Circle of Confusion

[Paul]

I am confused and going around in circles. Depth of Field, on a print has always been a seemingly straight forward concept to me. I take mostly landscapes but they don't always stretch into infinity so the DoF must be controlled to some extent. I use a tele-zoom so there are no DoF markings on the barrel. Ergo, I wanted to review the Hyper-focal Distance tutorial. I thought therefor, I should do the DoF one first. I do not find the DoF preview button on my camera particularly helpful. Along comes the Circle of Confusion. The more I read about it the more confused I become. So here goes.
Is the CoC when the shot is printed the same as the DoF?
Does the CoC only apply to the sensor?
Is the CoC a fixed dimension on the sensor? It seems to me that 0.01 of an inch is less than 1% of the size of my 350D sensor and the areas in a print which are in or out of focus, in relative terms, can be greater than this.
What advantage is there knowing the size of the sensor's CoC?
How does taking the CoC into consideration help?
It is possible that my questions are confusing enough to illustrate my real confusion.

HELP

Paul

[McQ]

Well, people often refer to the circle of confusion interchangeably for either (1) the print or (2) the camera sensor. The CoC for one can always be calculated based on the CoC for the other. Generally, the CoC is defined based on the desired print size and viewing distance, and then calculated at the sensor, and not the other way around.

Generically, a universal CoC can be defined based on a standard print size. This generic circle of confusion is really just an arbitrary threshold/cut-off for acceptable sharpness based on an 8x10 inch print viewed at about 1 foot with eyesight a little less than 20/20 (defined to be a blur diameter of 0.01 inches in that print).

You can always calculate what a CoC in print translates into on the camera's sensor by shrinking the print down to the sensor's size (ie, dividing by the enlargement factor). Your 350D, for example, has a sensor size of 22.2x14.8 mm. If you were to make a 22.2 cm x 14.8 cm print, and intended to view it at a distance such that the largest acceptable CoC in that print were 0.01 inches, then the circle of confusion at the camera sensor would be 0.001 inches in diameter.

Ultimately, it comes down to what you feel is and is not acceptably sharp. One you get the hang of the concept then try ignoring the numbers for a while. If you feel that the standard CoC is to relaxed (ie, too large), then shift the hyperfocal distance a little further toward the background to increase sharpness at infinity. Most landscape photographers do this since sharpness there is often more critical. Further, most also feel that the standard CoC is really not sharp enough for critical prints.

[Paul]

Thankyou for your rapid response. I regret my initial review of your answer still hasn't helped a great deal. I am definitely missing something with the CoC concept. Let me review the Hyperfocal distance tutorial and see if that helps at all. If not I will probably have to get you to help me again.
By-the-way, is that shot of a dining room taken in Cambridge University. Classy.

Many thanks,

Paul

[Paul]

I am afraid I am still uncertain what the CoC really is. I have looked at other sources but they have not been helpful.
To illustrate my problem let me use a hypothetical example. I take a picture between two extremly long rows of apple trees and I include an apple in the forground to ensure the viewer is aware of what the picture really is. When printed, the apple and the far distant trees are out of focus ( unsharp).I decrease the aperture size and this time everything is in focus.
So during this activity what happened to the CoC? Did it change size or remain the same?
If one assumes that the sensor, apart from its size, behaves essentially like a piece of film, and everthing in the picture is in focus the CoC must be as large as, if not larger, than, the surface area of the sensor. So when various sources mention CoC size, eg. you mention 0.01 or less and I have seen reference to my camera sensor having a CoC somewhere between 0.019 and 0.025mm (I assume these are diameters) it totally destroys my perception of what the CoC really is as these dimensions are quite small compared to the area of the sensor. I have also seen reference to the fact that the CoC varies in size with the aperture. So what am I missing? How does one use a variable sized CoC to calculate DoF? And, how important is it when taking pictures in the field?
When one doesn't understand something it is quite difficult to espouse a clear question. I hope I have managed to do it this time.

Thank you

Paul