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  technical marketing terms specific to Nikon lenses explained here

                          Glossary of Lens Terminology
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2011
23
Jan

Lens defects in modern lenses
Posted by admin under Articles | 2 Comments
On this page…

      Introduction
      Types of lens defects
      Some new lenses and reported defects
      Most vulnerable lenses
      Why so many new lenses have defects
      Test for lens focus shift
      Lack of focus shift with
       Nikon AFS 50mm f1.4G
      Lack of focus shift with
       Nikon AFS 14-24mm f2.8G
      What can be done by photographers
      Summary




Lens defects are increasingly being detected and reported by amateur photographers in high end
professional grade lenses.

Professional photographers understand their equipment and know what to do to work-around the
limitations and get the best out of their equipment. Now-a-days, it is the non-professional
amateur photographers who take the equipment to its limits and then report on the limitations of
the equipment.

Some people dismiss the work of these amateur photographers as 'pixel peeping' and accuse
them of testing the lenses in artificial conditions. The work of these amateur photographers has
helped highlight several defects which have been validated and subsequently corrected. So there
is a lot of value in their work. The lens defects are only of relevance to the most demanding
users.

Without this healthy criticism, we would not see new developments at the pace we are seeing
now.

This work is still of value to the less demanding photographer as it helps them avoid situations
where the lens does not perform as expected.




Types of lens defects
Even modern high end professional quality lenses exhibit lens defects. The 3 most common
types of lens defects seen now-a-days are:

      Focus shift
      Firmware issues
      Sample to sample inconsistencies

Focus shift is usually a design defect or a limitation of the particular lens design.

Firmware issues with lenses generally manifest as autofocus inconsistencies. The lens usually
works as intended in manual focus mode but exhibits Autofocus errors. The lens does not focus
precisely and accurately in AF mode.

Sample to sample inconsistency is due to lax quality control.




Some new lenses and reported defects
The table below shows some modern lenses and their reported defect.

                                 Reported
       Lens        Released                              Quotes from external reviewers
                                  defect
                                                "The problem is that it isn't consistent from shot-to-
Nikon AFS                     Autofocus not     shot and at different distances if you're more of a
                   2010
35mm f1.4G                    accurate          pixel-counter than a photographer." and
                                                "inconsistent and lousy results in repeated tripod-
                                             bound trials"
                                             "Focus shift is a legitimate concern, after all, this
Leica 35mm f1.4
                                             exactly what the new 35mm Lux ASPH with the
ASPH Mk 1       1994        Focus shift
                                             FLE addresses" and "The Version I 35 mm
Summilux-M
                                             Summilux ASPH was notorious for its focus shift"
Zeiss ZE/F                                   "It turns out that this lens has focus shift" and
                  2008      Focus shift
85mm f1.4                                    "exhibits focus shift at close working distances"
                                             "this is a lens that seems to have a fairly high
Canon EF 100-               Lens to lens
                  1998                       sample inconsistency" and "assuming you get a
400 f4-5.6L                 inconsistency
                                             good sample"



Note that all the lenses above are all exceptional lenses and probably the best you can buy. The
lens defects are only of relevance to the most demanding users.

It is possible that an individual photographer might not come across these defects. But the lens
performance might be less than expected for the more demanding user.




Most vulnerable lenses
The lenses which are most vulnerable to lens defects generally have these characteristics:

      Fast lenses with wide aperture
      Wide angle to medium telephoto lenses (less than 85mm)
      Complex zooms
      Focus at closer distances
      Dont have floating elements

So if you are buying a newly introduced fast lens less than 85mm in focal length, remember to
test its performance at close focusing distances.




