DOF Calculations

[Carrots]

Can someone just quickly explain the circle of confusion to me? It confusing me.

[polarwasp]

Photography Field Guideof national geographic has a good story on that.

[geriatric]

C of C is points of light that if overlapping with an adjacent point results in unsharp images. The most spectacular is unsharp images using a mirror lens.

Going back to my statement that C of C is tied up in the lens checking on D Flemmings calcs, the C of C is calculated by actual FL / equivalent FL * 35mm format. I would say that is all tied up with the lens. But then again I have read somewhere that it depends on Pixel size of the sensor.

[slipe]

Carrots wrote:

Can someone just quickly explain the circle of confusion to me? It confusing me.

There is only one perfect focus point. But there is a range in which the "circle of confusion" from not being in perfect focus is considered to be in focus. This is called depth of field.

This is a fairly simple explanation of circle of confusion: http://www.northnet.org/jimbullard/CoC.htm

I personally think DOF calculations are a blunt tool. They are based on the resolving power of film from 70 or so years ago and the resolving power of the human eye looking at prints that film could produce. The resolving power of the human eye hasn't changed, but the resolving power of the film has. And the resolving power between different digital cameras is vastly different.

Say a pro bought a 3Mp DSLR a few years ago with an APS sized sensor. And say he took group photos and printed them for schools etc. The circle of confusion caused by not having the perfect focus point would allow for a fairly wide relative DOF because the resolving power of the sensor and print he could produce would allow the circle of confusion to be fairly large and not be seen in the print.

That same photographer now has a 1Ds Mark II. He prints a 16 X 24 of a championship football team to display on the wall in the boosters club. The boosters are going to look closely to see the individual faces. A circle of confusion that was acceptable with his 3Mp camera would be completely unacceptable with his 17Mp camera printed at 16 X 24. He had better have the focus perfect or the faces will appear less than sharp on close inspection.

The sensor size is the same in both cases. The lens' sharpest aperture is probably the same – he might still be using the same lens. The DOF formula gives the same range. But the actual DOF is vastly different between the two cameras.