[rollei_list] Re: Sweet spot: f8/f11?
- From: Allen Zak <azak@xxxxxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Mon, 24 Oct 2011 12:35:01 -0400
On Oct 24, 2011, at 12:06 AM, Richard Knoppow wrote:
----- 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
---
It was this issue that brought me to the Rollei group in the first
place back in 1997. At the time, I had recently photographed an
orchestra on stage under ambient light and was surprised at the f5.6
off-axis resolution. Although not awful, it was noticeably less
sharp than the central 1/3 of the negative. For that occasion, the
Rollei had served as a backup camera for my Hasselblad, which at the
same aperture was sharp across the field. Until then, I had never
noticed any sharpness difference between the two Planars involved.
Usually, they were used well stopped down, and when at wider
apertures, it was for subject matter where the edges were out of
focus or unimportant. Edge resolution on my sample didn't really
clean up until f8-ish.
Before this Rollei Planar, my TLR-ography had been exclusively with
Tessar type lenses. Except for its critical sharpness at the center
wide open, my Planar performed otherwise not much different than the
better Xenars on several Rolleicords I had owned over decades. This
was a disappointment to me, since I had assumed that an advantage of
the Planar was better edge resolution at all but the widest f-stops.
Seeking answers, I did an internet search that eventually led me to
the Rollei site. There, I received information about alignment, film
plane variations, properties of various lenses (Planar vs.
Xenotar :D) and recommendations for service. According to Dr.
Fleenor, to whom I sent the camera for a CLA, the lens was fine and
in proper calibration. So I learned to live with it, assuming that
lenses were just like that, until several years later when I came
into an all too brief ownership of a Mamiya 6. Its 75 mm f3.5 lens
was crisp at center and edges wide open, and outperforming both
Rollei and Hasselblad until @ f8 on either one. Detail was amazing,
but even so, I preferred the Planar image rendering. There is more
to lens performance than wide aperture resolution.
Allen Zak
---
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