In our previous edition we saw the lenses that by their shape change the direction of light, either concentrating the light rays or scattering them. This phenomenon is known as Refraction. We also observe the luminous and resolute characteristics of the lenses, according to their focal length, the angles of vision and the possibilities that they allow to frame the image that you want to show and the proximity to which we want to be as observers or witnesses.
The different formats for reproduction of the moving image, both in 16mm, super16mm, 35mm or 70 mm in cinema, and in video, 4:3 and 16:9, require lenses that cover the respective frame ratio. The lenses used to film in the 16mm format, have a focal length or the equivalent of a little less than half of those used in 35 mm; in terms of weight and size, they also represent a decrease. In the case of video, these equivalences are closer to the 16 mm cinematic format.
Digital video cameras that include the 16:9 format, equivalent to the 1:66 of the cinematic screen, have lenses (zoom, fixed optics), which cover this format. The 16:9 format for video or 1:66 for cinema is wider horizontally than 1:33 or 4:3 for the standard television format respectively. The 16:9 format is used for high-definition television (HDTV). Therefore, the lenses must have on the area of the focal plane, the point where the light rays converge and the focus is defined, the ability to cover that area for which you are working.
In film photography, lenses that are used in 16mm, 1:33 frame format, with the traditional video equivalent of 4:3, cannot be used for super 16mm or 16:9 for video, unless they have the ability to cover this extra area in the focal plane. For this reason, lenses that only have the ability to cover a certain area of the focal plane fall short in wider formats such as 1:66 or 16:9.
On the other hand, lenses for photography in 35 mm, can be used and used very well in 16mm and super 16mm photography, as they are designed to cover a larger focal plane area that is 35 mm. Keep in mind that the focal length will be doubled, that is, a 50mm lens for 35mm photographic film, would be equivalent and would work as a 100mm in 16mm, covering a much larger area of the focal plane of 16mm, which implies that the center of the lens, where you get the best resolution, you're covering the area.
Also 35mm lenses can be used for super 16mm. In video, this relationship must be taken into account when using 35mm lenses in cameras that allow it by means of adapters. Fixed focal lenses are generally used. Care must be taken that the depth of focus is correct so that the lens has focus on the focal plane. In the new camera formats for digital video, which include the 16:9 wide image option, this aspect of the coverage of the focal plane area should also be considered, when the camera does not have the device that extends the traditional 4:3 format to 16:9.
Another lens design used in large-format cinematography is the cinemascope, which presents a projection of 1:2.35 ratio. These lenses called anamorphic have the characteristic of compressing the image wide. This format uses another lens that produces the opposite effect, returning the image to its normal proportion when projected.
Aberrations
In the basic knowledge of lenses, we must take into account aberrations. These are the distortions that a lens can present and refer to the reproduction of color, contrast, definition, focus and the geometry of the lines. It is well known that contrast and color reproduction, as well as depth of field, are affected when the lens works completely open, that is, when the iris or diaphragm is at its lowest numbering, allowing the maximum light that the lens can transmit. On the contrary, when the diaphragm or iris is more closed, the image is more contrasted and is obtained with it, greater depth of field and better definition.
The lenses of good quality and design manufactured with the best optical elements, minimize the influence of these aberrations. Angular lenses distort the shape of space and it is common to see in them, a distortion of parallelization in vertical lines. It is important to take into account these differences because the exchange of optics with different definition and contrast, give unexpected results that are easily captured by the eye of the beholder.
Another very common effect that distorts the image is that produced by light rays that are not in line with the axis of the lens. They are usually backlit rays that come from the angle opposite the lens and directly affect its elements, producing spots, rings or scratches in the frame, just in the opposite part where they come from. This is the effect called flash, or better known by the word flare. Special care must be taken, since in many cases it is imperceptible and appears as a small blurred spot, other times it is not produced by the light source, but by the reflection or brightness of it on some object within the set. In video cameras equipped with CCDs, it is seen as a strong ray of light, which in some camera manuals is called smear (stain), which in some cases can be produced by light supersaturation.
Spherical and Aespherical
The spherical and aespherical terms with which the lenses or optical parts of the lens are called, are related to the curvature of the arc that carries the cylinder of the optical piece. If the lens is spherical, it is because the arc is part of the circle that makes it up, if it is aespherical it is because the design of the arc is outside the circle. Aespherical lenses are used in wide aperture lenses, which ensure better quality with less distortion and are used as parts in combination with spherical lenses to reduce optical aberrations. The aberrations or deformations produced by spherical lenses are somewhat counteracted when the lens is worked with closed diaphragm or iris values.
As we can see, the design characteristics of the lenses are diverse according to their use in the reproduction of the image. But also in scientific observation, the study of microorganisms or in astronomy to observe at great distances. The facilities of technology today allow better precision both in design calculations and in the elaboration either in crystals or synthetic materials.
The lenses are like the lens of the eye, depending on their design, quality and transparency we will obtain sharper and more realistic images.
