Focus on an optical blind spot A closer look at lenses and the basics of CCTV optical performances,

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Focus on an optical blind spot A closer look at lenses and the basics of CCTV optical performances, by David Elberbaum M any security/cctv installers and dealers wish to know more about lens basics,

Similarly, most users are confused and cannot understand why CCDs and lenses are being referred to as ½", 1/3" or ¼", even though none of the CCD sizes has any relation to inch measurements and this

1 Focus on an optical blind spot A closer look at lenses and the basics of CCTV optical performances, by David Elberbaum M any security/cctv installers and dealers wish to know more about lens basics, lens terminologies and their values, or what lens standards such as C and CS mount are all about. Similarly, most users are confused and cannot understand why CCDs and lenses are being referred to as ½", 1/3" or ¼", even though none of the CCD sizes has any relation to inch measurements and this is when ISO mandates the transforming of all measurements to a metric base. Optics and the physics of light are two subjects that most schools, colleges and universities fail to teach well, as most students do not gain a general knowledge in optics which leads to "common sense" in day to day optical encounters. The reality is that the general public has little knowledge of optics and/or the physics of light. To most people, even to some of those dealing with CCTV, simple terms such as Aperture ratio (F), focal length (f), ND filter or Lux are well known terms, yet very few know what the terms or their values mean. I have agreed to contribute a series of articles to the "CCTV focus" magazine to help users of CCTV understand what is proper, better, wrong, or inferior. I have taken the liberty to explore the basics with illustrations and explanations for "Dummies" and have refrained from drifting into mathematical analysis or complex optical components details and/or how a lens is constructed. Such technical information is available in various publications, yet it does not help in day to day optical understanding, nor does it help in selecting lenses and/or designing a good CCTV system. Therefore my 16 November/December 2000

touching upon the lens structure in this and the following articles is for the sole purpose of explaining the lens parameters, so as to let the readers understand what is important and why.

1a, and the concave lens which diverges the light passing through it as shown in fig.1b. I will rely on the convex lens of fig.

understanding of lens basics. As seen in figs.

2 touching upon the lens structure in this and the following articles is for the sole purpose of explaining the lens parameters, so as to let the readers understand what is important and why. The lens basics There are two common lens elements, the convex lens which converges the light passing through it at its focal point as shown in fig.1a, and the concave lens which diverges the light passing through it as shown in fig.1b. I will rely on the convex lens of fig.1a which resembles a common magnifying glass to explain the lens basics, and I recommend that the readers use a common magnifying glass for some instant self-experimenting, which will lead to better understanding of lens basics. As seen in figs. 2 and 3 the focal point is a point where the light beam of a single light source, such as a distant light bulb or the sun (radiating what we can look upon as parallel light rays) will converge and form an image. In the case of sunrays it will be the point at which a spot is burned. To review this hold a magnifying glass between a white paper and a light bulb as shown in fig.2 and see how the light bulb Theory in focus image is formed (focused) at the focal point. The distance from the principal plane of the magnifying glass to the paper is the focal length. The focal length is measured and specified in millimeters such as 20mm or 100mm etc. It is similar to the focal length of CCTV lenses such as the 4mm, 6mm or the 12.5mm we regularly use, this similarity will be discussed later in more detail. If we direct the magnifying glass to a cluster of light sources instead of a single light bulb we can no longer consider the light beams as parallel and as seen in fig.4 the reproduced images of the different bulbs will be formed (up side down) at different positions on a virtual surface which is termed the focal plane. Repeat your experiment by directing the magnifying glass to a fluorescent lamp instead of a light bulb, the image formed on the paper will be that of the fluorescent lamp as shown in fig.5. The entire fluorescent lamp image is now formed and focused on the focal plane at November/December

statement that larger aperture, such as 40mm passes more light than 30mm will be true as long as the lens focal lengths are identical, such as the three lenses shown in fig.6.

This brings us to the next lens basic, which is the curvature or the radius of the curvative surface of the lens. the back of the lens.

A magnifying glass having a focal length such as 100mm can be made in different sizes, as shown in fig.6.

3 statement that larger aperture, such as 40mm passes more light than 30mm will be true as long as the lens focal lengths are identical, such as the three lenses shown in fig.6. When different lens sizes with different focal lengths are compared such simple comparison cannot be applied, as such comparison must also relate to the focal length. This brings us to the next lens basic, which is the curvature or the radius of the curvative surface of the lens. the back of the lens. In practice, the focal plane is a flat plane at the focal point and perpendicular to the lens principle axis. It is this plane where the image is formed onto a film or onto the CCD surface. A magnifying glass having a focal length such as 100mm can be made in different sizes, as shown in fig.6. The focal length of all the three lenses L, M and S is same (100mm) and their magnification power is identical even though their sizes are different. The difference will be observed in the brightness of the images formed onto the focal plane of the three lenses. Larger lens diameter passes more light through it and therefore offers brighter image at its focal plane. The outer diameter of the lens (its size) is termed the "Aperture". Larger aperture or larger lens diameter passes more light and delivers brighter images. The aperture is measured and specified in millimeters and a 18 November/December 2000 The lens power A larger radius (less curvative) has longer focal point and narrows the angle of view, while smaller radius (very curvative) has shorter focal point but wider angle of view as seen in fig.8. The lens with shorter focal length converges more light and has higher power, known as dioptre by opticians. The shorter the focal length the greater is the lens power as illustrated in fig.9. By now we have covered the lens basics including the focal point, focal length, focal plane, angle of view and the lens aperture. We have also reviewed the amount of light passing through the lens, which depends upon the lens size and its focal length, all of which are fixed basics of a given lens. There are two other basics relating to the lens and its iris (or aperture) which are the

brightness of the formed images (the F-number) and the depth of field.

