Broadcast Video Lenses
How it all works....

The Picture of a Lens

The Structure and Principals of Lenses

A zoom lens is a lens that can have it's focal length continuously change without losing it's focus. The name, zoom, comes from the strong visual impression that makes it appear as if the viewer were "zooming" towards an object or image as if in a fighter plane. How does does this all work? Here is an explanation.
When you hold a fixed focal length lens in your hand, you know that changing the distance between the lens and the object, changes the size of the image along with the focus of the image. So,each time you move, you have to refocus and reframe the image . If two lenses were combined, by moving them in coordination it is possible to change the distance between the lens and the object and still retain the focus of the object.
The zoom lenses used in broadcast or professional video cameras are much more complex than the "two lens" theory, but the basic principle remains the same; move one part of the lens system to change the size of the image, and move another part to keep it in focus.

At the wide-angle end of the zoom, the variator is moved forward, creating a retrofocus type of lens structure. At the telephoto end, the variator is moved back, so the lens structure resembles the telephoto type. To keep the image in the same position as the two lens groups move, the lens groups must move along precise curves determined by the laws of geometric optics. The motion of the variator and compensator is controlled by the barrel cam mechanism. The normal hand-held zoom lens has a divergent variator and a divergent compensator. The track followed by the compensator takes it forward, then back.

Lens Mechanics

The inner barrel has a linear guide groove (linear cam), and the outer barrel has a curved cam groove matching the track of the lens motion (curved cam). When the outer, curved cam barrel is turned, the variator and compensator move following the curved cam grooves.

If the correct cam curve, designed for a particular lens, is not followed precisely, focus will be lost during zooming. Because of this need for precision, the cams are machined to micron tolerances by computer controlled machining tools.

Optical Abberation

Optical Abberation Demo

A zoom lens must also correct optical aberration so that the image will stay sharp when zoomed. The paths of the light rays passing through the internal lens groups undergo complex changes during zooming. To correct for aberration at all focal lengths, the aberrations caused by each of the lens groups must be minimized. The aberrations that the individual lens groups cannot correct on their own must be carefully balanced by another lens group, so that one lens group corrects another.

Lens Structure Example

The optical path of a hand-held zoom lens has a four part structure. The first group is called the focusing group, because it is used to focus the image.

The First Lens Group

The second group is the variator that changes the image size.

The Second Lens Group

The third group is the compensator that maintains the focus.

The Third Lens Group

The fourth group is a stationary lens group called the relay lens.

The Fourth Lens Group

Depth Of Field

The "text book" explanation is as follows:

"The zone in front and back of the image plane in which the defocus is less than the permissible circle of confusion is called the depth of focus."

"The depth of field is the one within which the subject forms an image that is within the depth of focus. Anything within the depth of field will appear as sharp as if it were in focus."

A simpler explanation is to tell you that the depth of field is that area behind and in front of the subject that remains in focus at any given f-stop or range of the zoom lens.
The larger F-numbers,(smaller aperatures),give greater depth of field. In addition to the F-numbers changing depth of field, you will find that depth of field is greater at the wide angle end of the lens and smaller at the telephoto end.

Focal Length

Fundamental Optics

If parallel rays of light pass through a convex lens, they will converge to a single point on the optical axis. This point is called the focal point or focal plain of the lens. The focal length of a fixed focal length lens is indicated by the distance from the center of the lens to the focal point/plain.

A lens has two focal points, one on the object side, called the primary focal point, and one on the image side, called the secondary focal point. When the term "focal point', is used alone, it means the secondary focal point.

The lenses used on video cameras are compound lenses, consisting of several individual lenses combined so as to correct "aberrations". However, they function like a single lens located at an imaginary point called the "principal point".

The focal length is the basic factor used to calculate the image position and magnification of a lens. The focal length of a video lens is important as a parameter describing the angle of view of the lens. The focal length and "principal point" of a zoom lens are changed by zooming, so as you zoom, you change the angle of view of the lens. A short focal length gives a wide angle of view, and a long focal length gives a narrow angle of view, which causes the image to be magnified.

Lens specifications contain a large quantity of figures/specifications. To interpret them and to tell whether the lens is right for a specific purpose, requires some basic knowledge of optics.

Image Size
The first item to check in the specifications is the image size. There is no point fitting a 1/2" lens on a 2/3" camera. The image it forms is too small, the image will cover the complete area of the image sensor.

The image formed by the lens is round, not rectangular. The range of the image is called the "image circle".

In a video camera, the CCD sensor occupies a rectangular area, which touches the "image circle". The size of the image sensor is the actual image size. Current video cameras use two main sizes of image sensors, with one minor size not commonly used. There are also two main series of zoom lenses, one for each image size. The first letter of the lens designation indicates the image size.

The ratio of the width of the screen to the height of the screen is called the aspect ratio. In most standard definition television it is usually 4:3. The wide screen standard definition television and high definition aspect ratio is 16:9, which more closely resembles the film aspect ratio.

Zoom Ratio

The zoom ratio is the ratio of the focal length at the telephoto end of the zoom to the focal length at the wide-angle end. The zoom ratio indicates how much the size of the image on the monitor can be changed. If a zoom lens has a zoom ratio of lOx, the image it gives at the telephoto end will be magnified exactly 10 times as much as the image at the wide-angle end. You can use the model numbers on the lens to determine it's zoom ratio. An A16X8.5 or J16X8.5 is essentially, at the telephoto end, 16 times 8.5 millimeters.

