An Interesting Distance Meter

The purpose of a distance meter in photography is to enable photographers to focus their lenses upon objects without measuring the intervening space with a scale or tape. Focusing is necessary on account of the variation in distance between the lens and image in proportion to the distance from lens to object. Critical focusing is most necessary when the diameter of the lens opening is large compared to its focal length. In general, any lens operated near its maximum speed needs to be accurately focused for near objects.

Fig. 1 Distance Meter [for author's eyes and arm length]

Fig. 2 Appearance of Distance Meter when Eyes are Focused on Object Beyond

The optical distance meter collects light through two receiving systems which are so coupled that the two images thus formed may be combined into one. Proper adjustment of this device may be arranged to set a pointer on a scale, or to focus the camera lens directly. It would seem logical, therefore, to use our own pair of eyes directly as an optical meter, and this may be done in several ways. One of these is the use of an individually calibrated scale held out at arm's length, and sighted upon to measure the eyes' convergence while looking at an object.

Calibration of the Distance Meter

From cardboard, a triangle is cut with three-inch base and eight-inch altitude. Starting from a window, or any light source, the shortest distance shown on one's camera focusing scale is laid off, and one stands at this point facing the window or light. The triangle is held up before the eyes at arm's length toward the window, using a natural amount of muscular effort, such as can be duplicated at will. With the eyes focused on the window, one is aware that the triangle appears to be double, because each eye sees from its own point of view. While one is still holding focus on the window, it is possible to put a straight edge across the triangle parallel to the base such that it appears to intersect the edge of the two triangles at their common intersection. Finally a line is drawn where the straight edge lay. This completes the calibration for one distance.

The entire focusing scale of one's camera is calibrated in the same way on the cardboard triangle by backing off from the window each distance shown, and then determining where the two apparent triangles intersect and drawing a horizontal line through that intersection. The accuracy of the meter decreases with distance, but so does the necessity for critical focus. The accompanying drawings show how the scale looks when it is completed and how it appears when the eyes are focused on a point beyond it.

How to Use the Distance Meter

In practical use, the scale is held vertically at arm's length with the eyes focused on the important part of the object. The thumb nail is slipped up to the apparent intersection of the two triangles, after which the eyes may be focused on the triangle to see what line the thumb nail is covering, and the camera is focused to the corresponding value. Apparently nothing could be simpler, but there are some disturbing influences of which one should be aware.

Our eyes deceive us. Somtimes we think we are looking carefully at an object, but our eyes are actually focused on a point in midair. The remedy is to take one or two readings rather quickly, not giving our eyes time to tire or waver, and repeating until consistent results are obtained. Also it must be remembered that the pupil of the eye is not a point, nor is the opening the same size in bright as in dim light. So a certain lack of definition appears at the edge of the scale, and approximately the same brightness must be used when reading from the scale as when calibrating. This effect is reduced by calibrating with a moderately bright light, and by looking at a light of equal intensity just before taking a reading.

A mathematical relation and justification for this meter is shown in Fig. 3, and, as may be seen, the eye separation is very important for the longer distances. The significance of this is that if the gage is calibrated in a light room, and used in a light room the eyes do not change their separation. However, in dark places the pupil of the eye spreads, thus giving one scale value too large and another too small, depending upon which is read.

The other personal variable, that of holding the scale out always at the same distance, is very easily overcome by a very little practice, using a natural position and comfortable muscular effort. Errors in holding the triangle are most important at close distance.

This device is not adapted to commercial production because it must fit a certain pair of eyes, and a particular arm. It costs nothing, and may be made in half an hour, but when used with due care, it transforms a fine pair of eyes into a good distance gage that needs no apology. Extensive use of this meter in photographing children at play with the camera wide open, up close, and set for speed, has produced many satisfactory negatives, and demonstrated the usefulness of the device.

Fig. 3 Curves Showing Length of Lines on Gage in Relation to Distance to Object for 27 Inch Arm and Various Eye Separations.