a  slight  increase  in  density.  This  is  called  the threshold and indicates the least amount of exposure that will produce a noticeable change in density. Straight-Line Section Further to the right (fig. 2-6) is a section of the curve that appears to be a straight line. Note that in many cases there will not be a well-defined straight line. This section of the curve has a constant slope, and  in  addition,  the  gradient  of  the  slope  here  is greater than in any other part of the curve. In the straight-line section of the characteristic curve, there is an equal increase in density for an equal increase in exposure.  This  is  the  most  important  part  of  the curve. Subject exposures that fall on the straight line produce  constant  density  differences. Shoulder  Section The  upper  section  of  the  curve  (fig.  2-6)  is  a convex, curved line that gradually decreases in slope. This section of the curve is called the shoulder. Like the  toe  area,  equal  changes  in  exposure  do  not produce equal differences in density. Tones of the subject falling very far up in this section are blocked; that  is,  reproduced  with  densities  too  heavy  for printing  or  maximum  detail.  For  normal  exposures, bright  highlight  tones  of  the  subject  tend  to  be reproduced in the shoulder section of the curve. Exposure Although   sensitometry   is   a   tool   of   the   lab technician, it also is significant to the photographer. Notice  how  this  applies  to  exposing  film. For example,  when  a  uniformly  lighted  gray  card  is photographed,   there   is   a   single   exposure, corresponding to a single point somewhere on the log-H axis. When the light on the surface of the gray card is increased and another photograph is made (maintaining the same camera settings), the exposure and the log H also increase. This causes a shift to the right   on   the   log-H   axis. There   should   be   a corresponding increase in density, and the two factors again should plot on the characteristic curve. Extending this to a scene with a large number of luminances (reflectances) or high-luminance ratio, the tonal differences in the subject, the lighting, and the camera  settings  determine  the  film  exposure  that produces varying amounts of densities. These density differences must be related to the log-H differences that produce them; that is, density differences in the negative must be considered in their relationship with the tones of the subject. The  density  differences  in  a  negative  can  be partially   controlled   by   placing   the   exposures corresponding  to  the  subject  tones  in  the  correct section of the characteristic curve. This is done by adjusting the camera settings correctly, providing that the range of tones in the subject is not too great. Emulsion  Latitude The emulsion latitude is the exposure range where there  is  a  proportional  relationship  between  density differences and log-H differences. In other words, it is  simply  the  range  of  exposure  covered  by  the straight-line  section  of  the  characteristic  curve  (fig. 2-6).   The   latitude   of   an   emulsion,   therefore, determines the brightness range of the subject that can be   reproduced   proportionally.   Latitude   may   be expressed either as the difference in log-H values between  the  two  extreme  points  of  the  straight  line or  as  the  exposure  ratio  between  these  same  two points. The emulsion latitude of light-sensitive materials varies  according  to  the  purpose  for  which  it  was designed—from   1:400   or   more   for   long-scale panchromatic film, and from 1:20 or less for process film. For any given emulsion, the emulsion latitude varies  according  to  the  contrast.  The  emulsion latitude decreases as the contrast increases. Log Exposure Range The useful exposure range includes part of both extremities  (toe  and  shoulder)  as  well  as  the straight-line section of the curve. For ground pictorial film the useful exposure range of a sensitized material is greater than the emulsion latitude, since portions of the toe and shoulder regions of the curve are used without sacrificing print quality. The   approximate   lower   limit   of   the   useful exposure  range is    the    density    point    on    the characteristic curve that is not less than 0.10 above the gross fog of the film. This point is referred to as minimum useful density. The upper limit is generally located at 90 percent of maximum density on the shoulder of the curve and is referred to as maximum useful density. In practice, many photographers use a 2-12