The finite minimum size of a dot is dependent upon the glass or the plastic materials the lens is made of, the lens surface accuracy, the focal length, the

Yet, even with good lens materials and production techniques it is literally impossible to maintain the minimum dot sizes of all the images that are converging on

The focal plane is never an absolute perfect plane and the converging dots do not pass through the lens uniformly, therefore the dots may converge somewhere in

4 brightness of the formed images (the F-number) and the depth of field. The depth of field An image of a dot formed on the focal plane has its finite minimum size such as 1.5µm or 2.0µm. The finite minimum size of a dot is dependent upon the glass or the plastic materials the lens is made of, the lens surface accuracy, the focal length, the aperture, the lens thickness and more. Yet, even with good lens materials and production techniques it is literally impossible to maintain the minimum dot sizes of all the images that are converging on the entire focal plane. The focal plane is never an absolute perfect plane and the converging dots do not pass through the lens uniformly, therefore the dots may converge somewhere in front of or behind the focal plane as shown in fig.10. It becomes apparent from fig.10 that the converging dots in front of and at the rear of the focal plane are forming larger circels at the focal plane itself, i.e., onto a film or the face of a CCD. The larger circulars, known as the "circles of confusion", must be kept below a given size, or smaller than a grain of a film or a pixel of a CCD. 2.5µm is normally the maximum "circle of confusion" for high quality photographic film. Shown in fig.11 are the same objects of fig.10, but further distanced apart November/December

a distance along the principle axis throughout which an object may be placed or moved around while its converged image onto the CCD

The "circles of confusion" of image 1 and 3 are larger than those of the circle of confusion shown in fig.

In simple words, object 1 and object 3 of fig.

To simplify, the depth of field is The Aperture ratio or the F number Another lens basic is the iris, which controls the amount of

5 a distance along the principle axis throughout which an object may be placed or moved around while its converged image onto the CCD face remains in focus. from each other along the principal axis. The "circles of confusion" of image 1 and 3 are larger than those of the circle of confusion shown in fig.10 and are larger than the maximum acceptable circle of confusion, i.e. larger than a pixel of a CCD. In simple words, object 1 and object 3 of fig.11 will be out of focus when they are outside a perimeter along the principle axis known as the "depth of field". To simplify, the depth of field is The Aperture ratio or the F number Another lens basic is the iris, which controls the amount of light passing through the lens via a variable center hole as shown in fig. 13. The smaller the opening of the iris is, the smaller the amount of light passing through the lens, thus reducing the brightness of the image formed on the focal plane. As shown in fig.14 it also becomes apparent that a lens with a variable iris acts upon the 20 November/December 2000

passing light as if the lens has a variable outer diameter or a variable aperture.

Theory in focus Another fundamental importance of the iris performance is in its relationship with the depth of field. As shown in fig.

extending the depth of field. The smaller the aperture is, the smaller the circle of confusion is and the greater the depth of field is.

The F number As explained above the amount of light passing through the lens depends upon the ratio of the lens aperture and its focal length.

6 passing light as if the lens has a variable outer diameter or a variable aperture. Accordingly the specified aperture of lenses with iris does not refer to the lens' outer physical diameter but to the maximum opening of the iris. Theory in focus Another fundamental importance of the iris performance is in its relationship with the depth of field. As shown in fig.14 the covering of the lens with a plate having a small opening reduces in fact the lens aperture, which in turn reduces the size of the circles of confusion, thereby extending the depth of field. The smaller the aperture is, the smaller the circle of confusion is and the greater the depth of field is. This is an extremely important item of lens performance that will be discussed in the following articles in more detail. The F number As explained above the amount of light passing through the lens depends upon the ratio of the lens aperture and its focal length. This ratio is known as the aperture ratio or the F number, it is defined as: Accordingly the F number of a lens with 100mm focal length and 70mm aperture is = 1.4 or F1.4. When the focal length and the aperture diameter of a lens are of same value, such as 20mm focal length and 20mm aperture, the F number is = 1 or F1.0, which indicates that the lens passes the November/December

light in full to form an image having the same brightness as the object itself.

The brightness of the image is therefore inverted and proportional to the F number as follows: etc.

next article the focal plane, i.e. the "backfocus" of the CCTV lenses, the sizes of the CCDs and the resolution or the

from Japan. The most common F numbers in use are F1.0, F1.4, F2.0, F2.8, F4 and F5.

7 light in full to form an image having the same brightness as the object itself. Accordingly, the larger the F number is, the lower the brightness is. The brightness of the image is therefore inverted and proportional to the F number as follows: etc. I will discuss in the next article the focal plane, i.e. the "backfocus" of the CCTV lenses, the sizes of the CCDs and the resolution or the maximum circles of confusion affecting the CCD-lens combinations. David Elberbaum is the president of Elbex Ltd. from Japan. The most common F numbers in use are F1.0, F1.4, F2.0, F2.8, F4 and F5.6 etc and these F numbers are commonly imprinted onto the iris ring of a lens. This simplifies the brightness setting, so as to cut the brightness by half at each step of the ring as follows: See our new web face :-) Last month we had over 11,700 visitors. CCTV focus magazine => CCTV Labs Pty. Ltd. => 22 November/December 2000

Lenses. A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved.

$Lenses. A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved.$ PHYSICS NOTES ON A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved. Types of There are two types of basic lenses. (1.)