Tim:
Circle of confusion should be a simple concept to understand if
reduced to simple geometry.
Assume on a piece of paper we have a single point somewhere to the
left and two points exactly on the right side edge.
The we draw two lines, one from each of the right side points to and
through the left hand point. So what we have is sort of a triangle
lying on its side, pointing left with the sides extending past the vertex.
So, now consider that the vertex is the exact focus point of the lens
represented by the distance between the two right side points. If
you move away from that point either towards the "lens" or away from
the "lens", the point kind of spreads out to the width between the
lines. That spread is the circle of confusion (represented here as a
distance between the lines) and will be seen in the image, not as a
point ( like the focus point ) but as a smeary circle or blurred
point. Depth of field is determined by how much of this blurring you
can tolerate in the final image. In a large format negative where
the final print will be a 2x enlargement, You probably can't detect a
blurring of perhaps 2 thousandths of and inch ( that would make tiny
points into 4 thousandths inch dots .) In a 35mm negative enlarging
to the same print size, we would need 10x enlargement and the blurry
dot size on the negative couldn't be larger than 4 ten-thousandths of
an inch, only 1/5th the acceptable size on a 4x5 negative.
Now to get back the image 'sharpness', you can get this blurred dot
by either being slightly out of focus ( near or far ) or by image
aberration ( the lens just doesn't resolve that perfect point light
source to perfect dot on the negative or a combination of
both. Large format lenses don't have to reach the resolution values
that 35mm lenses do to get the same quality in the final 8x10 (or
larger print). That's why, for the same aperture, large format
lenses are cheaper than comparable 35mm lenses with the same angle of
view ( discounting the added cost of the additional shutter in the
large format lens.
Large format users are perfectly happy with a maximum aperture of f/8
in a 300mm lens, but 35mm users wouldn't even consider buying a
comparable 75mm lens with a max aperture of f/8 - they would think is
unusable. On the other hand, a 300mm F/2 lens for 4x5 would be
astronomically priced if even available but a 75mm f/2 lens could be
a reasonable 35mm lens to have
I think I rambled on too much.
Cheers,
James
At 04:39 PM 4/15/2007 -0400, you wrote:
Tim,
The "sharpness" of lenses is expressed in many different ways but a
common way is to characterize them by line pairs per millimetre at
the image. Another is by contrast at a given number of lines per
millimeter at the image plane. Another yet is something that Pop
Photo uses and that is Subjective Quality Factor - which essentially
is something that describes how large an enlargement you can make
from the negative before it starts to look unsharp to the "standard
observer" from a given distance (I think that is what SQF is but a
not positive).
Depth of field is usually not connected with lens sharpness. The
assumption being that the lens can perform better than the
characteristics of the "circle of confusion" associated with depth
of field determinations.
Confused? Me too!
cheers
andy
Tim Corio wrote:
How are "resolving figures" represented? Is there some standard unit of
measure for sharpness? I've heard of circle of confusion, but was
confused by it. Perhaps I need to take the time to understand it.
James Schenken
