Digital Lens FAQ version 0.3
Joseph S. Wisniewski
Revision: 0.3, August 30, 2003 (rough draft, under construction)
Author: Joseph S. Wisniewski
Home: http://www.swissarmyfork.com/digital_lens_faq.htm
Copyright 2003, all rights reserved
Index
Brief Introduction
1) What defines a "digital lens"?
1.1) Longer "throw"
1.2) Reduced coverage circles
1.3) Reduced chromatic aberration
1.4) "Digital Friendly" features
1.5) Increased sharpness (resolution)
1.6) "Digital friendly" focal lengths
2) What are the different "digital lenses"?
2.1) Sigma "DG" lenses
2.1.1) Sigma 14mm wide angle lens
2.2) Nikon "DX" lenses
2.2.1) Nikon 12-24mm f4.0 DX
2.2.2) Nikon 16-55mm f2.8 DX
2.2.3) Nikon 10.5mm f2.8 DX fisheye
2.2.4) Nikon "digital friendly" lenses
2.3) The "four thirds" system
2.3.1) "four thirds" characteristics
2.4) Tamron "Di" lenses
2.5) Canon "EF-S" lenses
2.6) Schneider-Kreuznach "digitar" lenses
2.7) Pentax "DA" lenses
3) Do we really need digital lenses?
4) How about "Wide Converters"?
4.1) Kodak patent 5,499,069
4.2) Will a "wide converter's" extra glass elements hurt performance
5) How well do teleconverters work with DSLRs?
A) Glossary
Acknowledgements
References
Disclaimer
Revision History
0.1 May 8, 2003
0.2 August 24, 2003
0.3 August 30, 2003
Brief Introduction
By now, the average DSLR owner has heard the phrase "digital lenses"
often enough to be familiar with the term, but the explanations, facts,
and myths are so conflicting that one really does not know what to
think. The common myths and flying factoids include:
· Old film SLR lenses don't have enough resolution for digital
· DLSRs always exaggerate chromatic aberration
Here, then, is my pathetic attempt to shine some light into this area…
1) What defines a "digital lens"?
There is no real agreement among lens makers and digital camera users as
to what constitutes a "digital lens". A "digital lens" could be
described as a lens with one or more of these characteristics: long
"throw" (strong retrofocus or telecentric design), reduced coverage
circle, reduced chromatic aberration, or increased resolution. It may
also have other "digital friendly" features, such as a double lens cap
or lens shade that provides coverage for both a full sized film frame
and a reduced size digital frame, or a focal length range that provides
the DSLR equivalent of a popular 35mm film lens.
1.1) Longer "throw"
Film is sensitive to light that strikes its surface at virtually angle,
even angles far from perpendicular. For example, some ultra-wide-angle
lenses such as the Voiglander Heliar (for 35mm rangefinder cameras) and
the Schneider Super Angulon (for large format cameras) actually reach an
angle of 60 degrees from perpendicular in the corners of the image.
These lenses are symmetric in construction (which is wonderful for
controlling lens aberrations) and their "exit pupil" (the point in space
from which light seems to originate) is very near the "optical center"
of the lens.
Digital sensors are more selective, being most sensitive to light that
arrives perpendicular to the sensor. Most sensors use "microlenses", a
tiny lens in front of each pixel of the sensor, to increase their
sensitivity. Such sensors normally lose 50% of their sensitivity when
the light diverges just 15 degrees from perpendicular horizontally (from
the data sheets for the Kodak KAI-11000CM "full frame" and KAF-5101CE
"four thirds" sensors). To avoid noticeable vignetting, light should be
within 12 degrees of perpendicular, horizontally. Even sensors that do
not have microlenses have a noticeable decrease in sensitivity by the
time light diverges 20 degrees from perpendicular.
This means that any conventional symmetrical wide-angle lens would not
work on a digital camera. Fortunately, for the last 50 years, the
popularity of SLR cameras caused the creation of many "retrofocus"
lenses, lenses that have "virtual" exit pupils much farther away from
the film than you would expect from their focal lengths. This was done
to increase "back focus" (the physical distance between the rear lens
element and the film or sensor) so that the rear elements of the lens
would physically clear the swinging mirror in the SLR cameras. It has
the benefit of moving the exit pupil to a point around 50 mm from the
film plane. Light from a 50mm exit pupil will strike the film within 19
degrees of perpendicular, a big improvement from 60 degrees for the
Heliar. So digital wide-angle photography is possible.
