Wilbur Wong

21-Oct-2005, 09:32

I think I have a better understanding now from the earlier thread.

Specifically in the use of wide or superwide lenses, the "circle of confusion" works in conjunction with the "cone of acceptable focus". My understanding is that the smaller the f stop, the narrower the "cone" will be, and therefore will increase the tolerance of how far from the plane of focus, the film may be permitted to deviate (given the same tolerances for eventual print sizing etc.).

I think I understand that this angle of the cone is independent of focal length and is related strictly to focal ratio (f stop). Is that correct?

If the above is true, isn't there an additional factor which comes into play with wide angle lenses? Billiard balls elongate into elipses in the corners of wide angle shots, I think in inverse of the tangent of the angle off axis from the center of the picture. Doesn't the cone of focus also elongate similarly? It seems to me that if it does, than the tolerance for depth of focus becomes even less at the edges of a wide angle photograph as the film will fall off of that cone with less distance from the plane of sharp focus.

Of course this is where I have increasing difficulty viewing the image, both visually on the ground glass, and with a loupe. I admit to doing as JJ mentioned in a previous post that many photographers simply stop down as far as possible, with some frequency.

What do you think?

Specifically in the use of wide or superwide lenses, the "circle of confusion" works in conjunction with the "cone of acceptable focus". My understanding is that the smaller the f stop, the narrower the "cone" will be, and therefore will increase the tolerance of how far from the plane of focus, the film may be permitted to deviate (given the same tolerances for eventual print sizing etc.).

I think I understand that this angle of the cone is independent of focal length and is related strictly to focal ratio (f stop). Is that correct?

If the above is true, isn't there an additional factor which comes into play with wide angle lenses? Billiard balls elongate into elipses in the corners of wide angle shots, I think in inverse of the tangent of the angle off axis from the center of the picture. Doesn't the cone of focus also elongate similarly? It seems to me that if it does, than the tolerance for depth of focus becomes even less at the edges of a wide angle photograph as the film will fall off of that cone with less distance from the plane of sharp focus.

Of course this is where I have increasing difficulty viewing the image, both visually on the ground glass, and with a loupe. I admit to doing as JJ mentioned in a previous post that many photographers simply stop down as far as possible, with some frequency.

What do you think?