Why so many new lens have defects
In pre-digital days, high end professional grade lenses did not have so many defects. The
incidence of defects among new lenses is increasing. This is because of:
      Demands of digital imaging – In the 1990s, most people would take pictures on film
       using compact camera and prints pics at 5×7 inch size. The requirements of consumer
       photography were very modest. Consumer and serious photographers today routinely
       view pictures at 100% maginification on high end monitors. They order digital prints
       several inches wide. It is the non-professional photographers who have become more
       demanding
      Convergence of technologies – In the 1990s, there was a clear distinction between lens,
       the image capture medium(film) and the box to hold it all together(camera). People
       would routinely buy the three components from different manufacturers and use them
       together. In the digital imaging, every component is optimized to work best with specific
       set of components only. Lenses have microchips, firmware and lens profiles which are
       read by the camera and are used to focus the lens and apply corrections to the final image.
      Complexity of modern lenses – there has been a shift from simpler prime lenses with
       modest aperture to complex zooms and very fast primes
      Most modern AF digital SLRs have screens which are optimized for bright image with
       slow f4-f5.6 lenses. The focusing screen of most SLRs is not good enough to show the
       correct focus with a f1.4 lens. So the small error when focusing manually or using AF is
       not obvious in the viewfinder but very obvious in the final print or on screen. If the
       photographer tries to focus a f1.4 lens with a consumer grade camera, he will most likely
       not be able to focus correctly. This would be reported as inability of the lens to focus
       correctly.




Test for lens focus shift
The standard test for focus shift is the ruler test. The lens is focussed on a point on the ruler.
 Three picture are taken wide open and stopped 1-2 stops. If the plane of sharpest focus shifts
from the point the lens is focussed, then the lens is said to exhibit focus shift.




Lack of focus shift with Nikon AFS 50mm f1.4G
The Nikon AFS 50mm f1.4G is one of the very few lenses with no reported lens defects.

In 2010, the Nikon 50mm f1.4 is the most modern 50mm lens in production and so there are very
high expectations of this lens. Nikon has done a very good job with this lens.

Three pictures below show the lens at 50mm f1.4, f2 and f2.8 on Nikon D3100. The lens is
focussed at mark '19' on the measuring tape and pictures are taken at f1.4, f2 and f2.8. If there
was any focus shift, the point of sharpest focus would have moved from the '19' mark on the
measuring tape. The lack of focus shift in such a sharp and fast lens is commendable.
 Move your mouse over the aperture below. Note the lack of focus shift as the aperture changes



                              f1.4 f2 f2.8
Lack of focus shift in Nikon AFS 14-24mm f2.8G
The Nikon AFS 14-24mm f2.8G is one of the very few lenses with no reported lens defects. Part
of the reason for the lack of focus shift is that the lens is usually used wide open at f2.8 – so
focus shift is less of an issue. At smaller aperture, there is usually sufficient depth of field to
mask any focus errors. However, even at very close distances, there is virtually no focus shift
thanks to the excellent lens design.

No wonder that this lens is an optical legend.

Three pictures below show the lens at 14mm f2.8, f4 and f5.6 on Nikon D3100. The lens is
focussed at mark '17' on the measuring tape and pictures are taken at f2.8, f4 and f5.6. If there
was any focus shift, the point of sharpest focus would have moved from the '17' mark on the
measuring tape. The lack of focus shift in such a complex and wide angle lens is commendable.




 Move your mouse over the aperture below. Note the lack of focus shift as the aperture changes



                              f2.8 f4 f5.6
Three pictures below show the Nikon 14-24mm lens at 24mm f2.8, f4 and f5.6 on Nikon D3100.
 The lens is focussed at mark '17' on the measuring tape and pictures are taken at f2.8, f4 and
f5.6. If there was any focus shift, the point of sharpest focus would have moved from the '17'
mark on the measuring tape. Again, the lack of focus shift is commendable.




