## 76. Relative Values Of Different Markings On Diaphragms

Relative Values Of Different Markings On Diaphragms. There are two methods generally used in marking diaphragms or stops. The first expresses the ratio which the diameter of the opening bears to the focal length of the lens. For example, F. 16 means that the diameter of the opening is 1-16 of the focal length. By focal length we mean the distance between the lens and ground-glass when a distant object has been focused. In a rapid rectilinear lens the distance from the diaphragm to the ground-glass will be correct. The focal length is dependent upon the curvature and combination of the lens.

77. The second method of stops is the Uniform System, the numbers of which bear the same ratio to each other as the area of the diaphragms which they designate. The Uniform System has been generally adopted by the manufacturers of hand cameras and amateur outfits. We here give a comparative table:

 u. s. 1 equals....................... F. 4 u. s. 4 equals................... F. 8 u. s. 8 equals................... F. 11 u. s. 16 equals.............. F. 16 u. s. 32 equals............... F. 22 u. s. 64 equals.................. F. 32 u. s. 128 equals............... F. 45 u. s. 256 equals....................... F. 64

## 78. Kind Of Plate To Use

Kind Of Plate To Use. As better results are produced with ordinary plates by the average photographer, until one becomes thoroughly familiar with the workings of the ordinary plate, it is advisable not to attempt the use of specially prepared plates requiring more careful treatment. For general out-door work the ordinary plate should be used. If one has sufficient experience to use any plate, better results will be produced with the orthochromatic plate. In any case the plate used should be a fast one, as it will per-mit you to make quick exposures even on buildings of dark or non-actinic colors. When working in crowded thoroughfares quick exposures are always necessary, so the fastest plate must be used. While in a fast plate the latitude of the exposure is greatly reduced, yet one soon becomes accustomed to the speed of a plate, and the exposure resolves itself into a matter of secondary importance. The fast plate permits the use of small diaphragms. This is an important item in architectural work where a great deal of clearness and detail are absolutely necessary.