What Wired Didn’t Understand about Depth of Field
It's not about analog vs. digital, it's frame size vs. focal length
By Art Adams | August 16, 2008
In the Sept. '08 issue of Wired Magazine, in an article about the RED camera called "A Star is Born," the author states that the difference in depth of field between film cameras and HD cameras is due to "analog versus digital," with no other explanation. I've written this for him and for anyone who wonders why 35mm film and large sensor HD cameras have so little depth of field.
Here we see a hypothetical 25mm lens mounted on a 35mm film camera or an HD camera with a 35mm-sized sensor. The lens casts a circle of light, and the sensor/film gate "samples" the center of the image. The depth of field remains roughly the same for both cameras because the film gate and the sensor are roughly the same size.
Here's the same lens, shooting the same subject at the same distance, but with a Super 16mm film gate or 2/3" sensor. All that's changed is that we're now sampling a smaller section of the image projected by the lens. We've gone from a wide shot to a close up just by changing film or sensor size, but the depth of field remains the same for a subject that distance from a 25mm lens. And while we'd expect a lot of depth of field for a wide shot, we don't expect it on a close up--but try to throw the background out of focus on a 25mm lens in Super 16mm or on a 2/3" HD camera. It's hard to do.
The difference in viewing angle isn't strictly 2:1 between 35mm and 16mm (35mm-sized sensor and 2/3" sensor) but it's close enough (around 2.4:1) to give you an idea of what's going on. And if you're wondering why you can't ever get the background out of focus on a 1/3" chip camera, it's because you're sampling an even smaller section of what the lens sees, so even shorter focal lengths are required. For example, while the 25mm lens above started out as a wide shot and became a head-and-shoulders shot simply by stepping down in sensor size, another step down in sensor size will make it a tight closeup--but still with a lot of depth of field.
To shoot the same shot on all the cameras we've spoken about (35mm, 2/3" and 1/3") we'd be looking at using (approximately) the following lenses to keep the same angle of view as in the top diagram (a wide waist-to-head shot):
35mm film = 25mm lens (a lot of depth of field)
2/3" = 12mm lens (tremendous depth of field)
1/3" = 6mm lens (near infinite depth of field)
All will yield the same size shot, with the camera at the same distance to the subject, but with drastically different depths of field.
If we shot a close up of this same person with an 85mm lens, here's what would happen:
35mm film = 85mm lens (not much depth of field)
2/3" = 42mm lens (twice the depth of field as 35mm film)
1/3" = 21mm lens (four times the depth of field of 35mm film, twice that of 2/3" sensor)
With each step we are doubling the depth of field... which is why you'll never get much out of focus when shooting with an HVX-200 or DVX-100.
Go to page two for some handy depth of field and angle of view rules...
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