Ordinary retrofocus lenses move the exit pupil just far enough to clear
an SLR mirror. A lens can be made more "digital friendly" if the exit
pupil is moved farther than this. For example, the Nikon 17-35mm f2.8
lens has an exit pupil that ranges from 98mm (at the wide 17mm setting)
to 78mm at the 35mm telephoto setting. The Sigma DG lenses also feature
such "extreme" 80mm exit pupils. Camera and lens companies refer to such
lenses as "telecentric". This is a slight exaggeration: a true
telecentric lens has an exit pupil an infinite (mathematically, anyway)
distance from the sensor. This causes the light to arrive perpendicular
to the sensor. It is also costly, and creates problems, so 80-90mm "near
telecentric" lenses are considered the norm.
This table shows that a lens with an exit pupil 52mm from the film plane
(typical of Nikon or Canon wide angles) has the potential for severe
vignetting on a 1.0x or 1.3x crop camera, and noticeable vignetting on
1.5x or 1.6x cameras. (1.7x and 2.0x cameras are essentially immune to
such vignetting). Increasing the exit pupil to 80mm means that the 1.0x
camera may have objectionable vignetting, but 1.3x to 2x cameras will
have no vignetting at all.
FOV crop factor Light angle with a 52mm exit pupil Light angle with an
80mm exit pupil
1.0 19.1 12.7
1.3 14.9 9.8
1.5 13.0 8.5
1.6 12.2 8.0
1.7 11.5 7.5
2.0 9.8 6.4
1.2) Reduced coverage circles
Lenses designed for 35mm film photography project a circular image at
least 43mm in diameter (the diagonal measurement of a 24mm x 36mm film
frame). Digital cameras usually have sensors that are smaller than the
35mm film frame, the infamous "crop factor". This is due primarily to
the cost of making larger sensors. The nice thing about lens design is
that you can take any lens design and simply scale it up or down as much
as you want to cover any particular format. So the Tessar (a classic
"normal" lens) can be scaled to a 45mm lens that covers a 35mm film
frame, or a 420mm lens that covers an 8x10 view camera frame. Weight, of
course, is proportional to the "scaling factor" raised to some power
between 2 and 3.
The only problem here is that the exit pupil of the lens gets closer to
the sensor when you scale a lens design smaller. That is correctable by
making a retrofocus lens: altering the rear part of the lens (maybe
adding an extra element or two) to make the light appear to come from a
point farther away from the film plane. This increases the "back focus",
the distance from the lens to the film plane so the lens physically
clears the SLR mirror, and moves the exit pupil farther from the sensor,
so the lens is more "digital friendly".
But the minor additions necessary to increase the back focus don't add
an outrageous amount of size and weight to a lens. There is no reason
why a 1.5x smaller "DX" lens can't have a diameter about 66% that of a
good 35mm design such as the 17-35mm f2.8, and about 1/2 the weight.
Length should still be a bit shorter, assume you can take an 80mm long
lens, scale it down 66% to 53mm, then add another 20mm at the back for a
couple of achromats to increase back focus, and you're still 10mm
shorter than an equivalent 35mm lens.
This "scale and extend" method of increasing the coverage circle should
make for a cost effective lens.
1.3) Reduced chromatic aberration
Chromatic aberration is the most annoying flaw in a lens used on a
digital camera. It causes different colors to come to focus at different
places on the imager. Both Bayer sensor and Foveon sensor cameras
perform forms of interpolation (guessing) based on colors. If the lens
produces colors that are not properly aligned, this guessing process is
disrupted, and the results are annoying. Color fringes are brighter and
clearer than they would be on an equivalent film camera. Sometimes they
glow like neon or acquire really strange colors. So, it is important to
have good chromatic aberration performance, even if this means lens
design compromises that increase other (less annoying) aberrations.