The F-Stop/Number
An item of equal importance with the focal length is the F-stop/number, which indicates the brightness of the image formed by a lens. A smaller F-stop means a brighter image. The F-stop is closely related to the depth of field. For a given focal length, the larger the aperture of the lens is, the smaller its F-number is.
The larger the zoom ratio is, the more the size of the image can be changed. It is important to select an appropriate zoom ratio. A large zoom ratio is desirable, but it also makes the lens bigger and heavier.
The iris ring of most lenses are marked with a series of numbers with a ratio of 1:1.4, 1.7, 2, 2.8, 3.5, 4, 5.6, 8, 11, 16, 22. The brightness of the image is inversely proportion to the square of the F-number. Each time the ring is turned one number up the F scale, the brightness is decreased by half. As the iris ring is turned down one number, the brightness is increased by twice.

F-Drop (Ramping)

If you have zoomed with a zoom lens open to full aperture, you may have noted a drop in video level at the telephoto end. This is called the F drop or "ramping". The "entrance pupil" of a zoom lens changes in diameter as the focal length is changed. As you zoom toward the telephoto end, the entrance pupil gradually enlarges. When the entrance pupil diameter is equal to the diameter of the focusing lens group, it can not become any larger, so the F-stop drops. That is the reason for the F drop.

To eliminate F drop completely, the focusing lens group, (the elements in the front of the lens), has to be larger than the entrance pupil at the telephoto end of the zoom. It has to be at least equal to the focal length at the telephoto end divided by the F-number. To reduce the size and weight of a zoom lens to make it easy to use for hand held cameras, we have a trade off that makes it common to have a certain amount of F drop or ramping at the telephoto end. For better composition effect, however, in some studio zoom lenses the focusing group is made large enough that no F drop occurs. F drop is a major determinant of the value of zoom lenses used in live on-site sports broadcasts, which require a long focal length and must frequently contend with twilight or inadequate artificial illumination.

As many people know, movie camera lenses are rated by a T-number instead of an F-stop.The F-stop expresses the speed of the lens on the assumption that lens transmits 1OO% of the incident light. In reality, different lenses have different transmittance, so two lenses with the same F-stop may actually have different speed. The T-number solves this problem by taking both the diaphragm diameter and transmittance into account. Two lenses with the same T number will always give the same brightness.

Glass Compensation

A television camera contains a beam splitting prism, filters, and other glass blocks. Its lens has to be corrected so that it will deliver optimum performance when these glass blocks are inserted. Different television cameras have different beam-splitting prisms, so the lens glass compensation has to be matched to the type of camera. Currently, most camera manufacturers have standardized their 2/3" prism compensation and design for their entier line of 2/3" cameras. This allows for camera matching between the studio type and the hand held cameras and allows a user to combine both types of cameras for a production.

When the prism mounted behind the lens differs from the designed glass compensation, the main effects are increased spherical aberration and longitudinal chromatic aberration. Longitudinal chromatic aberration caused by different glass material the letters and numbers at the end of the lens designation indicate the glass compensation type. If the designation is 18x9134, for example, the letter B indicates that the lens is glass compensated, and the number that follows indicates the type of compensation.

Chromatic Abberation

Longitudinal Chromatic Abberation
Lateral Chromatic Abberation
This form of aberration causes the different color wavelengths to focus on different image planes. It corresponds to the lens tracking error. In a zoom lens, the amount of the longitudinal chromatic aberration varies as the lens is zoomed. The aberration is largest at the telephoto end. If corrections for longitudinal chromatic aberrations are not put in the lens, a color tracking error will occur on the red and blue channels. This will cause color blurring, even when the lens tracking adjustment is optimal. In a long-focal-length, high zoom ratio lens, chromatic aberration is the greatest problem, particularly with the secondary spectrum, which is a high-order chromatic aberration. The chromatic aberration of a lens is usually corrected at two wavelengths. The secondary spectrum is the residual chromatic aberration left at the wavelength midway between these two. Lateral chromatic aberration occurs because the magnification of the image differs with the various color wavelengths. In a video camera it causes what appears to be a registration error. Lateral chromatic aberration also has a secondary chromatic aberration, making it difficult to correct all three of the red, blue and green wavelengths at the same time.
Two-wavelength correction is inadequate in a television camera that has three (red, blue, and green) channels. The secondary spectrum also has to be corrected. The main cause of the problem is the residual chromatic aberration of the focusing group of lenses. It is difficult to solve because of inherent limits in the dispersion (wavelength characteristic of refractive index) of optical glass. The secondary spectrum of many manufacturers lenses is corrected by using fluorite crystal, which has a better dispersion than ordinary optical glass. This is why many lenses appear to have a tinted front element that differs by manufacturer.

This has been some basic and advanced information regarding video lenses, and why we see what appear to be problems with our lenses on occasion. From what we have heard from the various manufacturers of cameras, the capability of the current generation of cameras out performs the current generation of lenses. And with CCD cameras, it's harder to make a lens that focuses all three channels on one focal plane at a time, throughout the entire zoom range. This is why you may see more abberations with your lens currently, while a few years ago you wouldn't see a thing. Hopefully this little bit of lens information is interesting to you, and makes a bit of sense. If you have any questions, please feel free to write us. Keep shooing out there!

Comments to Camera Dave
Updates on Lens Theory -
Camera Dave - 1999

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