Article: 19390701046


Better pictures will result from thorough understanding of the laws which govern the working of your camera lens.
Wide Margin for Clips
Popular Photography
AMATEUR photographers have come to realize that in order to make better pictures they must understand their camera thoroughly, especially the lens and the manner in which it functions. Photographic optics is not difficult to understand.



Amateur Photographer, Chicago, III.

Better pictures will result from thorough understanding of the laws which govern the working of your camera lens.

AMATEUR photographers have come to realize that in order to make better pictures they must understand their camera thoroughly, especially the lens and the manner in which it functions.

Photographic optics is not difficult to understand. True, the formulas and terms used may appear formidable, but once you tackle them their simplicity immediately becomes obvious. An article which appeared recently in POPULAR PHOTOGRAPHY explained the workings of four simple formulas which the amateur can put to good use. [March, 1939 issue, page 38. Ed.] Now, how about such terms as resolving power and circle of confusion, and how are they related? What is the difference between depth of focus and depth of field?

The term, resolving power, is used to describe that quality in both lens and film which permits very fine detail of a subject to be photographed without merging or overlapping and consequent loss. A circle of confusion, or more properly, a disc of confusion, is the circular spot of light formed in the focal plane when a point source of light is thrown slightly out of focus.

Before attempting to define or discuss the other two terms—depth of focus and depth of field—a few essential definitions must be introduced. We could do without them, but we can do better with them and they are terms which no good photographer should shy away from. The first is object space, that conical portion of space included in the angle of view of the lens and extending from the lens to an indefinite distance. The image space, strictly speaking, is a cone of space similar to the object space but lying behind the lens. Practically it is limited to the space within this cone between one and two focal lengths behind the lens as shown in Fig. 1. The reason for this practical limitation becomes clear when a specific case is considered. For example, with a 6" lens any point in the object space from 12" in front of the lens to infinity can be

focused sharply somewhere in the image space between 6" and 12" behind the lens. Only when working closer to the object than 12" is it necessary to use more of the image space than lies between these limits.

The next term to hurdle is object point. It must be remembered that a photographic image of an object is not formed as a whole, but that each microscopic point of the object reflects light to the lens individually. An object point, therefore, is the smallest portion of the surface of an object which is capable of reflecting light to the lens. Each object point has a corresponding image point where the rays of light reflected from the object point are reunited after traveling different paths through the lens system.

The object plane is that plane lying at right angles to the axis of the lens which includes the object in critical focus. In

Fig. 2 the object point D__

is critically focused by the lens Q and its image D lies, therefore, in the focal plane. Under these conditions all other object points in the circle PORS will be in critical focus in the circle P’O’R’S’.

We are now in a position to define a disc of confusion more precisely and, we hope, more

clearly. The image of a thi'ee-dimensional space is also in three dimensions. If the object has length, breadth, and height, its image in space also has length, breadth, and height. Only objects lying exactly in the object plane are brought to critical focus on the film. Other object points which lie nearer or farther from the lens will form critically sharp images either behind or in front of the focal plane in which the film lies, and on the film itself where the pencil of rays strikes, there will be a small disc of light of confused pattern which is called a disc of confusion.

In Fig. 3 the object point A is in critical focus at A’. The more distant object point B is brought to focus in front of the focal plane at B’ and on the focal plane itself is a disc of confusion B”. In the same way, the nearer object point C is brought to focus at C’ behind the focal plane and the converging rays as they pass through the focal plane form the disc of confusion C”. The size of these discs is very important; since if they exceed certain limits, the picture secured will not be sharp to the human eye. Unfortunately, this limiting size depends upon several variable factors and cannot be fixed definitely.

This brings us to a definition of depth of field. If the discs of confusion B” and C” in Fig. 3 are of maximum admissible diameter and equal, then the distance between object point B and object point C

Fig. 2. Diagram showing object plane and focal plane in relation to the camera lens.
Fig. 1. Conical portion of space included in angle of view and extending to infinity is the object space. A similar cone of space behind lens is the image space.
Fig. 3. Illustrating the manner in which various points of an object are recorded on the image plane. Note the relationship of "depth of field" to the "discs of confusion."

(Continued on page 72)

(Continued from page 59)

is by definition the depth of field. Object point C is the near point and lies in the near depth plane; object point B is the far point and lies in the far depth plane.