1.4) "Digital Friendly" features
Aside from these features, the lens may have other "digital friendly"
features. The Sigma 8mm fisheye, 14mm ultra-wide-angle, and 15-30mm zoom
have "double lens caps". The "main" lens cap is a
1.5) Increased sharpness (resolution)
1.6) "Digital friendly" focal lengths
One of the difficulties in using regular "film" lenses on a digital
camera is that the focal lengths become a bit uncomfortable. For
example, every lens manufacturer offers a 24-70mm, 28-80mm, or something
similar, because it is a very handy focal length range for event
coverage, or just general-purpose photography. In fact, this is
considered a "standard" zoom theses days. With crop factors of 1.3x to
2.0x, these lenses becomes more like 45-120mm zooms. The wide end isn't
"wide" anymore; it's merely "normal". The Olympus 14-56mm, Nikon
16-55mm, Canon 18-55mm, and Pentax ??-??mm are the first lenses to offer
equivalents to the "standard" zoom on a cropped camera.
2) What are the different "digital lenses"?
So far, there are several different "digital lenses" either in
production, or announced for near future production. Nikon has the "DX"
lenses. Kodak and Olympus have the "four thirds system". Sigma has "DG"
lenses. Canon has "EF-S" lenses. Tamron has "Di". Schneider offers the
"digitar" lenses for digital medium and large format. Each of these
offerings approaches digital in a different way.
2.1) Sigma "DG" lenses
Sigma says that the DG lenses are "designed specifically for digital SLR
cameras" and that the lenses "feature superior light distribution, so
that there is minimal light fall-off or vignetting, even when used at
maximum aperture". Aside from this, they are not saying what constitutes
"designed specifically for digital". From Sigma's lens data sheets, and
from measurements on several Sigma lenses, we see that the most
noticeable design feature is that the DG lenses have large back focus
distances. This means the light from the lens is nearly perpendicular to
the sensor, even in the far corners of the sensor. This prevents
vignetting (dark corners) and reduces chromatic aberrations (color
fringes). This is especially important on cameras with large sensors and
cameras that use "micro lenses" on their sensor to increase sensitivity.
The Sigma 20, 24, and 28mm DG lenses have exit pupils from in the 80mm
range. Equivalent Nikon and Canon lenses have exit pupils in the 50-55mm
range. The lenses do not feature reduced coverage circles, or noticeably
less chromatic aberration than wide angle lenses from other
manufacturers.
It's strangely ironic that Sigma's DG lenses appear to be optimized for
performance on any digital camera, except Sigma's own. The Sigma SD-9
camera has the smallest sensor in the industry, and it's one of the few
cameras to not use microlenses, so lenses with exit pupils in the 50mm
range are not a problem for the SD-9.
2.1.1) Sigma 14mm wide angle lens
This is probably the very first "digital friendly" lens, so it deserves
special mention. The Voigtlander Heliar proved that a relatively small,
cheap 12mm lens could be built for a rangefinder film camera. In
designing the retrofocus 14mm for SLR cameras, Sigma appears to have
taken the SRL design one step further. It has two "digital" features, a
65mm exit pupil, where 50 would have done fine (and probably reduced
distortions on film a little bit) and the double lens cap. The 14mm f3.5
and f2.8 predate Sigma's "DG" designation. I don't think Sigma would
want to try to "re-brand" it a "DG" lens after it was already on
dealer's shelves.
2.2) Nikon "DX" lenses
Nikon has announced that there will be four of these, featuring reduced
coverage circles, reduced aberrations, and lens hoods matched to the
1.5x crop of the Nikon D1, D100, and D2 series cameras. They are high
quality (and expensive) lenses.
The lenses are part of Nikon's professional line, with multiple aspheric
and low dispersion elements to control aberrations, constant apertures,
and fairly small zoom ratios.
2.2.1) Nikon 12-24mm f4.0 DX
The 12-24mm DX has a reduced coverage circle, since a 12-24mm zoom isn't
really practical for 35mm full frame. It is essentially a conventional
18-35mm lens, scaled down by a factor of 1.5x, with additional
enhancements to make it more "digital friendly", and improvements to
decrease aberrations.
2.2.2) Nikon 16-55mm f2.8 DX
2.2.3) Nikon 10.5mm f2.8 DX fisheye
2.2.4) Nikon "digital friendly" lenses
Nikon ties with Sigma as producing the first "digital friendly" lens
that I have direct experience with. The Nikon 17-35mm f2.8 AF-S appears
to have been designed to fill the needs of both film and digital
photographers. Its aberrations are so well controlled that it defines
the state of the art in ultrawide zooms. And its exit pupil location is
impressive, ranging from 78mm at the 35mm telephoto setting to 98mm (at
the wide 17mm setting). This is long enough to insure virtually
undetectable vignetting on a 1.5x crop camera, and excellent performance
even on a full frame camera.