 Move your mouse over the aperture below. Note the lack of focus shift as the aperture changes
                             f2.8 f4 f5.6
What can be done by photographers
      Read up on the lens before buying. Most popular lens review sites and magazines
       usually give a positive review of every lens they test as they need the advertising from the
       big manufacturers. Some of the smaller and less popular sites run by hobbyists report on
       lens performance under more demanding conditions.
      Avoid situations where the lens exhibits the defects – easier said than done since most
       defects manifest as focus shifts and AF errors
      Verify the issue exists and isolate the problem – is it the lens or the camera or incorrect
       calibration of camera-lens, or the software on your desktop to process the RAW files
      Use in camera calibration tools to calibrate lens – most professional cameras allow the
       photographer to calibrate the camera to compensate for AF errors
      Try different samples. Return the lens and ask the retailer to give you another lens
      Dont rely on manufacturers to do anything. Expect them to deny the issue. If it can be
       fixed with a firmware update, then they probably will fix it. But nothing more.
      Send lens for re-calibration along with the camera to the service center. The
       knowledgeable technician can calibrate the lens to the camera. It is possible that while the
       lens and camera are individually within tolerances, the camera-lens unit might be out of
       tolerance. Such a recalibration is almost always done at a cost to you but this might be the
       last resort. It is also not unknown for the lens to be returned with a big bill but no work
       done or no improvement
      Sell the lens and get an alternative




Summary
Most common lens defects being found now-a-days are focus shift, AF errors and sample to
sample inconsistency

Lens defects are being found in high end professional grade lenses by demanding users. These
demanding users provide valuable feedback which is of value to all photographers.

Most common lens defects being found now-a-days are focus shift, AF errors and sample to
sample inconsistency.

Some lenses like the Nikon 50mm f1.4G and 14-24mm are remarkably free of lens defects like
focus shifts, AF errors and sample to sample inconsistency.
If a photographer is not happy with their high end lens, then it is worth isolating the cause of the
defect. Then try and calibrate the lens yourself using in camera tools. You can also send the lens
for recalibration at the manufacturers service centre. At the last resort, you may have to search
for an alternative sample or use a different lens.




Thanks for reading. Your comments and feedback will be appreciated.

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Coatings
Distortion

Light Falloff

Vignetting

Flare and ghosts

Diaphragm blades

Turns of the AF Screw

Color Rendition

Spherical Aberrations

Chromatic Aberrations

Diffraction

back to top

Coatings

Coatings are treatments applied to the outer surfaces of lens elements to reduce
reflections. They are a layer (single coating) or layers (multicoating) that are a fraction of
a wavelength of light thick.

Without coating most glass reflects about 7% of the light! This is important since coating
eliminates ghosts, low contrast and other annoying flaws caused by light reflecting
inside the lens to places it shouldn't go. Reducing reflection also increases
transmittance. Increasing transmittance is critical to allowing zoom lenses to work. A
window only has one piece so losing 7% of the light isn't important. A zoom lens may
have a dozen or more pieces of glass ("elements"), so without coating so little light
would make it through that zoom lenses would not be practical.

Coating was invented and kept as a military secret by the Germans in World War two.
Thus no coatings were used before the 1940s, although some people noticed that old
lenses left sitting around acquired a tarnish that served a similar purpose. After the
1940's camera lenses started to be coated, and by the 1950's most were.

In the 1970's multicoating became popular, which works even better than a single layer
coating.

It is easy to see what kind of coating you have:
No coating: bright white reflections from the glass, just like a window, drinking glass or
most TV sets. (Better TVs and computer monitors are coated so you see less reflections
from room lighting.)

Single coating: dimmer blue or sometimes amber reflections when looking straight into
the glass.

Multicoating; much darker reflections of many colors, especially green or dark red. Hint:
Multicoated Schneider lenses are usually marked "MC"




This photo shows the three types of coatings. Note the reflections in each filter.

The filter on the left is an uncoated Tiffen filter. Note the bright white reflection.

The top filter is a single coated Nikon filter. Note the darker blue reflection.

The right filter is a multicoated Hoya HMC filter. Note the much darker green reflection.
       Uncoated lens typical of disposable cameras. Note the white reflection.




