International photographer (Jan-Dec 1941)

l6lVIIVI. dEpARTMENT Light Meters — Their Use and Misuse Amateurs reading this and expecting to be told what perfect pictures will result just because they use a meter to determine their exposure are going to be disappointed. Nothing could be further from the truth because the meter is not a robot, any more than is the camera itself. A good photoelectric exposure meter is an accurate instrument for the purpose of measuring the intensity of the light and computing the exposure, but because of its high order of accuracy it must be used intelligently, and with a full understanding of the processes involved. It must be emphasized that merely aiming a meter in the general direction of the object we wish to photograph will not result in a perfect exposure, even if we take into consideration the correct values for the film speed. To begin with, let us determine what, exactly, is a "correct" exposure. Actually, there are several "correct" exposures for any one object, and we may consider the several successive values to be called the latitude of the film. They vary with the type of the film used, and the amount of development given that particular film. Below the lowest value of this latitude we GOERZ KINO-HYPAR LF.NSES t f:2.7 and f:3 / for regular and color movies of surprising qualify. High chromatic correction . . . Focal lengths 15mm to 100 mm — can be fitted in suitable focusing mounts to Amateur and Professional Movie Cameras. COERZ Reflex FOCUSER — Patented — for 16mm Movie Camera users — voids PARALLAX between finder and lens — provides fullsize ground-glass image magnified 10 times. Adaptable to lenses 3" and up. Also useful as extension tube for shorter focus lenses for close-ups. Extensively used in shooting surgical operations, small animal life, etc. I COERZ Parallax-Free FOCUSER i and FIELD FINDER CONTROL ' for Filmo 121 and Simplex-Pockctte, no more / off-center pictures, magnifies 4 and 8x. : For Driailrd information Addrest \ 'l Dept. II' 1 1 j ■ C. P. Coerz American Optical Co. '• 317 East 34th St., New York t American Lens Makers Since 1899 have under-exposure; and above the highest value permissible in the latitude we have over-exposure. In sensitometric parlance, latitude is the region of normal exposure, and now that we glance back upon it — that probably would have been the best definition in the first place! To determine what this region of normal exposure is, a series of tests are made in the lab. called sensitometric strips, which are a series of exposures increasing from a very low value to a very high one, in a logarithmic progression. When these strips are developed to a specified value they are placed in an instrument capable of measuring the density of each of these steps of different exposure on the strips by means of the amount of light passing through the film — called a densitometer. These densitometer readings are plotted on a graph as the ordinates against the logarithms of exposure as the absiccas. A "curve" results — which is curved on the top and the bottom, with a straight line connecting them, in the middle. It is this straight line that represents the region of normal exposure for that particular film developed in the given amount of time, or the line that represents the latitude of the film. Along this line, any increase of exposure causes a proportionate increase in density. If our exposures go into the upper curve, or the "shoulder," the increase in density is not proportionate with the exposure, and a distortion in the contrasts results. The same holds true in the lower curve, or the "toe." If we take a scene where all the principal objects are within the range of exposures represented by this straight line, the contrast of tones is faithful to the scene in reality. But, if we place our exposure either too high or too low on this line then some of the highlights will go into the "shoulder" if we are on the high end of the scale, or some of the shadows will go into the "toe" if we are too low on the low end. It becomes, then, a problem of placing our exposures in such a position on the scale that the h'ghlights stay below the "shoulder," while the shadows stay above the "toe." And here is where the meter comes in. In our opinion, the two meters on the market that represent a very high order of accuracy are the General Electric and the Weston. Both of them utilize a photoelectric cell to measure the intensity of the light, and in both cases this light is translated into electrical energy, which minute quantity is measured by a very sensitive meter. The stronger the light, the stronger the electrical current generated by the cell, and hence the higher the reading on the scale. But here is where their similarity slops. Primarily designed for reading reflected light — the light reflected from the subject — its scale is calibrated in candles per square fool the unit of brightness. Since the scale of the Weston meter is a logarithmic one, there is one big advantage and one big disadvantage. The advantage lies in that the calculator is arithmetical, and hence very easy to read. Added to that, the entire sensitometric curve is practically laid out on the calculator, and if the development factor is known it is possible to place the exposure at any predetermined point on the straight line portion, or the region of normal exposure. We can predetermine the density of the developed negative. And make sure that none of our highlights (except catchlights, or "kicklights") go outside of the normal exposure reigon on the high side, nor any of the shadows on the low side. The disadvantage lies in the fact that since the scale is a logarithmic one, any errors made in the reading of the meter — of the light portion of it — will be logarithmic, and hence show up as serious errors. In the General Electric the scale is an arithmetical one, the unit of mensuration being the foot-candle. Here a slight error in the evaluation of the light is an arithmetical error, and as such will not show up as gravely — but the calculator is logarithmic, and as such a little more difficult to read. In this calculator we do not have the sensitometric curve spread out before us as we do on the Weston calculator. But there is an advantage the General Electric meter has which is not found with the Weston. The General Electric has been designed primarily for incident light readings — the measuring of the intensity of the light itself, rather than the light reflected from the object. In this method the meter is pointed toward the camera, with the light falling upon the sensitive cell in the same manner it falls upon the subject. It is to be remembered that light striking an object at an angle will appear to have a lesser intensity than light striking perpendicularly, so for this reason it is important to measure the light as the camera will see it. While the General Electric meter has been adapted to read incident light only up to 70 footcandles — from there on they go into the reflected light method — it is possible to secure a multiplier which fits over the sensitive cell and lets only 10% of the light reach the sensitive surface. This raises the 70 foot-candle scale to a 700 foot-candle scale. A tip to those using this method: The calculator on this meter only provides for the 70 foot-candle scale for use with the incident light readings. If you wish to use the calculator with the multiplier, then, instead of taking 1/50 of a second — if that is the speed you wish to use — on the calculator, take 1/5. And use the "Dim Light Arrow." What is being done in effect is to multiply the calculator by ten in the same (Continued on page 27) 24