Put a Little Depth into Your Field (page 2)

Trying to get an appropriate depth of field may sometimes be tricky. I recall a struggle we once had in trying to film a chemistry experiment. In that situation, our field size, i.e. the amount of the world we wanted to show, was strictly fixed. We needed to show the entire experiment, no more, no less.
When we set up the shot, we found that the depth of field was insufficient. We soon learned that if we pulled the camera farther away, we got more depth of field, but then we had to zoom in to get the correct field size, which lost us the depth we had gained. If we moved in we gained from the shorter focal length but lost from the closer point of focus. As long as we kept the field size the same, we got the same depth of field regardless of where we positioned the camera.
The only thing left to change was the aperture. To solve the problem we had to pour in some more light so that we could shoot with a smaller aperture setting.
From this we learned two things. 1) Modifying the aperture by controlling the light (or the shutter speed, is the easiest way to change the depth of field. 2) Under any circumstances, if you need to adjust your depth of field, it is best if you can change only one factor to do it.

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