(First in a series of three articles)
One of the biggest mysteries of the video production world is in the camera lens. While it is clear that it is one of the most essential elements of the production chain, the lens is also one of the least understood parts. While many filmmakers and videographers understand well how a camera operates, they still tend to think of optics as something "magical" that, in one way or another, works.
For the camera operator it is important to understand not only what happens when they zoom in with the lens but also the "why" happens that way. Here's a reason: Considering that many of today's lenses have advanced electronic components that offer menu systems included, these systems can help the operator perform actions with the lens that they would never have been able to perform in the past. However, camera operators, with a better understanding of the lens, will benefit more from these powerful digital controls than those with poor knowledge of lens operation.
But before we delve into its functionality, let's stop and look at some of the different categories of lenses: These include:
- Broadcast and professional lenses. "Broadcast-grade" represents the most advanced in lenses with functionality for HDTV and SDTV. This type of lens also offers a full range of digital controls and the possibility of switching between 16:9 and 4:3 formats. The "professional-grade", on the other hand, can offer a very high quality but are made for applications where such expensive optical and mechanical elements are not needed, such as a long telephoto and very wide wide angle capabilities.
- Study/field lenses, ENG and EFP. Studio/field lenses, also known as "box style," are designed for use with studio or portable cameras, and usually come in a "box" or housing. A lens with a very large telephoto capability, such as the Canon 100xs or 86xs, falls into this category. An ENG (Electronic News Gathering) lens is lighter, more compact and designed to be completely portable; for camera operators who must have mobility and always be ready to record at the time of the news. For its part, an EFP lens, although it also has some portability, has greater telephoto capabilities than an ENG lens but is usually too heavy to rest on an operator's shoulder for a long time (hence the benefit of using a good tripod).
Virtually all lenses manufactured for professional or broadcast applications are zoom lenses. With a zoom lens you can change the focal length continuously without losing focus. The term "zoom" comes from the strong visual impression this creates. A zoom lens is identified by two key numbers in its specification, something like 100x9.3 or 17x7.7. The first number indicates the zoom rate, or the degree to which the focal length can be changed from a wide shot to a close-up during zoom. The second number indicates the focal length at the widest point, which is the distance from the optical center of the lens to the front surface of the lens imaging device to which the image appears in focus (with the lens set to infinity). The higher the zoom rate, the larger the telephoto capability; the lower the focal length, the greater the amplitude as a wide angle.
Something else to consider: If a camera operator changes the distance from the lens to the object they are filming, the image size also changes. The position from which the image is formed also changes, which means that the image must be re-focused each time the lens is moved. But if two lenses or groups of lenses are combined to move synchronously, it is possible to change the magnification without destroying the focus. In other words, one part of the lens system is moved to resize the image while another part is moved to maintain focus.
A zoom lens has at least two moving parts: the drive and the compensator. The part of the lens that moves to resize the image is the drive. While the compensator is what moves to keep the focus. A typical handheld zoom requires another group of lenses called the "focus group" on the front. The image moves from the focus group to a static group of lenses called the relay group and moves to a beam-splitting prism. To keep the image in the same position with the movement of the drive and compensator, the lens group must move following very precise curves determined by the laws of geometric optics and controlled by the guides marked on the lens barrel mechanism. Camera guides are essential for maintaining focus and are marked with tolerance measured in microns.
At the same time, a zoom must correct optical aberrations in such a way that the image remains sharp when zoomed in on it. These defects are caused by the path of light rays that present complex changes during the zoom action. This is why it is important that a high-quality lens minimizes imperfections in each group of lenses, which, in turn, are carefully balanced to correct each other.
There are many other key specifications in zoom lenses. The first feature to observe is the size of the image. The CCD in a broadcast or professional lens is 1/2 or 2/3 inches, so the dimensions of the lens must be corresponding. As the image formed by the lens is round (not rectangular like the shape of a television screen), the range of the image is called the image circle. In a television camera the CCD sensor occupies a rectangular area inscribed in the image circle and its size is what defines the size of the image.
In addition to the focal length, mentioned above, an equally important specification is the F-number. This expresses the brightness of the image that the lens composes. As this number is reduced the image becomes brighter. The F-number is related, very closely, to the depth of field of the lens since for a given focal length, the greater the aperture of the lens, the lower its F-number. The iris ring (stop ring) of the lens is marked with a series of numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. It's all part of a complex formula. However, ultimately, each time the ring is brought to a higher number on the F scale the brightness is reduced by half.
Another basic feature of the lens is the Minimum Object Distance (MOD), which defines the maximum proximity you can have to your target. In broadcast and professional lenses this can be measured from the front corner of the lens which is the most prominent surface of the focus group. Due to the small space in a studio, studio lenses require a short MOD. On the other hand, telephoto zoom lenses don't need such a short MOD. If you need to get closer than your MOD lenses it is possible to use a macro mechanism. If there is no macro function switch to a close-up lens.
A key quality in a lens is the flange-back. This constitutes the distance from the surface of the lens mount tab to the image plane, which must correspond exactly to the distance from the surface of the camera flange to the surface of the CCD. As each camera has a specific flange-back the user must choose a lens with the same flange-back to accompany it. In connection with this, back focus is the distance from the most prominent surface behind the lens to the image plane and is designed to avoid contact with the camera filter and dust-protecting glass.
It is convenient that the camera operator is familiar with the power of resolution when using a lens. This last one tells you your performance with the image. Resolution power is defined by focusing on a pattern of lines with various widths and then observing how far the lens can still separate and reproduce the lines. Note, however, that a lens with high resolution power does not necessarily offer good image quality, it only expresses the limit value of the lens. Therefore, another way to evaluate the total performance of a lens is through its modulation transfer function, or MTF. The MTF expresses the ability to reproduce contrast, displayed through a graph with spatial frequency, on the horizontal axis, and the ability to reproduce contrast on the vertical axis.
The last basic aspect that camera operators should be aware of is chromatic aberration. This arises from dispersion when the refractive index of the crystal differs from the wavelength. The first type of chromatic distortion is longitudinal distortion, a focus error that occurs when RGB information is not focused on the same point. The second type is lateral distortion, a recording error of RGB information. The third, and final type of chromatic distortion, is geometric distortion due to the twisting of straight lines. Canon lenses, for example, correct chromatic distortion using fluorite glass. This crystal has a different dispersion than the common optical glass, in the focus groups of the zoom lenses.
Operators must remember that what happens inside the lenses is not magic or a mystery to be solved, it is simply physics and the center of constant research and development. Anyone who wants to know more can contact a lens professional and ask them! We like to talk about our favorite topic: lenses and how to use them to achieve the highest quality in the image.
In the next edition Gordon Tubbs will write about the future of lenses examining the latest technological advances in this field.
