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Just for fun I am putting this
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A zoom lens is a lens that can have
it's focal length continuously change
without losing it's focus. The name, zoom, comes from the strong visual
impression that makes it appear as if the viewer were "zooming" towards
an object or image as if in a fighter plane. |
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How does does this all work?
Here is an explanation. |
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When you hold a fixed focal length
lens in your hand, you know that
changing the distance between the lens and the object, changes the size
of the image along with the focus of the image. So,each time you move, youhave
to refocus and reframe the image . If two lenses were combined, by moving
them in coordination it is possible to change the distance betweenthe lens
and the object and still retain the focus of the object. |
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The zoom lenses used in broadcast
or professional video cameras are much more complex than the "two lens" theory,
but the basic principle remains the same; move one part of the lens system
to change the size of the image, and move another part to keep it in
focus. |
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At the wide-angle end of the zoom,
the variator is moved forward, creating a retrofocus type of lens structure.
At the telephoto end, the variator is moved back, so the lens structure resembles
the telephoto type. To keep the image in the same position as the two lens
groups move, the lens groups must move along precise curves determined by
the laws of geometric optics. The motion of the variator and compensator
is controlled by the barrel cam mechanism. The normal hand-held zoom lens
has a divergent variator and a divergent compensator. The track followed
by the compensator takes it forward, then back. |
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The inner barrel has a linear guide
groove (linear cam), and the outer barrel has a curved cam groove matching
the track of the lens motion (curved cam). When the outer, curved cam barrel
is turned, the variator and compensator move following the curved cam
grooves. |
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If the correct cam curve, designed
for a particular lens, is not followed
precisely, focus will be lost during zooming. Because of this need for
precision, the cams are machined to micron tolerances by computer
controlled machining tools. |
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A zoom lens must also correct optical
aberration so that the image will stay sharp when zoomed. The paths of the
light rays passing through the internal lens groups undergo complex changes
during zooming. To correct for aberration at all focal lengths, the aberrations
caused by each of the lens groups must be minimized. The aberrations that
the individual lens groups cannot correct on their own must be carefully
balanced by another lens group, so that one lens group corrects
another.
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The "text book" explanation is as
follows: |
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"The zone in front and back of the
image plane in which the defocus is less than the permissible circle of confusion
is called the depth of focus." |
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A simpler explanation is to tell you
that the depth of field is that area behind and in front of the subject that
remains in focus at any given f-stop or range of the zoom lens. |
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The larger F-numbers,(smaller
aperatures),give greater depth of field. In
addition to the F-numbers changing depth of field, you will find that depth
of field is greater at the wide angle end of the lens and smaller at the
telephoto end.
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Lens specifications contain a large
quantity of figures/specifications. To
interpret them and to tell whether the lens is right for a specific purpose,
requires some basic knowledge of optics.
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The first item to check in the
specifications is the image size. There is no
point fitting a 1/2" lens on a 2/3" camera. The image it forms is too small,
the image will cover the complete area of the image sensor. |
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The image formed by the lens is round,
not rectangular. The range of the
image is called the "image circle". |
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In a video camera, the CCD sensor occupies
a rectangular area, which
touches the "image circle". The size of the image sensor is the actual image
size. Current video cameras use two main sizes of image sensors, with one
minor size not commonly used. There are also two main series of zoom lenses,
one for each image size. The first letter of the lens designation indicates
the image size. |
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The ratio of the width of the screen
to the height of the screen is called the aspect ratio. In most standard
definition television it is usually 4:3. The wide screen standard definition
television and high definition aspect ratio is 16:9, which more closely resembles
the film aspect ratio.
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If parallel rays of light pass through
a convex lens, they will converge to a
single point on the optical axis. This point is called the focal point or
focal
plain of the lens. The focal length of a fixed focal length lens is indicated
by the distance from the center of the lens to the focal
point/plain. |
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A lens has two focal points, one on
the object side, called the primary focal point, and one on the image side,
called the secondary focal point. When the term "focal point', is used alone,
it means the secondary focal point. |
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The lenses used on video cameras are
compound lenses, consisting of
several individual lenses combined so as to correct "aberrations". However,
they function like a single lens located at an imaginary point called the
"principal point". |
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The focal length is the basic factor
used to calculate the image position and magnification of a lens. The focal
length of a video lens is important as a parameter describing the angle of
view of the lens. The focal length and "principal point" of a zoom lens are
changed by zooming, so as you zoom, you change the angle of view of the lens.
A short focal length gives a wide angle of view, and a long focal length
gives a narrow angle of view, which causes the image to be
magnified.
The focal length is the basic factor used to calculate
the image position and magnification of a lens. The focal length of a video
lens is important as a parameter describing the angle of view of the lens.
The focal length and "principal point" of a zoom lens are changed by zooming,
so as you zoom, you change the angle of view of the lens. A short focal length
gives a wide angle of view, and a long focal length gives a narrow angle
of view, which causes the image to be magnified.