Single coated lens typical of simple cameras today and most lenses from 1950s-1970s.
                        Note the typical blue and amber colors.
          Modern multicoated lens. Note the typical green and magenta colors.

back to top

Distortion

Ideally straight lines would stay straight in a photograph. In reality they often curve a tiny
bit in or out when they run along the sides of an image.

If they curve out from the center of the image they are called "barrel" distortion, as these
lines would mimic the shape of a wooden barrel.

If they curve inwards, this is referred to as "pincushion" distortion, since the shape
mimics a pincushion or pillow as seen from above.

Most zooms have problems with this. Just find a straight line (like the horizon of the
ocean) and line it up along the top or bottom of your viewfinder. Zoom in and out,
keeping the line referenced along the edge of your finder, and oh-my-God, it will change
its shape very slightly as you zoom.

Most zooms have barrel distortion at their shorter end, and pincushion distortion at their
longer end. They usually have a point in the middle where they are pretty free from
distortion. It's good to know where this point is so you can use it when photographing
something that has straight lines that you want to remain straight.

Most view camera and rangefinder lenses have little to no distortion. You easily can
check the view camera lenses on your etched lines of your ground glass.

Most wide angle SLR lenses have some barrel distortion. Ignore the idiots who try to get
you to believe that that's just "the curve of the Earth." Since you can change the
direction of that curve by changing the position in your image it's obviously not a
constant feature of our planet.
Don't make a final determination of distortion based on what you see through your
viewfinder, since most viewfinders have some distortion. For instance, the Nikon F2 has
a distortion free finder, but most other Nikon cameras have pincushion distortion
deliberately designed into their finders to compensate for the barrel distortion typical in
the 50mm f/1.4 lens and almost all of Nikon's wide angle fixed-focal-length lenses. You
need to shoot film and look at it directly if you are looking for distortion. Don't try to
project your slides to measure this, either, since projector lenses can have distortion,
too. Some of the newer AF cameras like the F100 have more complex distortion
signatures in their viewfinders making any TTL guestimate of distortion difficult.

I measure distortion by photographing straight lines, be they a wall or the horizon, and
then laying a straight edge over my chrome on a light table. I then drop a big loupe over
all this, and if I can't see any variation from straight on the film, pronounce that as no
distortion. Of course instruments could measure levels so low I can't see them, but as
an artist if I can't see it I don't care.

Kodak carousel zoom lenses are popular and awful. You can see the distortion vary as
you zoom from one end to the other.

Distortion is a complex phenomenon. It cannot be specified with a single number,
although most reviews you read do just that. It varies with:

1.) Distance to subject (reproduction ratio)
2.) Distance away from center of image.
3.) Focal length setting on a zoom lens

To specify this clearly requires a series of graphs of distortion vs. angle or distance from
the center of the image. On each graph one needs several lines corresponding to
different reproduction ratios, and if you have a zoom you need different graphs for each
focal length.

Therefore when you see Mamiya quoting the distortion of their superb 43mm f/4.5 lens
for the Mamiya 7 as "0.04%" you need to know that what they aren't telling you is that
that's the figure measured at the edge of the frame, and that they forgot to let you know
that the distortion at other parts of the frame can be 0.2% or more. In practice none of
this is visible (I start to see distortion at about 1%), so don't worry too much.

Just know that if you want to see well specified distortion you can get data from
Schneider. Schneider's website has great distortion curves for their lenses, and they
can publish them because their lenses don't have enough to worry about.

Simple distortion can usually be fixed in Photoshop with the "spherize " and "pinch"
filters. Just increase the canvas size to about twice the size of your image and
experiment. More complex, higher order distortions require fancier filters to correct.

back to top
Light Falloff

Most lenses, due to mechanical vignetting, lose some illumination towards the sides of
the image at full aperture. This goes away when the lens is stopped down a stop or two.

A tradeoff is made in practical photographic lenses to save size and cost by not
designing each and every element to as large a diameter as would be required for the
entire full aperture to be visible from all angles as seen by the lens. If lenses were
designed this way they would be twice as big as they are today, all for an almost
invisible performance improvement at the full aperture which usually isn't used anyway.

