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Depth of Field is an obsession for some, but it's just a repeatedly poorly explained Circle of Confusion for most of us. Between the optical physics and the algebra lies a very simple truth with Two Variables you can control to get more selective focus with your camcorder.
Depth of Field is simple. It might even help if you think of it as Depth of Focus or Selective Focus.
Having a deep (or wide) Depth of Field means everything is in focus -and- you have no selective focus.
A shallow (or narrow) Depth of Field means only your subject is in focus -and, therefore- you have a great deal of selective focus.
For most of us point-and-shooters, a really deep Depth of Field is nice, because then we don't have to worry about focus at all - everything is in focus. Fortunately, one of the primary factors that determines the Depth of Field is the aperture, which is inherently really small in a small camcorder.
Artistic types and wannabe artistic types like more control... to keep some stuff in focus while leaving other stuff out of focus. Indeed, selective focus is one of those details that Hollywood has the time, money and personnel to pay attention to. And, Hollywood also has big cameras and big cameras mean big apertures with potentially shallow Depths of Field.
This tiny square represents a 1/6" CCD, which is pretty typical for a consumer camcorder, although larger 1/3", 1/2" or even 2/3" sensors are typical of more expensive models. 35mm film is larger still, Ultra Panavision 70mm even bigger and then there's the gigantic Imax frames. Larger formats require larger aperture lenses and the inherent potential for more control over selective focus.
The horrible equations that explain all of the optical physics start with the Hyperfocal Distance, which is the nearest you can focus and have an infinite Depth of Field. The focal length - or ZOOM - is an important variable, as is the f-number or aperture. Now we've got to calculate the Near limit of the DoF, where another important variable pops up, the subject distance from the camera. Then we calculate the far limit of the DoF, subtracting the Near Limit from that and substituting our hyperfocal distance equations back in gives us...
Look: if you really want more Selective Focus options, you really only have to worry about: Zoom, Aperture and Subject Distance.
And since our control over the aperture is limited to opening it up, that leaves just Zoom and Subject Distance.
- As we zoom in on the subject, the Depth of Field gets shallower AND
- if we move the camera close to the subject we also get a shallower Depth of Field
Which leaves us with an interesting conundrum: If we zoom in to get a shallower Depth of Field, then physically back the camera up, to keep the subject the same size - we get a deeper Depth of Field. In other words: The two cancel each other out.
Fortunately, the two variables don't cancel each other out in a simple, direct way, which means you can move the camera back and zoom in to exploit the laws of optical physics:
- I'm about 50 feet from the background here, and the camera is in nice and tight.
- I'm about 50 feet from the background here, but the camera is about 20 feet away and zoomed in.
There are also some other very good, interesting and artistic composition, framing and relative perspective reasons you might want to back the camera up and zoom in.
If you want increased selective focus, separate your subject from the background as much as possible, and then Zoom In or Get the Subject Close to the Camera. This may be one reason you tend to see Flowers in Depth of Field demonstrations all of the time.
And if you want to win a foreign film award, just remember the words of the famed producer and director Billy Wilder and make sure you "Shoot a few scenes out of focus."
References:
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