I love civilized topics.
In still photography depth of field control was vital to good photography. Cameras used to have a depth of field scale on each lens and there was a manual control to stop down the lens to the exposure set F stop in order to visually see the depth of field, as viewing from SLR camera was always done at the maximum aperture for brightness purpose but that wasn’t accurate as after pressing the shutter for the exposure the lens would stop down to the selected aperture.
"Exposure value" was also an important term that I haven't heard lately. This is the combination of shutter and F stop that will properly expose the image. You can go in either direction within the value if you needed depth or speed. On Hasselblad lenses you set first the exposure value then you could move both together for either depth or speed without changing the exposure. On modern automatic cameras the same can be accomplished with automation by selecting either the aperture or shutter priority. On digital camera we can also select the ISO speed, but going upward with the speed will effect the quality of the image.
Coming from still photography depth of field has always been one of my main priorities, even behind the video cameras. Video doesn’t have all the options that we have with still cameras, but nevertheless there are methods of controlling depth of field.
The size of the chip has a lot to do, this is why prosumer and now also some low end professional cameras with 1/3 chip are in my opinion good only to create visual records but not good to manipulate the picture for creativity. These shortcoming is what separate professional from amateurs equipment. Those who insists that they are both alike do not really understand how short these small cameras are on technical creativity.
Having said all this I’ve been doing a lot of experimenting trying to get the most, better yet the least depth of field out of small cameras, but first you must understand the control we have with full size cameras.
First, the longer the telephoto you use the shallower the depth will be, this goes with all cameras. This is why personally I only use my wide angle lenses when I need a wide angle shot, long lenses is what I always prefer to use. We can not use shutter speed on video cameras as this will effect the actual video, but we can control how much light will enter the camera with the selection of ND (neutral density) filters as most professional cameras has a ND control with four settings; and I use these setting constantly. If I shoot sports action, especially when using the 2X converter I will want the most depth, in this case I use the lowest setting on my ND filters trying to keep my lens at an F11 stop, sometime I even put a plus 3db. in order to stop down the lens . When I do interviews I reverse that, I want my aperture not to be smaller than 2.8, and with cameras like the HDX900 that’s very good in low light I have to put a 2 ND filter and also use a -3db. Of course the distance of camera to subject and subject to background is vital for a good and controllable depth, but that’s another topic.
On small camera, and I have a few of those, I was able to achieve something close (not too close) by cheating the camera in how much light was there by placing ND filters in front of the lens. The results were clearly visible.
For optimum low depth right now there’s nothing better than the new generation of DSLR, The only time I use my Canon 5D is when I need very very low depth. Caution because those also have different chip or sensor size. The 5D Mark II has the equivalent of a full size 35mm film, while the newer 7D, although it has many more video functions it also has a 20% smaller (cheaper) sensor, and that makes a BIG difference.
As far as rack focus goes it is a very nice effect when done properly and with good equipment. It is, at least when I do it, a pre set effect, meaning I try it several times without recording and placing objects and camera at optimum distance. The most effective one, and we’ve been seeing this done a lot on Planet Earth, is close up rack focusing between objects and/or subjects. For these I use my lens maximum aperture.