We have been using the term critical focus pretty freely—just what do we mean? The object point A (in Fig. 3) and all object points in the same plane are in critical focus. All other object points lying between the near depth plane and the far depth plane are by definition in focus. Object points located between the lens and the near depth plane, and object points beyond the far depth plane, are out of focus.

Do you know what a sharp print is? Of course you recognize one quickly enough when you see it, but how would you describe it? The difference between a print which is sharp and one which is out of focus is defined by the limitation of human vision. The human eye is very closely analogous to a camera. There is a lens which may be focused on nearer or farther objects, a diaphragm (pupil) which becomes smaller in intense light and opens up in faint light, and a focal plane (retina) on which the image is cast.

The size of the image on the retina depends both upon the size of the object and upon its distance from the eye. These two factors may be considered as one if we

object size

combine them in a ratio,—jr~7--, and


for small angles this ratio is almost exactly equal to the tangent of the angle subtended by the object at the eye. Consequently, instead of considering the two factors which affect the size of retinal images separately, we may consider merely the angle and from it derive the size of the image.

Now it is an experimentally determined fact that if the angle subtended by an object is one minute of arc (the 60th part of a degree) or less the object appears as a point, or if it has any form at all, such form is due to optical imperfections and bears no relation to the true shape of the object. In order to be clearly delineated, an object must subtend an angle of five minutes of arc, and this arc may be accepted as the threshold of clear vision. An angle of one minute of arc gives an image size on the retina of approximately M?ooo inch. The usual explanation of this failure of vision is that the sensitive structures in the retina called rods and cones have a surface area of about this same diameter. When an image covers several cones or rods, the brain receives a pattern of nerve impulses which corresponds point for point with the shape of the image; but when the image is so small that it covers only one rod or one cone, there can be no pattern and consequently no delineation of shape. This interpretation may or may not be correct, but the experimental fact remains.

At a viewing distance of 12 inches a disc of confusion of Moo" diameter subtends an angle of abolit 3 minutes, and a disc of Moo" subtends an angle of exactly

1 minute 8 seconds. Both of these sizes lie below the threshold of clear vision when viewed at 12 inches, and their physiological effect would be that of points. The importance of these facts becomes apparent when an analysis is made to discover what an “out of focus” print is.

When a contact print is made from a negative, each point in perfect focus makes a point on the print, but each disc of confusion makes an identical disc on the print. The total effect of a large number of these points and discs makes the picture seen in the print. Now, if the

print is viewed at a distance of 12 inches, the discs will appear as points if they are less than Moo" in diameter and the picture as a whole will appear sharp. If, however, the discs are larger than Moo", or if the print is viewed more closely, the discs will not appear as points and the picture will have the typically fuzzy appearance which we call “out of focus.” It is clear, therefore, that because of the limitations of vision, the viewing distance is an important consideration in fixing a maximum size for the disc of confusion.

Now let’s get back to our first term, resolving power, and consider it in relation to the others.

In order to show the relation of these terms, let’s imagine an experimental setup. Since a point has no dimensions, it is impossible to provide a perfect point source of light, so we must be satisfied with an approximation. In Fig. 4 is shown one type of setup to give this approximation. The high power bulb throws a strong illumination on the opal glass and the iris diaphragm allows the size of the spot of light to be altered at will. Let’s set the iris diaphragm at V2" diameter, place the lens of the camera 1,000 inches away and focus on the spot of light. The image will show up as a small disc of light. Next, keeping the spot in exact focus, let’s gradually reduce the size of the spot by clos1 J iris diaphragm in our light box \w is closed down, the

image ij-. r^A^V^Jly/9«duced in size—but this proce'siK .v.9 ' ¿¿itinue indefinitely.

A point reached hfrally where further reductiöïtfc*^ ; ’»e £he hole does not

result in reduch*«,. size of the image,

indicating thmfc^Vi limits of the resolving power of the leire have been reached.

If this tiny image is now examined with a microscope, it will appear as a small disc very bright in the center and fading off to darkness at the edges. Around the disc will be seen several concentric bands of

Fig. 4. Setup for determining resolving power, depth of focus, and depth of field.

(Continued on page 74)

light. See Fig. 3. This pattern is due in part to diffraction of the light rays as they pass the lens diaphragm in the camera. Since only the outer rays which pass the edges of the diaphragm in the lens are affected, the larger the diaphragm opening, the smaller the percentage of the light which is affected. Thus the resolving power of a lens is greatest at its widest opening, providing other lens faults are fully corrected.

