Chapter 3 of "Instruction Manual for RB Super Graflex &c"
The lenses supplied by Graflex with Graflex cameras are selected as the best available for the requirements of both black-and-white and color photography.
Most lenses on Graflex cameras are in barrel mount, (either normal or sunk), although certain models will accommodate lenses in shutters The RB Super D Graflex also accepts lenses in automatic diaphragm mount. (See page 7.)
Lenses (Figure 9) are normally equipped with an iris diaphragm between the cells, which forms a many- sided opening whose diameter is altered by the rotation of a ring outside the lens. The diaphragm controls 1 the amount of light passed by the lens, and 2 the depth of field.
|The size of the opening of the diaphragm is indicated by a series of numbers associated with the diaphragm-control ring; these are the f/ numbers, expressing the relationship between the focal length of the lens and its effective aperture--(not the diameter of the diaphragm opening). The rated speed of a lens is its maximum relative aperture, as indicated by the f/ number engraved on the front cell-usually as a part of the name of the lens. Equal f/ numbers have substantially equivalent light-transmitting values on all lenses regardless of focal length. The larger openings, passing larger quantities of light, are associated with smaller numbers such as f/2.9, f/3.5, or f/4.5. The smaller openings, passing less light, are represented by larger numbers like f/22 or f/32. Full information about the use of the diaphragm will be found under the heading "Correct Exposure" on page 20.|
The focal length of a lens is the distance, when it is focused on infinity (very distant objects), from the plane in which the image is formed to the rear nodal plane of the lens (in standard lenses, generally located near the diaphragm) . Like the maximum relative aperture (f/ number), the focal length is usually engraved on the lens cell. It may be expressed in inches, millimeters or centimeters.
The size of image given by a lens at any particular working distance varies with the focal length: for instance, if a 3-inch lens gives a 1-inch high image of an object, a 6-inch lens in the same position will give an image 2 inches high of the same object.
The "normal" lens for a given size of negative has a focal length approximately equal to the diagonal of the negative. The Revolving Back Graflex and other reflex cameras require focal lengths slightly greater than "normal" to allow clearance for the mirror when it swings up.
The greater the focal length the farther the camera can be placed from a subject for obtaining a given image size, with improved perspective.
The greater the focal length of a lens, the less its depth of field for a given working distance and aperture. This is discussed fully under the heading "Depth of Field" on page 19.
When it is not possible to approach close enough to a distant or semi distant subject, as in sports photography, long focal-lengths are desirable to give as large an image as possible on the negative. Telephoto lenses are long-focus lenses specially designed so as not to require as great bellows extension as ordinary lenses of the same equivalent focus, thus making the camera more compact and usable. Telephoto lenses can be used with most models of the Graflex; full information is under "Interchanging Lenses" on page 8.
These lenses have focal lengths materially shorter than "normal" for the negative they are designed to cover, special optical design making possible a sharp Image over the entire area of the negative. They cannot be used in Graflex, or other reflez, cameras because they do not provide clearance for the swing of the mirror.
The coating of lenses is one of the most recent accomplishments in the optical industry. A coated lens will have a brownish-purple color on the surface. This color is similar to the tarnish or iridescent color that was previously considered detrimental in lenses. The process of coating deposits a metallic salt, such as magnesium fluoride, on the surface of the lens in an extremely high vacuum.
The coating of lenses is done only on the air glass surfaces and not on the cemented surfaces. The advantages of coating photographic lenses are an increase in transmitted light by a reduction in light loss caused by internal reflections, a definite increase in brilliance or contrast of the Image and the elimination of flare spots due to multiple reflections within the lens. Every surface of a lens that is coated will increase the light transmission However, with most standard lenses, this Increase In light transmission will not be noticeable due to the exposure latitude of most of the films currently available. The added brillIance due to the coating of the lens surfaces should be noticeable and should result in finer photographs.
The coating which is now being applied to lens surfaces is almost as hard as the average optical glass. While no special precautions are necessary for cleaning the surfaces of coated lenses, they should be handled as carefully as all fine lenses. All surface grit and dust should first be removed with a soft camel's hair brush before the lens is cleaned with a fine lens tissue or a soft cloth such as a well-laundered linen handkerchief. Do not apply pressure on the cloth. Simply breathe on the surface of the lens and wipe gently with easy circular motions. If a lens cleaning fluid (only those specifically recommended by lens manufacturers) is to be used, do not apply it directly on the lens surface. Instead place a drop or two on the lens tissue or cloth and wipe the lens with the moistened material.
Although we may focus most precisely on a certain portion of an object, we all know that things slightly nearer to the camera as well as some slightly farther away may also appear sharp in the final print. The field of sharp focus extends farther behind the object than in front. This range of distances within which all objects appear equally sharp is called the depth of field.
As in the case ot stopping motion to be considered in the following section, our acceptance of sharpness will depend upon the nature of the print and how it is viewed; and the sharpness to be demanded of a negative for making such prints must depend upon the amount of en largement contemplated. Depth of Field, therefore, is not a precise physical quantity which can be measured. Most lens manufacturers publish tables under the heading "depth of field," or less correctly "depth of focus," which some photographers find useful. The great majority of camera users, however, find observatton of the imaqe on the ground glass and the following easily-remembered facts to be enough of a guide for all practical purposes.
If you want the greatest possible amount of material to appear sharp in your picture, stop the lens down as far as you can without requiring an exposure time so long that movement becomes objectionable. If, on the other hand, you wish to minimize the background or nearer objects by purposely throwing them out of focus, open up the lens to one of its larger apertures. If you want the maximum sharpness at the precise point on which you focus without regard to objects nearer or farther away, choose a lens aperture approximately half way between the greatest and the smallest which your lens can give.
When using the automatic diaphragm of the Super D Graflex, bear in mind that the depth of field will be greater when the lens has stopped itself down than when you focus wide-open.