Bob Carnie said:

Hi Ed

would this acceptable area on both sides as you suggest be called the *circle of confusion*.

being the small area of depth of focus that the human eye will accept as sharp.

I suggest we are both talking about the same thing here, My only concern is too not confuse the issue by considering * depth of field * as an applicable method of setting up an enlarger because the 1/3-2/3 idea is unrealistic when focusing on a flat playing field.

We are not talking about the same thing.

A circle of confusion has been defined as: "When we try to focus all the rays of light from a lens into one point, we cannot. The best we can do is to form a circle of light. That circle's deviation from an infinite point - is the "Circle of Confusion". It can be reduced by eliminating some of the rays toward the outside extremities of the lens - restricting the image to the central rays... in other words, using a smaller f/stop."

Formula for determining Depth of Focus / Field:

First calculate the "Hyperfocal Distance - the distance where the lens, when focused at infinity, will produce an acceptably sharp image of the nearest object.

Hyperfocal Distance = ((FxF)/f) x (1/C)

Where:

F= Focal length of the lens

C= Circle of confusion

f = f/stop = diameter of aperture / Focal length of lens

An example = A 50mm lens at f/8, with a Circle of Confusion of 1/100mm:

H= ((50 x 50) / 8) x (1 / (1/100))

H= 312.5 x 100

H= 31250mm; 31.25 meters

Everything from 31.25 meters to infinity will lie within a Circle of Confusion (acceptably sharp) of 1/100 mm.

It is obvious that reducing the f/stop (increasing numerically) will decrease the Hyperfocal distance.

Now, Depth of Field:

For the nearest sharp plane:

(H x D) / (H + ( D - F))

Where:

D = Distance focused upon

H = Hyperfocal distance

F= Focal length of lens

and the farthest sharp plane:

(H x D) / (H - ( D - F))

Working the same lens, same aperture and and the same circle of confusion; focused at 25 meters:

Np = (31.25 x 25.0) / ((31.25 - (25 - .05))

Np = 12.4 meters

The Farthest plane:

Fp = (31.25 x 25.0) / ((31.25 - (25.0 - .05))

Fp = 124.0 meters.

Someone check my math!!!

I haven't cracked those books for one hell of a long time!!! I just realized HOW long!!