You can check for this visually. Just look through the front of your lens at full aperture
from as far away as you can get. Look straight into it and you'll see a round aperture
through which you can see the focusing screen. Now rotate the camera a bit to look
through the lens to the sides of the focusing screen. Instead of seeing a round aperture,
you'll most often see a smaller hole cut off in part by the edges of the lens elements.
The smaller this aperture gets, the more falloff you get.

If you stop down your lens a stop or two and repeat the process you'll not see any part
of the smaller aperture cut off. In this case you don't have light falloff.

This effect is most visible with long lenses because you are more likely to have 1.) a
solid background and 2.) be using them at full aperture in good light.

Ideally the falloff is gradual and not too noticeable. On some lenses it starts abruptly
and becomes more visible as one approaches the edges. The Nikon 80-200 f/2.8 AF-S
zoom is pretty bad at this.

In wide angle lenses designed for view cameras and rangefinder cameras there usually
is light falloff at all apertures. This is because, unlike SLR retrofocus lenses, these
lenses are not designed to compensate for 1.) the effect of the longer distance the light
needs to travel from the rear nodal point of the lens to the far edges of the film, and 2.)
the fact that at the far edges of the film that the light is hitting the film at an angle other
than 90 degrees.

In these cases one may use a center filter that is darker in the center than at the edges
to compensate. Center filters cost about $300 and don't have much visible effect unless
you enjoy photographing evenly lit walls. The advantage to these lenses is that they do
not have any of the distortion or loss of sharpness in the corners that retrofocus lenses
may have. Their design gives you sharpness and freedom from distortion in trade for
some light falloff. You can correct falloff with a filter or in printing, but you can't correct
distortion or lack of sharpness.

Back to top

Vignetting
Vignetting is a darkening of the corners. Wide angle lenses on view cameras tend to do
this wile other lenses don't. It's not a bad thing. Artists like Ansel Adams often
deliberatley add this in printing to keep your eyes from wandering off the image.

Like everything else in photography, try it yourself on film and see if you are concerned
about a particular lens and filter combination.

In addition to film tests, another way is to take the film out of the camera, set the shutter
on BULB and look through each end of the lens. You ought not see the open aperture of
the lens cut off by the filter as you peer through it at large angles. Shooting some film is
still a better way.

Back to top

Flare and Ghosts

These are the various blobs and things you see in an image when the sun or other
brilliant source of light is in the image. All lenses handle this very differently.

Back to top

Diaphragm Blades

3: Poor. Seen only on some motion picture lenses and old electric eye cameras.
4: Poor. Seen only on point-and shoots, digital cameras and cameras like the Olympus
XA. See an example of the potentially poor bokeh in the top photo of the Nikon 100E
lens here.
5: So-so. Standard for Hasselblad, Mamiya and older Canon, and 1960s and 1970s
Copal shutters for view cameras.
6: So-so. Standard for 1950s and 1960s Nikon SLR; standard for most discount lenses,
standard for Contax SLR
7: Very Good. Standard on Nikon SLR since 1977, standard on Minolta. Standard on
modern Copal #0, #1, and #3 shutters for large-format lenses.
8: So-so. Seen on better discount lenses and some Canon.
9: Great. Standard on long telephoto Nikon, and recently on some Nikon normal zooms.
10 or more: Good to Great. Standard on 1950s and earlier non-SLR cameras. Also
standard on Leica rangefinder lenses through today.

Back to top

Turns of the AF Screw (Nikon)
Figure 1.) The slotted screw head is really the rotational coupling for the camera body's
                                       AF motor.

Most AF Nikkors, except the AF-I and AF-S lenses, focus with a motor in the camera
body. The camera body motor couples to the lens with what looks like slotted,
retractable slotted screwdriver that pokes out of the camera's lens flange into a slotted
recess in the lens mount. The camera's coupling retracts when you press the lens
release button, and stays retracted if you mount a manual focus lens.

