[rollei_list] Re: Bright Screens
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 4 Jan 2012 08:46:46 -0800
----- Original Message -----
From: "Kirk Thompson" <thompsonkirk@xxxxxxxxxxx>
To: "Rollei List" <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, January 04, 2012 7:19 AM
Subject: [rollei_list] Re: Bright Screens
Thank you Richard for this thorough explanation of pitfalls
in the quest for sharpness.
Focus shift is even more a problem now that digital sensors
(perfect 'film flatness') reveal small lens disparities.
Leica recently re-designed the 35mm Summilux because it had
earned a bad rap for focus shift. Sean Reid, on the best
website for review of lenses for Leica, now tests all lenses
for focus shift. Several of the Cosina Voigtlander lenses
in Leica mount have done poorly in this respect.
One hears that Tessars can resolve as well as Planars if
stopped down to f8. No doubt this is where DOF overcomes
Tessar focus shift.
Fortunately for photographers, absolute sharpness isn't the
only value in a lens. Other aspects of how a lens 'draws'
or renders can be more important than how well it reveals
pores & beard hairs.
Kirk
Resolution has more to do with how well the several
lens aberrations are corrected. Some aberrations are stop
dependant and get better at smaller stops. Think of the
angle of the light though the lens: the smaller the angle
the light rays make with the surfaces of the lens elements
the less they are deviated and the less effect whatever
aberrations there are make on them. This is not true of all
aberrations, for instance chromatic aberration is not
improved by stopping down.
The Tessar is capable of very good performance, for an
f/3.5 Tessar both spherical aberration and its other
aberrations are about gone at f/8.
The ultimate limit of resolution is the diffraction
limit which is dependant on the effective size of the lens:
the larger the opening in the lens the higher its resolution
is. So, as one stops a lens down there are two conflicting
effects: the resolution due to diffraction becomes less; and
the resolution due to reduction of aberrations becomes more.
At some stop the resolution is maximum for that particular
lens. For many practical photographic lenses of high quality
this stop is around three stops down from maximum opening.
However, it is also dependant on the coverage angle expected
from the lens since many aberrations vary with this angle.
As an example a Dagor, which is an inherently wide angle
lens, is about optimum for "normal" coverage of about 60
degrees at about f/22 but, at its maximum angle, around 85
degrees, it must be stopped down to about f/45 for the
margins to be sharp.
Since some lenses have better correction of aberrations
to begin with their "optimum" stop will be larger than
another, poorer, lens of the same inherent speed. When
lenses are stopped down beyond the "optimum" stop the
resolution, in general, falls off so at small stops lenses
of differing designs or even quality may measure the same
since the resolution will be limited more by diffraction
than by aberrations.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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