What happens inside the lens is not magic or mystery to solve, it's just physical.

(First of a series of three articles)

One of the greatest mysteries in the world of video production is in the camera lens. While it is clear that 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 to operate a camera, still they tend to think in optics as "magic" that in some way or another, it works.

For the camera operator it is important to understand not only what happens when they zoom with the lens but also the "why" happens that way. Here's one reason: Whereas many current lenses have advanced electronics that offer menu systems included, these systems can help the operator to perform actions with the lens could never play in the past. However, camera operators, with a better understanding of the lens, will benefit most from these powerful digital controls than those with a poor understanding of how the lens.

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But before delving into its functionality, let us pause to look at some of the different categories of lenses: These include:

  • Lenses of Broadcast and professionals. "Broadcast-grade" represents the ultimate in functionality lenses for HDTV and SDTV. This type of lens also features a full range of digital controls and the possibility of Switch between formats 16: 9 and 4: 3. The "professional-grade", meanwhile, can offer a high quality but are made for applications where optical and mechanical elements not as expensive as a long telephoto and wide-angle capabilities needed very large.
  • Lenses study / field, ENG and EFP. Lenses study / field, also known as "box style", they are designed for use with studio cameras or portable and usually come in a "box" or housing. A lens with a very large telephoto capacity, such as the Canon 100xs or the 86xs, falls into this category. A lens for ENG (Electronic News Gathering, or Electronic News Acquisition) is lighter, more compact and is designed to be completely portable; for camera operators who must be mobile and always ready to record at the time of the news. For its part, an EFP lens, although it also has some portability, has higher telephoto capabilities than an ENG lens but is generally too heavy to rest on an operator's shoulder for a long time (hence the benefit of using a good tripod) .

Virtually all manufactured lenses for professional or BroadcastThey are zoom lenses. With a zoom lens 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, or something like 100x9.3 17x7.7. The first number indicates the rate of approach, or the degree to which the focal length can be changed from a wide shot to a close-up during zooming. 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 is focused (with the lens set at infinity). The higher rate approach, the greater the ability of telephoto; the lower is the focal length, the greater the amplitude and wide angle.

Something else to consider: if a camera operator changes the distance from the lens to the object being filmed, the size of the image also changes. The position where the image is formed also changes, which means that the image should be refocused whenever the lens moves. But if two lenses or lens groups are combined to move synchronously, you can change the magnification without destroying the focus. In other words, a part of the lens system moves to resize the image while the other hand moves to keep focus.

A zoom lens has at least two moving parts: the drive and the compensator. The part of the lens that moves to change the image size is the drive. While the compensator is what moves to keep the focus. Zoom in typical hand takes on the front of another lens group called the "focus group". The picture becomes the focus group to a static lens group called the group relay (or relay group) and goes to a beam division prism. To maintain the image in the same position with the movement of the variator and the compensator, the group of lenses must move following very precise curves determined by the laws of geometric optics and controlled by the guides marked on the mechanism of the lens barrel. Camera guides are essential to maintain focus and are marked with tolerance measured in microns.

At the same time, a zoom optical aberrations must be corrected so that the image remains clear to zoom in on it. These defects are caused by the path of the light rays that have complex changes during the zooming action. That is important for a high quality lens minimizes imperfections in each lens group, which, in turn, are carefully balanced to correct each other.

There are many other key specifications in zoom lenses. The first feature to note is the size of the image. The CCD lens Broadcast or professional is ½ or 2 / 3 inches, so that the dimensions of the lens must be corresponding. As the image formed by the lens is round (non-rectangular shape as a television screen), the range of the image is called image circle. In a television camera CCD sensor it occupies a rectangular area registered in 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 F-number. This expresses the brightness of the image the lens up. As this number decreases the image becomes brighter. He F-number It 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 smaller its F-number. The iris ring (stop ring) Lens is marked with a series of numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Everything is part of a complex formula. However, ultimately, each time the ring to a higher number on the scale F brightness is reduced to half leads.

Another basic feature of the lens is the Minimum object distance (MOD are its acronym in English), which defines the maximum proximity may have regarding your goal. lenses Broadcast and professionals this can be measured from the front vertex of the lens surface is the most prominent focus group. Due to limited space in a study study lenses require a short MOD. Moreover, the telephoto zoom lens do not need such a short MOD. If you need to get closer to your lenses MOD can avail themselves of a macro mechanism. If there is no macro switch over to a close-up lens function.

A key quality in a lens is the flange-back. This is the distance from the surface of the flange of the lens mount to the plane of the image, which must correspond exactly to the distance from the surface tab of the Camera but also the CCD surface. As each chamber has a flange-back specific user must choose a lens with the same flange-back to accompany her. In relation to this, back focus It is the distance from The most prominent surface behind the lens to the plane of the image and is designed to avoid contact with the camera's filter and dust-protecting glass.

It is desirable that the camera operator is familiar with the resolving power when using a lens. The latter indicates your performance with the image. The resolving power is defined by focusing a pattern of lines with various widths and then see how far the lens can still separate and reproduce the lines. Note, however, a lens with high resolving power does not necessarily offer a good quality image, only expresses the limit value of the lens. Therefore, another way to assess the overall performance of a lens is through its modulation transfer function, or MTF. The MTF expresses the ability to reproduce contrast, shown through a graph with spatial frequency, on the horizontal axis, and ability to reproduce contrast on the vertical axis.

The last basic aspect that camera operators must know is chromatic aberration. This arises from the refractive index dispersion when the crystal differs from the wavelength. The first type is the longitudinal chromatic aberration distortion, focus error that occurs when the RGB information is not focused on the same point. The second type is the lateral distortion, an error log RGB information. The third and final type of chromatic aberration, geometric distortion is due to twist straight lines. Canon lenses, for example, correct chromatic distortion using fluorite crystal. This crystal has a different from the common optical glass in focus group zoom lenses dispersion.

Traders should remember that what happens inside the lens is not magic or mystery to solve, it's just physical and the center of constant research and development. Anyone who wants to know more can contact a lens professional and ask! We like to talk about our favorite topic: the lenses and how to use them to achieve the highest quality in the image.

In the next edition Gordon Tubbs write about the future of the lenses examining the latest technological advances in this field.

Author: Latin Press



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