2.3) The "four thirds" system
This may be the most confusing "digital lens" system of them all. Some
consider "four thirds" to be an entirely new, digitally optimized
system. Others consider it to be a simple repackaging of existing
Olympus OM lenses. We're still sorting it out. Right now, no one that I
know of has access to the "open specification" for the 4/3 system (as
announced jointly by Kodak and Olympus) so this section will refer
specifically to characteristics of the Olympus "E-System" flavor of
"4/3". This appears to be a rather advanced digital lens system, having
virtually every desirable "designed for digital" characteristic.
2.3.1) "four thirds" characteristics
Again, keep in mind that these may be Olympus "E-System" specific
features, and not parts of the 4/3 system in general.
Olympus refers to the E-System as "telecentric". They define this as
restricting the incident angle to within 6 degrees of perpendicular at
the corners of the image. While not truly telecentric, this does require
the exit pupil to be at least 85 mm from the sensor, over twice what is
required for the 21.8mm image circle of the 2x crop "4/3 system".
The lenses also claimed to have very well controlled aberrations and
high resolution. The diagrams furnished by Olympus seem to support this,
showing complex, modern designs with aspheric and low dispersion
elements.
Lens hoods, of course, match the sensor, since there is no film body
"counterpart" to the digital "E-system".
Another interesting "digital" feature is that each lens contains a
memory chip which carries image correction parameters (barrel and
pincushion distortion, vignetting, and we would assume chromatic
aberration), allowing the camera to correct these distortions digitally,
on the fly.
2.4) Tamron "Di" lenses
Tamron has two "Digitally Integrated" lenses. According to Tamron:
Di is a designation Tamron puts on lenses featuring optical
systems designed to meet the performance characteristics of
digital SLR cameras. Lenses provide improved image quality for
users of both traditional film-based and digitla SLR cameras.
Di =
1. Improved Resolution
2. Minimized Peripheral Light Fall-off
3. Compensation of Ghosting & Flare
4. Reduction of Chromatic Aberrations
This is a nice assortment of features. The first two Di lenses are the
28-75mm f2.8 zoom, and a 180mm f3.5 1:1 macro. These lenses are long
enough so that reducing the coverage circle would not noticeably improve
their size, weight or performance. And f2.8 zooms in that range from
24-28mm up to 70-105mm have exit pupils sufficiently far from the sensor
that they do not need additional telecentric redesigns.
2.5) Canon "EF-S" lenses
At the moment, there is only one lens in this system, the 18-55mm
announced for the Canon 300D (Digital Rebel, KISS) body. It may not be
proper to say that this is a "designed for digital" lens, or even a
"digital friendly" lens. But EF-S is an interesting technique to
increase the wide-angle zoom range of a low cost "kit zoom" lens, and it
is only applicable to digital cameras, so we will cover it here. EF-S
takes advantage of the fact that a 1.6x crop digital camera can have a
much smaller mirror than a 35mm "full frame" film SLR. Full frame
cameras have mirrors that are approximately 36mm wide and 33mm deep.
This means that no part of the lens can come closer to the sensor than
about 35mm, or the moving mirror will strike it. Until now, most DSLRs
have used "borrowed" film camera parts, including the mirror. This is
because it's more cost effective to use parts that were already tooled
and tested for million unit/year film cameras, than to design new
components for ten thousand (maybe 100,000) unit/year digital SLRs. The
Canon 300D changes that, being the first DSLR actually designed to "go
gold" and sell a million units the first year.
A 1.6x crop digital only needs a 22x21 mirror. This allows a lens to
approach within 25mm of the sensor. In general, this is not much of an
advantage. But for wide-angle zooms, it offers something unique. A
wide-angle zoom typically zooms from its longest setting to its widest
one by moving an inner group of lenses forward, and the rear group of
lenses back towards the sensor. If there's more room to move the back
element farther back, the zoom can zoom even wider. This also reduces
the coverage circle, but this isn't a problem if you've got a 1.6x crop
DLSR. This has allowed Canon to take an existing lens, the 22-55mm
f4-5.6 (or at least start with a similar design), and stretch it into
the 18-55mm. And this allows them to hit a $99 (US) target for a "kit
lens", similar to the 22-55 on film cameras. It is a comfortable lens
for fans of 28-80mm lenses on 35mm film cameras, offering the equivalent
of 29-88mm coverage.
