What modern cameras (DSLR or mirrorless) have automatic depth of field exposure setting functionality? It would be desirable for example to specify a desired DOF based on subject size (or 'depth') and have the camera automatically adjust aperture based on the current focal distance and lens characteristics.
I've read that Canon had a DEP/A-DEP (Auto Depth of Field) exposure setting which was dropped from their lineup.
Here's the problem with automatic Depth of Field (DoF) calculations: The camera doesn't know the intended display size and viewing distance. These two factors determine the acceptable circle of confusion from which DoF is then calculated. The same exact image file will have different Depth of Field when displayed at different sizes and viewed from the same distance!
In the past cameras that had some type of automatic DoF setting operated on the assumption of standard display conditions: an 8x10 inch print viewed from 10-12 inches by a person with 20/20 vision. The digital age has so radically changed the way we view images that this is now, more often than not, a false assumption. On one extreme, most web based images are displayed at a much smaller size. On the other, viewing a 20MP image at 100% on a 23" (58cm) HD (1920x1080) monitor is like viewing a 56x37 inch (142x94 cm) print! For this reason, most camera makers at present don't seem to think that an automatic depth of field mode is useful. Most advanced users probably feel the same.
To fully understand this we need to understand what DoF is and more importantly what it isn't. There's a common misconception that everything within the Depth of Field (DoF) is equally in focus. This is not the case at all. There is only one plane that is in focus for any position of your lens' focus mechanism.
In a way, depth-of-field is an illusion. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.
Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focal plane. That is why DoF calculations must include the display size, viewing distance, and acuity of the viewer's vision in order to give a meaningful result.
Depth of field discussions:
The factors:
The roots of this discussion: A person with 20/20 vision viewing a disk from a distance 3000 times its diameter sees a dimensionless point. The shape of the object is unidentifiable. Now 1/3000 the distance works out to 1 meter diameter viewed from 3000 meters. However, for photographic purposes, due to optical flare, film turbidity, grain structure, and noise, we use a far lower standard. For photographic work the angle sustained is 3.4 minutes of arc. We are talking 1/1000 of the viewing distance as seen from 10 inches This is a circle size of 1/100 of an inch (0.25mm from 250mm).
If the image is viewed from 20 inches, this works out to about 2/100 of an inch viewed from 20 inches (0.5mm viewed from 500mm). This is the stuff of depth-of-field tables.
Now todays miniature cameras yield an image that is mostly useless unless enlarged. To get an 8x10 image from a full frame (24mm by 36mm) the degree of enlargement is about 8X. Thus the size of the circle at the focal plane of the camera must be 1/800 of an inch in diameter or smaller (0.032mm).
Based on all this gobbledygook, it is common practice for the industry to use 1/1000 of the focal length to construct depth-of-field charts and tables. The Leica standard is 1/1500 and the Kodak standard for precision work is 1/1500 of the focal length.
I think camera makers could adopt 1/1000 of the focal length for the size of the circle of confusion and build auto DOF routines. They could make the circle size selectable by the user.
