product conforms closely to the desired tonal values (based on a D-min of 0.40). Trigradient Control Curves As   stated   previously,   the   trigradient   tone reproduction  method  is  based  on  three  tone-control curves (or printer curves): high contrast, medium contrast, and low contrast. Basically, the printer curves are produced the same way that sensi-strips are made for process monitoring. Instead of using a sensitometer to expose the film, the contact printer is used to expose the film through a step tablet. Since the amount of exposure is unknown, the horizontal axis of the curve indicates the density of the step tablet. The vertical axis represents the density produced after the duplicating film is exposed through the step tablet and processed. Each set of printer curves consists of a family of response  curves  that  graphically  display  the  various tones produced in the duplicating material when it is printed   under   various   exposure   and   processing conditions.  Each  curve  is  labeled  with  the  exposure setting used on the printer to create the curve (figs. 4-30, 4-31, and 4-32). These response curves enable the density range of the imagery to be altered, so the density range of the final product conforms closely to the desired density range (1.00). The exposure of the printer  is  important  since  it  determines  the  placement  of the tonal values of the imagery being reproduced on the sensitometric  response  curve  of  the  duplicating material.   Ideally,   all   tones   should   fall   on   the straight-line portion of the response curve to ensure that the images are reproduced uniformly. The D-max of the imagery being duplicated is used to determine the required exposure since it reproduces as the D-min in the  reproduction. Trigradient Tone Reproduction Procedures The following procedures are used in the trigradient tone reproduction system: 1. Obtain the three sets of tone control curves (high, medium, and low contrasts) for the particular duplicating materials being used. 2. Determine the D-max and D-min densities on the roll of imagery to be duplicated. Remember, the D-max and D-min are the areas in which you want to retain detail, not necessarily the areas of highest and lowest density. Do not use specular highlights or completely black areas to represent the D-max and D-min. 3. Determine the density range in the original, and determine  whether  the  tonal  range  should  be  increased, retained, or decreased. Do this to obtain the desired density range (usually 1.0) on the duplicate. 4. Based upon results of step 3, select the set of response curves closest to the density range (high, medium, or low). 5. On the horizontal axis, locate the density of the step tablet that corresponds to the D-min density that you selected. Go up from this point until you intersect the 1.40 line and draw a tick mark. 6. On the horizontal axis, locate the density of the step tablet that corresponds to the D-max density that you selected. Go up from this point until you intersect the 0.40 line and draw a tick mark. 7. Using a straightedge, select the characteristic curve closest to your straight line. If the curves cross, choose the curve closest to your D-max because this density controls the exposure of the D-min on the duplicate. 8.  Expose  and  process  the  duplicate  film  according to your determination in step 7. 9. Read the D-max and D-min of the duplicate film.  If  these  values  are  not  within  established tolerances (0.05, for example), another duplicate must be made. Minor adjustments in exposure or processing may have to be made. For example, if the D-min is too high, then less exposure is required. If the D-min is within tolerance but the D-max is too low, then more development is required (to raise the contrast). The entire tone control system is based upon data produced when the system is established. For the system to be accurate and reliable, all of the major variables must be controlled so they can be carried out on  a  repetitive  basis. CAPTIONING AERIAL PHOTOGRAPHY Captioning   and   slating   hand-held   aerial photography is as important as captioning and slating still and motion-picture photography that is taken on the ground.  In  captioning  aerial  photography,  however,  the caption information should be expanded to include the camera lens focal length, the altitude, and the direction of the aircraft from which the picture was made and the time of day the photograph was taken. 4-40