[rollei_list] Re: Sweet spot: f8/f11?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 23 Oct 2011 21:06:27 -0700
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, October 23, 2011 5:17 AM
Subject: [rollei_list] Re: Sweet spot: f8/f11?
On Sun, Oct 23, 2011 at 8:02 AM, Marc James Small
<marcsmall@xxxxxxxxxxx>wrote: (snipped)
The Carl Zeiss MTF data seems to set out that the 2.8/80
Planar used in the
GX and FX cameras achieves its best performance from
f/5.6. I am not aware
of similar scientific tests done on the earlier versions
of this lens or on
the Xenotar. In my personal experience, both lenses as
used in the 2.8F and
earlier cameras peak from f/8 but, again, my own
experience is hardly any
more a scientific test than were the musings of Burt
Keppler.
Marc
My experience with the 80 Planar is that it's best between
f/5.6 and f/8
Eric Goldstein
Several things change with f/stop: the illumination
accross the field, coma, spherical aberration, depth of
field/focus. While astigmatism does not change its effect
depends on depth of focus. Depth of focus also affects
whatever effect film flatness has on the image. Since
magazine tests are of the entire camera system, usually
without any investigation of aerial image quality or any
attempt to optimise the camera the "optimum" stop can be
affected by lack of coincidence in a TLR or the effect of
spherical aberration in a SLR or rangefinder camera where
the finder system is adjusted for the lens wide open or at
some other stop. Note also that "optimum" for a view camera
may depend on the coverage expected of the lens. Generally
the marginal sharpness of a lens is affected more by the
stop than the center of the image. This is not much of a
consideration in a TLR or other camera using either a fixed
lens or, when lenses are interchangible, lenses of different
designs for different focal lengths. However, for lens like
say a Dagor which can be used as a normal, long focus, or
wide angle lens, the optimum depends on the size of the
field its expected to cover, smaller stops being necessary
to reduce the spherical aberration and coma at wide angles.
The five and six element Planar type lenses used on
Rolleis probably have maximum resolution near the center of
the field at nearly wide open but will need to be stopped
down two to three stops to get sharp at the margins. An
f/2.8 Planar/Xenotar is probably optimum at around f/5.6 to
f/8, an f/3.5 Tessar, which has a greater amount of oblique
spherical, will be sharpest at around f/11 to 16.
Tests that show resolution being higher at some
intermediate field position may indicate high order
aberrations or, more likely, lack of film flatness.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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