>"Neuman - Ruether" <d_ruether@hotmail.com> wrote in message

>

>> I have never placed the Nikkor 500mm f8 (f8 1/2 in

>> practice, earlier-type) next to a standard-design

>> 500mm lens shot at f8 1/2 for comparison, but the

>> Nikkor mirror, with its hard-edged and complex-looking

>> out-of-focus area imaging (otherwise known as "bad

>> bokeh"...;-) and "doughnuts" does appear to have more

>> DOF than one would expect from a 500mm, with areas

>> that would normally be expected to look soft in the

>> image retaining a sense of detail and focus - and it

>> is often possible to take photos with it that appear

>> to have deep DOF, as when shooting a building face

>> at an angle, or a distant patch of ground, etc. I

>> once shot a series of building details (from tiny to

>> very large) with the Nikkor mirror for a couple of

>> magazine articles - and almost all the images appear

>> to have universal DOF, regardless of the shooting

>> angles...

>> David Ruether

On 30 Jan 2003 21:03:14 -0600, "Toby" <zdftokyo@ggol.com> wrote:

>

>The actual DOF is the same, but the double-edge produced by the mirror gives

>the sense that the OOF areas are sharper. If you doubt this paint or paste a

>black spot in the center of a filter and mount it in front of a normal

>refractive 500 mm. Come to think of it this might act as a quasi-diaphragm

>by limiting the size of the front objective and thus would increase DOF

>somewhat. Don't forget that an f8 mirror lens has an effective T-stop of

>about f10.

>Toby

Lenses with "over-corrected" spherical abberations (the

Nikkor mirror is one) will produce a "harder" rendering

of far-side out-of-focus image parts - and this will aid

in the appearance of increased DOF on the more important

side of focus (where the "finer-scale" detail is likely

to predominate); "DOF" is a visual thing, not a mathematical

thing, so the DOF *can* be different for two lenses set

up the same way, but with different optical characteristics

beyond the basic FL, etc. (and, yes, the "doughnuts" can

help with this...); and I did mention that the mirror was

not truly f8 ("f8 1/2")...