PHOTOGRAPHY BY POLARIZED LIGHT— McFARLANE 229 



be avoided in the following cases, if a single Pola-screen is used at the 

 lens: (1) Photographing obliquely through glass or water; (2) photo- 

 graphing a surface the detail of which is obscured by oblique reflec- 

 tions; (3) photographing where bright oblique reflections interfere 

 with good composition. All these cases of oblique reflection control 

 apply to nonmetallic surfaces. 



SKY CORRECTION 



Controlling the brightness of the sky is another field of usefulness 

 of the Pola-screen as mentioned before. There is a band in the sky 

 from which the light is strongly polarized. This band is at right 

 angles to the sun's rays, as shown diagrammaticaUy in figure 3. If 

 we imagine the line from ourselves to the sun as an axle, this band 

 forms a wheel in the sky around this axle, with us at its center. 

 Therefore, at sunrise the band is north, overhead, south; at noon, 

 near the horizon in all directions; and at sunset, north, overhead, 

 south again. The band swings from overhead west during the morn- 

 ing, from east to overhead m the afternoon, passing through every 

 part of the sky. The band is sufficiently wide to provide a background 

 in ordinary photography. Because the light from this band is polar- 

 ized, we can diminish the brightness of this part of the sky consider- 

 ably with a Pola-screen at the lens. The indicator handle of the Pola- 

 screen provides a simple means of setting the Pola-screen for maximum 

 effect. When this indicator handle points directly at the sun, that is, 

 when its shadow falls along itself, the darkest sky is obtained. No 

 effect is obtained when the camera points close to the sun or directly 

 away from it. Pola-screen control, of course, applies to a clear blue 

 sky, as the Pola-screen has no more effect on sky completely obscured 

 by clouds than does a color filter. 



The question arises, how does this method of darkening the sky com- 

 pare with the use of yellow or red filters? Granting that the picture 

 to be taken is in such direction with relation to the sun that the Pola- 

 screen applies, the use of the Pola-screen has two advantages. The 

 sky only is affected, since the light coming from the foreground object 

 is not polarized. Therefore the monochrome rendering of the fore- 

 ground object is not affected, as shown in the photograph of the yellow 

 brick building in plate 5. This rendering can be very seriously 

 affected by the use of yeUow or red filters. Yellow or red brick build- 

 ings, faces, and other objects reddish in color, photograph unnaturally 

 light; blue objects photograph unnaturally dark when deep yellow or 

 red filters are used. The second advantage in favor of the Pola-screen 

 is that the effect is variable. By merely rotating the Pola-screen at 

 the lens, any effect in the sky, from very light to very dark, may be 

 obtained. 