Aside from this, we would expect a lens cap that properly matched the
1.6x crop, and not much else from such a lens. Although tests show
surprisingly good resolution.
2.6) Schneider-Kreuznach "digitar" lenses
Schneider states that this series of lenses " provides the ultra-high
resolution required by today's CCD sensors, and by the next generation
as well. This assures the highest possible image quality".
2.7) Pentax "DA" lenses
Pentax DA lenses are similar to the Canon EF-S, in that they are
oriented around
3) Do we really need digital lenses?
The answer is a firm, definite "maybe".
3.1) "Fixing" the sensor
3.2) "Full Frame" cameras
4) How about "Wide Converters"?
The focal reducer (or wide converter) is a device that goes between the
lens and the camera to increase the field of view of a lens, just as the
common teleconverter reduces the field of view. Such devices are well
documented. Astronomers refer to them as "focal reducers". (Then again,
astronomers refer to "teleconverters" as "Barlow lenses", so they're a
bit of a strange lot. Always scurrying about in the middle of the
night…).
Here's a nice "focal reducer" link.
http://astro.martianbachelor.com/CB245/ReducerDesign.html
That's the basic principle. In practice, a single lens isn't going to do
it. A pair of achromats seems to be about the minimum that will get the
job done (just like teleconverters).
4.1) Kodak patent 5,499,069
Kodak has a patent on the basic concept of the "wide converter".
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-adv.htm&r=1&f=G&l=50&d=CR86&S1=4,591,234.UREF.&OS=ref/4,591,234&RS=REF/4,591,234
Claim 3 is very specific:
3. An optical adapter for an SLR camera having a camera body with a
mirror in an optical path during a viewing mode comprising:
a) a lens attachment optical system having a plurality of lens elements
arranged into optical units, wherein the lens elements of said lens
units have radii of curvature and spacings sufficient to create a
smaller size image when said lens attachment system is placed in a
converging beam created by a primary objective lens system and
sufficient back focal distance to clear the SLR camera mirror; and
b) an adapter housing having a first mounting member capable of
attachment to an objective lens barrel incorporating said primary
objective lens system and a second mounting member capable of attachment
to said camera body.
The key difficulty with this is that prior art (of which there is much)
all deals with astronomical "focal reducers", which all reduce back
focus. On a telescope, this is not typically a problem, since their
optical systems allow a little extra back focus, for placement of
accessories. It is a problem on an SLR "wide converter" because back
focus is minimal.
4.2) Will a "wide converter's" extra glass elements hurt performance
You'll often hear people say that extra elements hurt the picture,
adding aberrations, or reducing light. This, of course, is absolute
nonsense. While extra elements, applied improperly, will cause image
degradation, using them right will improve the performance of the
optical system. Yes, each element you add to an optical system adds some
aberrations, some attenuation, and some reflection (increasing the risk
of flare). But each element you add contributes to the system,
correcting some of the aberrations introduced by other elements. If the
system is properly designed the beneficial correction from each
additional element more than offsets its additional attenuation or
distortion. This is why you've probably never taken a picture with any
lens having less than four elements.
In the case of the wide converter, reducing magnification brings all
sorts of benefits. The prototype 0.66x "wide converter", for example,
reduces most of the aberrations of the main lens by 33%. This includes
flare (per unit area), chromatic aberration, softness (resulting from
coma, astigmatism, or spherical aberration), and any aberrations
resulting from centration problems (the lenses elements not all being
precisely lined up along the common optical axis). In fact, your simple
little 4 or 6 element system would have to increase any of these
problems 50% in order to overcome the aberration reducing effect of the
reduction in magnification.
As far as light loss, even the single coated prototype has a 2% per
surface loss, which is 8% for the four surfaces. Since the focal length
reduction provides a 112% increase in light density, an 8% loss is
negligable. There's still over a full stop of light gain.
5) How well do teleconverters work with DSLRs?
Generally, not as well as film. It has to do with "final magnification".
