A Lens that Produces an Astonishing Image
The DP2 features
an integral standard lens
A lens with a focal length of 40mm
to 60mm on a 35mm film camera is
known as a “standard lens” because
it delivers natural perspective, close
to what the human eye perceives. The
usual definition of a standard lens is
one that has a focal length close to the
diagonal length of the image format.
The focal length of the DP2's lens is
24.2mm, and the diagonal length of
the image sensor is 24.86mm. So,
the lens used in the DP2 really does
deserve to be called a standard lens.
In the past, standard lenses have
traditionally been of either the Tessar
or the Gauss type. The Gauss type is
basically used for larger apertures with
high performance. Its disadvantages
include susceptibility to saggital
coma flare when used with a point
light source, and a tendency for the
light volume to decrease towards the
periphery. The Tessar type, on the
other hand, has a simple structure,
making it easy to miniaturize.
However, its drawback is that it tends
not to deliver high performance at
larger apertures.
Adapting SLR lens technology
for our own purposes
Sigma believes that a photo is only as
good as the lens it was taken with.
So we decided that the kind of quality
we wanted for the DP2 would not
be attainable using the traditional
techniques used to design standard
lenses. We rethought the design
from the development stage,
and set out in pursuit of the highest
possible lens performance. We took
the bold step of using a retrofocus
lens of the type mainly used
as wide-angle lenses for SLRs.
Besides having the excellent
telecentricity vital in lenses for digital
cameras, this retrofocus lens has
many other advantages: for example,
this format makes it easy to suppress
field curvature and astigmatism,
and to ensure that the light volume
stays the same right to the periphery.
However the lens does need to be
longer. Therefore, in order to minimize
the total length of the lens while still
attaining ideal image quality,
we used two groups of lens elements:
the group at the front have a high
refractive index, and the group at the
rear are aspherical glass-mold lenses.
This way, we successfully developed
a standard lens with a relatively
modest overall length, but very high
optical performance.
A super-high-performance lens that's
flat as a pancake right to the edge
In order to maintain high resolution
and contrast from the center of the
screen right to the edge, and to allow
scope for those cool background-
blur effects, we aimed for the kind of
MTF and all-round lens performance
only available with single-index lenses.
We are confident that the DP2's lens,
improved time and time again in
the interests of delivering the best
possible finished image, encapsulates
Sigma's expertise as a leading
manufacturer of lenses.
The impact of the eye-popping
image quality delivered by the DP2's
lens is something you really need to
see for yourself.
The DP2's integral lens is of the retrofocus type.
Besides having the excellent telecentricity vital
in lenses for digital cameras, this retrofocus
lens has many other advantages: for example,
this format makes it easy to suppress field
curvature and astigmatism, and to ensure
that the light volume stays the same right to
the periphery. However the lens does need
to be longer. Therefore, in order to minimize
the total length of the lens while still attaining
ideal image quality, we used two groups of lens
elements: the group at the front have a high
refractive index, and the group at the rear are
aspherical glass-mold lenses.
This way, we successfully developed a standard
lens with a modest overall length, but very high
optical performance.
As well as employing a rear-focus system,
we used aspherical glass-mold lenses, enabling
aberration variation to be minimized.
And the elements that make up the DP2 lens
have been treated with a super-multilayer
coating that succeeds in keeping ghosting and
flare to a minimum.
That's not all. Some of the DP2 lens elements
are hyperchromatic, which enables chromatic
aberrations to be corrected to a large extent,
without impacting other aberrations.
This means that even with a wide-open
aperture, the images produced have minimal
color flare, and are nice and sharp and crisp,
right to the edge. The Foveon X3®, which
delivers amazing color resolution, also makes
a big contribution to the very high level of
“emotional image quality” delivered.
The construction of the special-design lens
Sigma Lens 24.2mm F2.8 / MTF Chart
10 lp
30 lp
Spatial Frequency Sagittal Line Meridional Line
The MTF chart gives the results at the wide-open aperture.
Glassmold Aspherical Lens
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
IMAGE HEIGHT (mm)
CONTRAST
0 2.5 5 7.5 10 12.43
18
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General
Weight and Dimensions
