A Lens that Produces an Astonishing Image
The Sigma DP2x feature
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 DP2x's
lens is 24.2mm, and the diagonal
length of the image sensor is 24.86mm.
So, the lens used in the DP2x
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 DP2x 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 DP2x'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 DP2x's
lens is something you really need to
see for yourself.
The DP2x'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 DP2x's
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 DP2x's 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)
C
O
N
T
R
A
S
T
0
2.5
5
7.5
10
12.43
18
