Chapter 4 of "Instruction Manual for RB Super Graflex &c"
While the final goal of most photography must be admitted to he a satisfactory print, the first goal of the camera user is a negative in which at least the main subject of interest is sharply defined and of a contrast and density which will permit making a print with good tone values. These two factors will depend upon the brightness range of the subject, the speed of the film, the processing subsequently given the film, and upon the lens and the shutter settings in relation to these quantities.
It is difficult, if not impossible to give specific directions for setting up a camera for the making of every type of picture. However, for your cuidance in estimating exposures you will find the tables supplied by film manufacturers to be very satisfactory for the majority of aver age situations. Such tables are usually included with film packs and rolls, and are generally available for sheet film and other materials on request to the manufacturers and their Dealers.
Since any exposure guide, no matter how complete, can represent only an approximation ot the many various light conditions you will encounter, we strongly urge that you eliminate a good deal of the com plication involved in the determination of the exposure by using a good exposure meter. The price of one of the better photo-electric models will be repaid many-fold in good negatives and general satisfaction. In any event, be sure to keep a small card on which to jot down a word or two about each picture: character of the subject, prevailing light conditions, shutter speed, diaphragm opening, and meter readings. These data will be ot great assistance in evaluating your resulting negatives, and will also assist you to broaden out into the use of other emulsions, diaphragm openings and shutter speeds.
But the exposure meter or exposure guide will only go so far as to present you with a rather wide choice of lens-aperture and shutter-speed combinations, telling you that if you choose f/11 (for example) then you must set the shutter for 25 (1/25); or that if you wish to use 1/100 in order to stop some fast motion then you must set the lens aperture to f/5.6.
"Increasing the shutter speed" means that a shorter exposure is given--as, for example, in changing from 1/50 to 1/100. In other words, the shutter passes light for a shorter length of time.
"Increasing the Exposure" means that a slower shutter speed is used, as in going from 1/100 to 1/50.
"A larger stop" or "opening up the lens" means that the actual diameter of the aperture of the lens is increased, and hence more light is passed by the lens in a given length of time. The smaller f/ numbers in dicate larger apertures--f/8 being larger than 1/16, 1/3.5 larger than 1/5.6, etc.
The relation between these combinations of diaphragm opening and shutter speed is really very simple, and knowing one of them is sufficient to give you any other without even a pencil. The rule is simply this:
every time you open up the lens one full stop (to the next f/ number except from f/5.6 to f/4.5 or from f/4.5 to f/3.6. The difference here may be considered as being 50%, or 1/2 stop.), double the shutter speed; and vice-versa.
The choice of a combination of shutter speed and diaphragm opening will be determined by the type of subject. If the subject is moving take the lowest shutter speed necessary to stop its motion and set the dia phragm accordingly; if the subject is static use the diaphragm opening giving the required depth of field and set the shutter speed for that value.
Suppose, for example, that you determine from your meter or the exposure tables that the correct exposure would be 1/50 at f/16. But your subject is going to move and you have estimated that 1 /200th will be needed to stop it on the negative. You know that this will require open ing up the lens so you proceed to do so, counting as you go from stop to stop: starting at the next larger will be f/11 for which you double the speed--(going to 1/100th); the following stop is f/8, for which you double the speed again--(going to 1/200). And there you have it: f/8 at 1/200 (on earlier models use 1/195 which is the nearest speed produced by the shutter) . The exposure meter dials will be found to bear the same relation between the combinations offered.
Like so many so-called "rules" the above one also has an exception, which applies to only some lenses at maximum aperture. Going from f/32 to progressively larger apertures, each calling for a doubling of shutter speed as we go down the list, the ideal diaphragm scale would be marked:
f/32, 22, 16, 11, 8, 5.6, 4, 2.8, 2, 1.4, 1
The lens on your Graflex probably has just these same stopi indicated from f/32 to f/5.6. But if its maximum aperture is f/4.5 that figure will follow the f/5.6 without being twice as fast; it is about 50% faster and would therefore call for a 50% increase in shutter speed instead of the 100% set by our rule. A similar caution must be observed if you have an f/3.5 objective, as it will be marked at f/4.5 and f/3.5 even though the 2-to-1 exposure factor does not lie between these two apertures. Lenses in automatic diaphragm are marked with half stop positions to facilitate critical control of exposure.
Any of these combinations which an exposure meter offers you will give the same density of negative; but the over-all sharpneaa of those negatives and the amount of subject that will appear to be in good focus will vary with the exact combination you choose. And the choice which the photographer must make will be a compromise between the need for stopping motion (calling for the higher shutter speeds and the larger lens apertures) and the desire to have the greatest amount of material in sharp focus (calling for smaller lens apertures and slower speeds) .
If the subject you intend to photograph is moving, its image on the plate will likewise move. It will move some during even 1/1000th second, although only about one-tenth as far as it will during 1/100th second. All the photographer can hope to do is to give a short enough exposure so that the motion of the image on his film will not be objec tionable. And just what constitutes "objectionable" depends both on the observer and upon the use to which the negative is to be put. A negative which is to be enlarged 10 times or to be printed In a magazine must be sharper than if it were to be reproduced in its original size. And a print which is apt to be examined through a reading glass must natural ly be sharper than one to be seen from a distance only, as a highway billboard.
Not knowing precisely how his negative may be used, the averaqe photographer must aim at "average acceptability," and it is on this basis that the following table has been compiled. It allows the photog rapher to take into account the four factors which are present at the time of his exposure: Namely, the focal length of his lens, the distance he will be from the subject when he releases the shutter, the velocity with which the subject will be moving at the instant of ezpasure, snd the direction of its motion relative to the direction in which he is pointing his camera.
Suggested exposures to stop motion at right angles to the camera when the subject moves 10 miles per hour
|APPROXIMATE FOCAL LENGTHS IN INCHES
|These speeds are only approximate, and have been "rounded off" to give numbers easy to multiply and divide. The need of greater accuracy is doubtful in view of the uncertain speed of the obiect to be photographed.
The above table applies to a subject moving 10 miles per hour at right- angles to the camera. In all probability your subject will be doing something different. If so, modify the shutter speed called for in the above table by the following rules:
*Double the speed of the shutter for double the velocity of the subject.
**Half the shutter speed for half the velocity.
*Double the shutter speed for half the distance to the subject.
**Half the shutter speed for double the distance.
*Double the shutter speed for double the focal length.
**Half the shutter speed for half the focal length.
Use one-third the shutter speed if the subject is coming directly
toward you or going directly away from you.
Use two-thirds the shutter speed if it is coming or going at 45 degrees.
When in doubt, use the next higher speed.
--1/100 instead of 1/50, for example.
**--1/50 instead of 1/1 100, for example.