The Depth of Field Myth

People talk about the “depth of field” with a lens at a given setting. I believe that depth of field is something of a myth. The actual depth of truely sharp field is, effectively, zero, so it really boils down to “point of focus”.

With paper prints made from film, there was a fixed resolution to the film, and a fixed, lower, resolution to the paper. So there was a zone in a print which all had the same apparent sharpness, and then a drop off in front, and behind, that zone. Thus the “depth of field” myth was born. When digital began to mature, some long-time photographers observed that, while the maximum sharpness in digital was now sharper than with film, there was less “depth of field” in digital prints.

Lets consider the focal sharpness to be a cartoon mountain, a triangle, up on one side, from the foreground, to the point of focus at the top, and down on the other side beyond the point of focus. Changing your aperture makes the sides of the mountain steeper or shallower, but it doesn’t actually change its shape. The mountain always came to a perfect peak at the top, but there is this cloud obscuring the top; a small cloud, high up, in the case of a chrome, but a bigger one, further down, in the case of a paper print. From our point of view, that made Mount Depth of Field look like it had a flat top on it; as there was a zone of even sharpness; caused by the film and paper resolution, not the lens.

The Mountain (Obscured by Clouds)

Now we move on to digital: the best of the sensors, the best of the lenses, the best of the printing systems, provide a significantly sharper set of capabilities. So the cloud gets smaller, and higher, and the mountain starts to look pointed on top. And people complain there is less depth of field in digital. No problem; run a gaussian blur cloud over your image, and the depth of field will magically return, along with the lack of sharpness that caused it in the first place.

I owe credit for much of this piece to a discussion with Vincent Versace. Only the mountain is totally my own. I suspect one of these days I’ll catch up with my reading, and find this already covered in a book; probably somewhere between Oz and Kansas, actually.

Credits: C. David Tobie, Copyright 2012. Website: CDTobie.com Return to Blog’s Main Page

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