Digital photography hits us very hard in this area. First, unless you
own an expensive and exotic "full frame" camera like Canon 1Ds or Kodak
14n, you have a crop factor to deal with: 1.3x, 1.5x, 1.6x, 1.7x, or
2.0x. Second, you're typically printing more stuff larger than you did
with film, or looking at it closer (1:1 pixel level on the computer
screen).
A) Glossary
Aberrations -
Achromat -
Airy disc -
Back focus - The physical distance from the back lens element to the
image plane (sensor or film).
CA - see "chromatic aberration"
CCD -
Chromatic Aberration -
Circle of confusion -
Coma -
Coverage circle -
Crop factor -
Diffraction -
Exit pupil -
Field reducer -
FOV multiplier -
Foveon -
Microlenses -
Pupil - see "exit pupil"
Rear node -
Retrofocus - lens that has its rear node actually outside the lens, and
an exit pupil farther away from the film plane than you would expect for
its focal length.
Symmetrical design -
Telecentric -
Teleconverter -
Tessar -
Vignetting -
Wide converter -
Acknowledgements
Special thanks to Richard Gosler, for the information on the Schneider
"digitar" lenses
References
KODAK KAF-5101CE Image Sensor Device Performance Specification, July 18,
2002, Revision H.
KODAK KAI-11000M KODAK KAI-11000CM Image Sensor Device Performance
Specification, December 9, 2002, Revision 1.
"Frequently Asked Questions...and some helpful answers", Sigma
Corporation, http://www.sigmaphoto.com/html/faqs.htm (captured May 1,
2003)
"Tamron USA, Inc - Di Site", http://www.tamron.com/di.htm (captured
August 29, 2003)
Disclaimer
A lot of trademarks appear in this FAQ. They are all the property of
their respective owners. I am not associated in any way with any of
these folks. I own no trademarks. I was given the name Joseph Stanley
Wisniewski, long ago, by a couple of nice people.
Revision History
0.1 May 8, 2003
Initial draft
0.2 August 24, 2003
Reformatted document
Expanded several sections
Added sections 2.2.1, 2.2.2, 2.2.3, 2.2.4, 2.3.1, 2.4, 2.5, 2.6, 3, 4,
4.1, 4.2
Added the glossary and acknowledgements
0.3 August 30, 2003
Added info on the Pentax DA and Tamron Di lenses
Added sections 1.5, 3, and 5
Changed major section headings to questions (it is a FAQ, after all)
Added the index, disclaimer, and revision history
Fixed minor section numbering errors
--
Make flutes, not war!
remove .nospam. to reply
Mark N
'1.4) "Digital Friendly" features
Aside from these features, the lens may have other "digital friendly"
features. The Sigma 8mm fisheye, 14mm ultra-wide-angle, and 15-30mm zoom
have "double lens caps". The "main" lens cap is a '
Regards, Mark
"Joseph S. Wisniewski" <w...@netfrog.nospam.net> wrote in message
news:3F5166B8...@netfrog.nospam.net...
> Here, then, is my pathetic attempt to shine some light into this area.
> night.).
Magnus W
> The Digital Lens FAQ
> Revision: 0.3, August 30, 2003 (rough draft, under construction)
Do you want comments on this?
Joseph S. Wisniewski
Always. Any kind of comments, from typos, to entire new chapters, to a
list of what you want in the FAQ.
Sorry about the late reply.
Joseph S. Wisniewski
>
> you forgot to finish section 1.4
>
> '1.4) "Digital Friendly" features
> Aside from these features, the lens may have other "digital friendly"
> features. The Sigma 8mm fisheye, 14mm ultra-wide-angle, and 15-30mm zoom
> have "double lens caps". The "main" lens cap is a '
>
OK, got it. Thanks.
Magnus W
OK, so I only had two comments about the finished sections after all :-)
> 1.3) Reduced chromatic aberration
> Chromatic aberration is the most annoying flaw in a lens used on a
> digital camera. It causes different colors to come to focus at
> different places on the imager. Both Bayer sensor and Foveon sensor
> cameras perform forms of interpolation (guessing) based on colors.
FoveOn sensors do no such interpolation. They will still be affected by
chromatic aberration, of course, but it will look more "film-like" and not
produce the dreaded purple fringe present on Bayer pattern sensors.
> isn't "wide" anymore; it's merely "normal". The Olympus 14-56mm, Nikon
> 16-55mm, Canon 18-55mm, and Pentax ??-??mm are the first lenses to
Pentax 18-35.