Was auto iris on, and are high contrast scenes being shot (where at least part of the scene would be "properly" exposed at or near wide open iris/F1.9)? If the lens is zoomed in enough for shallow DoF, the DoF indicator will vary if auto iris is activated and the camera senses light variations large enough to make it decide to change the iris.

The auto iris probably can hunt a bit when adjusting, especially if there are objects moving in front of the lens that are triggering the auto iris to act (or the camera is moving enough to make the camera rethink exposure). I don't use auto iris much, but I was able to make the DoF indicator "dance" with full manual focus engaged (indicator would widen/shrink/appear to shift left or right a bit rapidly) using very high contrast view zoomed in around 18mm, while moving the camera slightly when a bright/dark border was near the center of the view. Auto iris was trying to guess what the heck I was doing and couldn't make up it's mind, and the DoF Indicator was understandably varying with the iris changes. Made the indicator look like it had drank too much caffeine until I stopped playing slightly wobbly hand-held and straightened out (auto iris settled out).

You elaborated one of the possibilities I mentioned earlier, Mark.

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