[rollei_list] Re: Lens Resolution Charts
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 21 Feb 2011 14:31:38 -0800
----- Original Message -----
From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, February 20, 2011 5:06 PM
Subject: [rollei_list] Lens Resolution Charts
National Bureau of Standards Special Publication
374, METHOD FOR DETERMINING THE RESOLVING POWER
OF PHOTOGRAPHIC LENSES, is the Holy Grail for
such charts. The Air Force chart is quite
adequate but these have been reproduced,
generation until generation, so the ones
available today are probably twenty or thirty
generation copies, with a loss of sharpness at every
generation.
The NBS chart included in this Special
Publication is printed by the US Mint and is a
first-generation product.
Marc
The USAF chart is available as a TIFF or high res JPEG
on the net. A Google search found several sources. These can
be printed out in any size.
To test a lens using this chart really requires a group
of charts arrayed over a flat surface. The resolution
printed on the chart assumes a certain magnification (or
rather reduction) so the ratio of the final image to the
size of the chart must be known.
The pattern of the modern chart is such as to avoid as
much as possible "false resolution" but this must still be
guarded against in interpreting results. The way is to count
the number of bars; the number will be wrong where there is
a false reading.
By using an array of charts the variation in resolution
with image angle can be seen. While there is a simple
relationship between theoretical resolution and image
anglebased on simple diffraction at an aperture it is
affected by then nature of residual aberrations, the
geometry of the image, and the "illumination" of the
entrance window, any or all of which can change the
distribution substantially from that of a simple aperture
with zero thickness, which is what the charts show. There is
also the issue of film flatness where the resolution is
measured from a recorded image rather than from the aerial
image.
A test using the USAF 1951 chart can be quite meaningful
but one must be aware of exactly what it is measuring and
its limits (true of any measurement of anything).
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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