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Friday, May 26, 2006

Depth of Field and Depth of Focus

Contrary to common belief, these are not interchangeable terms. Both refer to a range of distance in front of or in back of an ideal plane, within which objects will still appear sharp. Depth of field, however, is that range in front of the lens, out in the world, corresponding to the subject plane, i.e., what you have the camera pointed at; depth of focus is that range in back of the lens, inside the camera, corresponding to the film or sensor plane.

Depth of field is usually measured in inches, feet, or yards, (or centimeters and meters, for those of you in Yurp and the rest of the world), and is of conceptual usefulness to photographers in visualizing a picture. Depth of focus is a technical calibration measured in microns that for the most part photographers can't do a whole lot about.

The exception to the latter is that view camera photographers often do have to fret about depth of focus. The ground side of ground glass backs, where the aerial image shows up, needs to be pretty precisely where the film will be once you insert a holder. Also, the film needs to stay in one plane as much as possible. Chip Forelli, who incidentally has a lovely portfolio in the current Lenswork magazine, uses tiny pieces of double-sided tape to stick his sheet film to the back of the holder when he knows he's going to be making long time exposures. Howard Bond figured out a simple way to measure his film holders for consistency—after which he threw out half his film holders and stopped getting the occasional mystifyingly soft negative that theretofore had dogged him.

Another way of putting this is that depth of field is the distance an object you've focused on can be moved closer or further away from the camera while still appearing sharp. Depth of focus is the distance the film or sensor can move toward or away from the lens and still have the image appear sharp.



Anonymous Peter said...

Depth of field is proportional to focus distance for the same aperture. The closer you are to something you're taking a picture of, the smaller the depth of field; the farther away you are, the greater the depth of field is.

That's akin to saying that it's proportional to object magnification, the bigger the thing is in the VF the shallower the DOF, the smaller the thing is the deeper the DOF.

11:16 AM  
Anonymous 01af said...

Most recommended reading about what DOF is and what it is not and what is going on before and behind the plane of focus can be found here:

The most important lesson learned there is: if you're interested in good sharpness at or near infinity then do not rely on hyperfocal focusing. Set your lens to infinity!

-- Olaf

11:36 AM  
Blogger Dibutil Ftalat said...

There is another, third determinant to the equasion: the size of the film/sensor. The smaller it is - the bigger DOF. Thus, no shallow DOF for miny-tiny digicams as well as too shallow DOF for 8x10 view cameras that fight with it by tilting..

3:02 PM  
Blogger Albano Garcia said...

You said aperture and distance to subject, but you forgot focal length!

7:05 PM  

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