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The zoom ratio is the ratio of the
focal length at the telephoto end of the zoom to the focal length at the
wide-angle end. The zoom ratio indicates how much the size of the image on
the monitor can be changed. If a zoom lens has a zoom ratio of lOx, the image
it gives at the telephoto end will be magnified exactly 10 times as much
as the image at the wide-angle end. You can use the model numbers on the
lens to determine it's zoom ratio. An A16X8.5 or J16X8.5 is essentially,
at the telephoto end, 16 times 8.5 millimeters. |
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The larger the zoom ratio is, the more
the size of the image can be changed. It is important to select an appropriate
zoom ratio. A large zoom ratio is desirable, but it also makes the lens bigger
and heavier. |
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An item of equal importance with the
focal length is the F-stop/number, which indicates the brightness of the
image formed by a lens. A smaller F-stop means a brighter image. The F-stop
is closely related to the depth of field. For a given focal length, the larger
the aperture of the lens is, the smaller its F-number is. |
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The iris ring of most lenses are marked
with a series of numbers with a ratio of 1:1.4, 1.7, 2, 2.8, 3.5, 4, 5.6,
8, 11, 16, 22. The brightness of the image is inversely proportion to the
square of the F-number. Each time the ring is turned one number up the F
scale, the brightness is decreased by half. As the iris ring is turned down
one number, the brightness is increased by twice. |
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As many people know, movie camera lenses
are rated by a T-number instead of an F-stop.The F-stop expresses the speed
of the lens on the assumption that lens transmits 1OO% of the incident light.
In reality, different lenses have different transmittance, so two lenses
with the same F-stop may actually have different speed. The T-number solves
this problem by taking both the diaphragm diameter and transmittance into
account. Two lenses with the same T number will always give the same
brightness.
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If you have zoomed with a zoom lens
open to full aperture, you may have
noted a drop in video level at the telephoto end. This is called the F drop
or "ramping". The "entrance pupil" of a zoom lens changes in diameter as
the focal length is changed. As you zoom toward the telephoto end, the entrance
pupil gradually enlarges. When the entrance pupil diameter is equal to the
diameter of the focusing lens group, it can not become any larger, so the
F-stop drops. That is the reason for the F drop. |
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To eliminate F drop completely, the
focusing lens group, (the elements in the front of the lens), has to be larger
than the entrance pupil at the telephoto end of the zoom. It has to be at
least equal to the focal length at the telephoto end divided by the F-number.
To reduce the size and weight of a zoom lens to make it easy to use for hand
held cameras, we have a trade off that makes it common to have a certain
amount of F drop or ramping at the telephoto end. For better composition
effect, however, in some studio zoom lenses the focusing group is made large
enough that no F drop occurs. F drop is a major determinant of the value
of zoom lenses used in live on-site sports broadcasts, which require a long
focal length and must frequently contend with twilight or inadequate artificial
illumination. |
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A television camera contains a beam
splitting prism, filters, and other glass blocks. Its lens has to be corrected
so that it will deliver optimum performance when these glass blocks are inserted.
Different television cameras have different beam-splitting prisms, so the
lens glass compensation has to be matched to the type of camera. Currently,
most camera manufacturers have standardized their 2/3" prism compensation
and design for their entire line of 2/3" cameras. This allows for camera
matching between the studio type and the hand held cameras and allows a user
to combine both types of cameras for a production. |
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When the prism mounted behind the lens
differs from the designed glass compensation, the main effects are increased
spherical aberration and longitudinal chromatic aberration. Longitudinal
chromatic aberration caused by different glass material the letters and numbers
at the end of the lens designation indicate the glass compensation type.
If the designation is 18x9B, for example, the letter B indicates that the
lens is glass compensated, and the number that follows indicates the type
of compensation.
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Lateral chromatic aberration occurs because
the
magnification of the image differs with the various color wavelengths. In
a video camera it causes what appears to be a registration error. Lateral
chromatic aberration also
has a secondary chromatic aberration, making it difficult to correct all
three of the red, blue and green wavelengths at the same time. |
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This form of aberration causes the different color
wavelengths to focus on different image planes. It corresponds to the lens
tracking error. In a zoom lens, the amount of the longitudinal chromatic
aberration varies as the lens is
zoomed. The aberration is largest at the telephoto end. If corrections for
longitudinal chromatic aberrations are not put in the lens, a color tracking
error will occur on the red and blue channels. This will cause color blurring,
even when the lens tracking adjustment is optimal. In a long focal-length,
high zoom ratio lens, chromatic aberration is the greatest problem, particularly
with the secondary spectrum, which is a high-order chromatic aberration.
The chromatic aberration of a lens is usually corrected at two wavelengths.
The secondary spectrum is the residual chromatic aberration left at the
wavelength midway between these two. |
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This has been some basic and advanced information regarding video lenses,
and why we see what appear to be problems with our lenses on occasion. From
what we have heard from the various manufacturers of cameras, the capability
of the current generation of cameras out performs the current generation
of lenses. And with CCD cameras, it's harder to make a lens that focuses
all three channels on one focal plane at a time, throughout the entire zoom
range. This is why you may see more abberations with your lens currently,
while a few years ago you wouldn't see a thing. Hopefully this little bit
of lens information is interesting to you, and makes a bit of sense. If you
have any questions, please feel free to write us. Keep shooing out
there!
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Updates on Lens Theory -
Camera Dave - 1999 |
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