Properly speaking, this tiny diffraction pattern is a disc of confusion, but the term is more commonly used to describe the larger disc which appears if the object—in this case the light source—is thrown somewhat out of focus. If we move the light source nearer the camera, without changing the focus, the disc becomes larger. In the same way, if it is moved back from the position of critical focus, the disc becomes larger. In practice there is a certain range in size of these discs of confusion which may be allowed without producing noticeable unsharpness in a print. Depth of field is the range within which the light can be moved to produce a disc not exceeding the permissible size. Now if we leave the object in its original location where it is sharply focused, and move the groundglass (not the lens) of the camera forward and backward, we find that the same thing occurs: the disc of confusion grows larger the farther the glass is moved from the point of critical focus. The range within which this may be moved without producing noticeable unsharpness in a print is known as depth of focus—a term which is often used incorrectly for depth of field. It is worthy of note that the effect on the size of the disc of moving the object forward or backward is minimized by stopping down the camera lens—the smaller the lens opening, the greater the depth of field.

If, now, we carefully refocus our light source and expose a negative in the camera and develop it, we shall find on comparison that the disc in the negative is somewhat larger than that shown on the groundglass—although both, of course, are microscopic in size. There are three reasons for this difference. First, the spot may be considerably enlarged by halation, which is reflection from the under surface of the film support; second, in passing through the emulsion, some of the light will be reflected from the microscopic crystals of silver bromide with which the emulsion is sensitized and this scattered light will enlarge the spot a little; third, upon development there is a certain amount of grain clumping. In the case of a single disc, this third factor would make no difference; but with the normal negative, composed of innumerable such discs, clumping may draw some of them together, thus reducing the resolving power of the film. Many amateur films are now furnished with a dark coating on the back surface which almost entirely prevents halation, and the increasing use of finegrain film and finegrain developers has done much to reduce clumping to a minimum. The result is a great improvement in the resolving power of film during the last few years. There was a time not so long ago when the resolving power

of the best photographic lenses far surpassed the power of film to record their images, but developments in film have overcome much of this difference.

If you have followed along with us this far, you may ask, “What is the practical application of this information? How will it enable me to make better pictux-es?” A good question—with a number of answers, some of which follow:

1. If you are taking a picture of an object in a single plane—say a copy of an oil painting or a big di'awing in which it is desirable to secure all the detail—open your lens to within two stops of its maximum apei'ture. That is, if you have an ƒ 3.5 lens, shoot at ƒ 5.6. Since your subject is all in one plane, you don’t need the depth of field secured by stopping down, and you secure more resolving power by opening the lens. If your lens were perfectly corrected, you could open it entirely; but few lenses are fully cori'ected at their maximum apei'ture.

2. When taking a picture of a subject with considerable depth—say a group of people—focus on one of the group about Mi of the distance from the nearest person to the farthest. Then stop the lens down as far as possible without lengthening the exposure too much, and you will secure the maximum depth of field under the circumstances.

3. If you are using a view camera and are trying to focus on the grolmdglass in a very dim light, your knowledge of depth of field and depth of focus will help you in two ways. If you can get someone to hold a lighted match for you about a third of the way back from the nearest object which must be in focus, you can focus it, stop down, and get a good picture. If you have no assistant handy, focus the best you can and set the lens at the smallest stop. The depth of field will usually take care of you.

4. If you are making a portrait shot against a distracting background, focus on the subject’s eyes, open the lens to its maximum aperture, and the background will be sufficiently blurred to be unobjectionable except with short focus lenses.

There are other ways of taking advantage of these optical facts. Once you make them a part of your photographic knowledge they are yours to command at all times.—

Wide Margin for Clips

FREE-LANCE photographers often find it convenient to use paper dios in attaching photos to typed copy when

......... same to

/ ’’.M'rK diaphragfif’m ou’*{’rlp

margin is left on one end of the print, this will provide space for the clip without damaging the print. In this way several photos can be mailed without becoming separated from the copy. When making either contact prints or enlargements it is a simple matter-to arrange whatever marking device you are using so as to allow a wide margin on one side of the print.— Glenn Petersen, Walton, N. Y.

Space for clip.