acutance -- resolution

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

> pamphlet from (Zeiss) explaining the difference 
>between acutance and resolution.  So do you think more acutance
>gives more sharpness to out of focus objects?

Zeiss has always supported engineers and allowed them to publish, and their
legacy of technical papers is as solid as their advertising is mushy.  If
it's worthwhile & somewhat old, any chance of scanning this pamphlet?

Higher acutance gives a sharper edge to large objects at any focus, but
fine detail that is out of focus will not have any edge to enhance.
Acutance is mostly used as a measure of performance on film, while
resolution has a larger meaning.  A. Cox, "Photographic Optics", describes
attempts at Kodak to define acutance, similar to contrast, with the
distinctness of borders between areas being the criteria,  but does not
mention how they measured it.  John Williams, "Image Clarity", says that
acutance is measured with a knife edge in contact with the film, not a
dioptric or catoptric test, but a numerical measure of how well the film
can preserve 'unexposure' adjacent to an exposed area.  Both authors use
acutance as a property of film.  Presumably the techniques used in MTF
measurements could apply, if the acutance-preserving ability of an optical
instrument were to be measured. 

Resolution measures fineness of detail, but fine detail need not have high
contrast at its boundary, and the eye sees a high contrast border as a
sharp object.  A lens with lower measured resolution but good preservation
of contrast can produce images that appear sharper than a high res, low
contrast lens. 
Cox also notes that it is possible for a lens to have 2 foci, one with
highest res, and one with the best acutance.

High resolution across an image is achieved by balancing aberrations.
Field curvature is the major problem in visual instruments, but in a
camera, f.c. just means that sharp images at the edge are closer objects
than those depicted in the center (except for astrophotographers, copy
photographers, etc).  Coma is perhaps the main enemy of resolution,
assuming high res at the center of the field.

Depth of field is 'usually' a simple derivation of focal length & aperture,
but there are complications.  One is that a catadioptric lens gives ring
shaped out of focus Airy discs, which can effectively throw the image out
of focus at a lesser 'depth'.

The meaning of acutance I find in these books (and many of the best
references on photographic optics do not use the term), is preservation of
contrast on film.  We are apples & oranges here if we don't specify if
we're talking film resolution (grain size, etc.) vs. optical resolution;
and contrast preservation by the instrument (baffles, flat black paint,
lens coatings, color correction, and aberration control) vs. acutance of
the film (a grain will undergo full exposure without the adjacent grain
changing).

>I don't know how they trade off acutance for resolution and vice-versa.

Aberrations are 'traded off', one apt metaphor for lens design is squeezing
a water balloon, if you push on one area, another will bulge out.  There
probably are some refinements of lens design, manipulating higher order
aberrations, that act to lower some aberration at the edge to increase res,
but raise some other aberration that in turn can lower contrast.  Most
noticeable effects are probably a function of the lower order aberrations,
where such tradeoffs seem more unlikely.  (???)

I have gone out on a limb on this post, and I certainly hope that readers
who can make corrections and additions will do so.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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