need a camera that can support a 4-leaf bayonet mount as 2/3" cameras only support smaller 3-leaf mounts.  Although there are adapters that will enable you to use 35mm lenses on a 2/3" camera, the are expensive, slightly reduce the image quality and often reduce the light reaching the CCD by sometimes as mch as 2 stops. That means that you'll need to either open the camera iris by 2 stops or increase the overall light level on set by 4x just to achieve the same exposure as if you weren's using the adapter.

Fast vs slow lenses
The “speed” of a lens refers to the maximum opening of the iris and how much light it lets through to the film plane.  Fast lenses are capable of opening to a T 1.4 or T1.3, whereas slow lenses only open to a T5.6 or a T8.  Fast lenses are generally more expensive, and are generally used when shooting outdoors or in low-light environments.
Lenses04Lens Problems
Cheaply-made lenses can degrade the quality of the image, resulting in any number of visual defects:
• Vignetting – Occurs when the corners and outer edges of the frame a darkened
• Loss of Contrast – Black objects appear lighter or slightly milky.
• Softening or Blurring – Inexpensive lenses may not transmit light as cleanly as glass lenses, causing enough light refraction to soften the image, reducing the clarity and overall sharpness.
• Distortion – Inexpensive lenses can cause spacial distortion, causing objects to be disproportionately portrayed at various parts of the frame. 
• Chromatic Aberration – This occurs especially at the long end of the lens (zoomed all the way in), wherein the refraction through the glass creates a color separation, essentially turning the lens into a prism. Most noticeable around high contrast objects like a tree branches against the sky, you can see a red halo around one side of the object and a blue halo on the other side. Some professional cameras have a feature that detects and minimizes the effects of chromatic aberration, but nothing will completely solve the optical problem like a good quality lens.

When budgeting your movie, remember that the lens is generally more criitical to the overall image quality than the camera or recording format.  Now before you go out and try throwing a set of Zeiss Super Speed Primes onto a 20 year-old VHS camcorder, understand that many 2/3" camera packages like an Sony SDX-900 or Panasonic HPX500 come with a stock, ENG (electronic news gathering) lens. Although these are generally good lenses for field use, they are not so sharp that they exceed the camera's imaging or recording capabilities.  If you're going to put your money anywhere, definitely invest it into a good lens package. By upgrading your lens package to primes, or even a higher quality zoom, you will see a significantly sharper picture.

jason-tomaricJason J. Tomaric is an Emmy-winning director and cinematographer in Los Angeles, and produces the online filmmaking resource, FilmSkills.com.  FilmSkills uses dozens of instructional videos from hundreds of working film industry experts to enhance students’ learning experience.