On Mon, 28 Apr 2003 16:16:52 -0700, "Gene E. Bloch" <gino37@nowhere.none> wrote:

>Neuman - Ruether wrote:

[...]

>> Hmmmm.... I've heard this opinion before about shorter

>> FL lenses having more restricted maximum possible resolution

>> due to diffraction, but I don't believe in it in practice,

>> at least for the apertures likely to be used with the

>> sensor sizes involved. I observe that lenses for 35mm, for

>> instance, with FLs over a wide range of from 8mm to 500mm,

>> all show a common characteristic: general optimization

>> of resolution around f5.6 to f8 in the center, with

>> reduction at smaller stops due to diffraction... If one

>> compares lenses of FLs with the same angle of view with

>> differing sensor sizes and check for the optimum stop

>> for each FL for the differing formats, there is some

>> variation, but this would be expected in the design,

>> given physical size and DOF considerations (there are

>> no 300mm f2.8 lenses made for 8x10, for instance, but

>> there are many of the same speed and equivalent angle of

>> view made for video cameras - but even with the great

>> disparity of actual FL, the optimum stops for these are

>> not very different [it isn't worthwhile to make a

>> large-format lens diffraction-limited at a wide stop,

>> since it will likely never be used at that wide stop,

>> and similarly, smaller-format lenses are rarely

>> diffraction-limited at their widest stops...]). In

>> other words, short of spending a fortune for VERY

>> high quality video lenses that may optimize at very

>> wide stops, most good lenses perform about the same,

>> regardless of size (and almost regardless of format,

>> in the center...).

>> David Ruether

>In addition to the issues you address, the relationship between resolution

>and lens diameter is in terms of angular resolution. Using Dawe's limit,

>familiar to telescope makers (diffraction-limited angular resolution in

>seconds of arc is approximately 4.57/aperture in inches) I compute about 77

>seconds of arc for a 3mm F/2 lens. This corresponds to 1870 lines across an

>assumed 40 degree wide field, or 935 line pairs. This is more than the

>number of CCD cells across the detector and the number of pixels across the

>miniDV format, of course.

>

>If we zoom in to, say, a 30mm focal length at F/2, we'd improve the

>resolution to 7.7 seconds by the above formula while reducing the field of

>view to 4 degrees. Still 1870 lines, still no problem.

>

>Looks like we shouldn't worry too much about shorter FL lenses, if I can

>trust my slide rule (well, $5 electronic calculator + my own brain,

>actually).

>

>Of course your remarks assume - or actually, demonstrate - that camera

>lenses are not diffraction limited at full aperture; practical experience

>and common sense say you are absolutely correct...

>

>Gino

>Gene Bloch

Thanks for the comments.