This couples to the mechanics inside the AF lens. The lens focuses as this rotates. On
most AF lenses you can see this rotate as you turn the manual focus ring with the lens
off the camera.

One may estimate AF speed by observing how far this coupling needs to turn to move
the lens' focus ring a certain amount. It's sort of like the gearing on a bicycle: some
lenses move the focus ring a lot when this coupling rotates, other lenses require a lot
more turns to get the focus ring to turn the same amount.

I standardize this test by seeing how far the focus ring moves in one complete turn
(that's two half-turns) of this screw.

For the same focal length, the closer a lens focuses in one turn of this screw, the faster
it will tend to autofocus. For instance, the non-D 80-200/2.8 AF lenses only focused to
60 feet in one turn, the first 70-210mm AF lens only got to 40 feet with one turn; the
current 70-210D gets to 15 feet in that same turn.

Remember, you aren't turning this screw. The camera does. If you want to check for this
yourself, 1.) take the lens off the camera, 2.) focus the lens at infinity, 3.) look at the
screw while turning the manual focus ring and turn that ring until the screw has rotated a
full turn, 4.) look at the focus scale and see where it is.

On lenses that have a rotating AF/MF switch you instead will need to turn this screw
yourself: 1.) take the lens off the camera, 2.) put the lens in the manual mode and focus
it to infinity, 3.) take your Swiss Army knife and rotate the screw one full turn, 4.) Look at
the scale and see where it stops.

Back to top

Color Rendition

back to top

Spherical Aberration

Focus Shift

Longitudinal Spherical Aberration (Coma)

back to top

Chromatic Aberrations

This is when all colors don't come to focus in exactly the same place. It has nothing to
do with color rendition, purity or saturation, which are related to color transmission and
contrast. When you have chromatic aberration you lose sharpness and may see various
colors fringes on bright edges.

See a great article by Zeiss explaining the different ways different lens design tackle this
here. These explanations are about the different ways one corrects color fringing, which
is completely different from color rendition (transmission)

Chromatic aberration is classified by which colors are affected (primary or secondary)
and in what direction (axial or lateral). Thus you talk about a kind of chromatic
aberration by combining the two terms, for instance,"secondary lateral chromatic
aberration."

Lateral Chromatic Aberrations are colored fringes seen on sharp, contrasty edges in
the sides and corners of pictures. It's something you'll see if you enjoy photographing
white lawn furniture against dark backgrounds.

This happens because lens' magnifications can vary very slightly with different colors.
This means the image can be very slightly different sizes at different colors. When this
happens, the colors don't line up perfectly and you'll see colored fringes towards the
sides.

This is the aberration most often seen today.

It doesn't vary with aperture, although a lack of sharpness will hide it.
Axial or Longitudinal Chromatic Aberration means colors focus nearer or farther
away. The problem is they come to focus at different places along the lens axis. You
see this manifested as different colors coming in and out of focus at different places.
This was last seen in photography with fast telephoto lenses and was cured with Nikon's
ED glass.

Primary Chromatic Aberration is where the far ends of the spectrum, red and blue,
focus differently. Modern lenses have this completely corrected. This is when light is
spread out exactly as a prism does. Photographic lenses are corrected so the red and
blue ends of the spectrum focus in the same place. That's the easier part.

Secondary Chromatic Aberration is what's left when primary chromatic aberration is
corrected. Now the middle of the spectrum, green, is doing different things than the two
ends, red and blue. This has never been completely corrected, with the exception of
true apochromatic lenses. The lenses sold to photographers for less than a month's
salary aren't really apochromatic. Nikon's ED glass likewise was invented to minimize
this. Secondary chromatic aberration manifests itself as green/magenta artifacts.

The only chromatic aberration seen commonly today is secondary lateral chromatic
aberration. Lay people call this "purple fringing" for the green/magenta artifacts seen at
the corners of images on bright, contrasty things.

Canon has a great write up here.

back to top

Diffraction

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