This page supplements a Part 2 of a series of articles evaluating Zapruder film alteration. The original YouTube link was used to capture the audience member shown.
Figure 1. The audience member in the lower left turns her head while the shadow on her head changes significantly. Played at 25% speed. Normal speed video spans 0.25 seconds.
Figure 2. Zoomed in view of Figure 1.
Figure 3. After a specific frame is darkened, the animation alternates between the original and darkened renderings. This demonstrates how a simple Photoshop operation (see Figure 4) can make the head shadow appear to grow.
Figure 4. The Photoshop Curves adjustment used to darken the head in the alternating part of Figure 2. Note that the author has observed this effect on numerous images. Many varieties of Photoshop adjustments will change the apparent area of the shadow.