Evidently most of the digital cameras with semiconductor
imaging sensors, so-called solid state cameras (in contrast to bulb
or film based ones) are single-chip camera systems. This is the
reason for the lower light sensitivity of color cameras. Because
compared with the naked black-and-white (better: monochrome)
version the surface of the according color sensor is coated with a
Red-Green-Blue (short: RGB) layer. And this swallows about 50%
intensity. Often even much more, because for correct color
separation and for sufficient modulation range (dynamic) the near
infrared (NIR, near IR) part of the spectrum must be suppressed by
an additional IR-cut filter (also IR-stop filter) up to about 1.1
micron wave-length. The reason is the transparency of the color
mask for light with longer wave-length and especially in this
spectral range the sensors are rather efficient. (Filterless
black-and-white cameras could easily be used as cheap substitutes
for night goggles.)
RGB stripe filter
Bayer mosaic filter
Due to the filter coating in stripe or mosaic pattern an
additional disadvantage arises: In a black-and-white sensor each
light sensitive cell contributes to the resolution directly. Each
single cell can cover a tint by its gray scale value between black
and white, thus each single cell represents one pixel (= picture
element). A color sensor of the same quantity of cells needs one
red, one (or two) green and one blue masked cell to represent a
tint. Thus each pixel consists of a RGB sub-pixel triplet of three
cells. Just arithmetical the resolution in relation to the quantity
of cells is reduced to one third only. These facts are held back
when talking about resolution and the quantity of cells is told to
be equal with the quantity of pixels. However, through complex
algorithms the loss in resolution caused by the color coating can
be enhanced even in real time. Nevertheless, hardly more than 2/3
resolution of the according black-and-white sensor is achieved.
The images on the left show parts of color filters. Each cell
represents a (sub) pixel. The twice as much green cells are used to
enhance the resolution due to human perception capabilities (middle
of the spectrum; sensitivity). The electronics for control and
address circuits are usually placed in the black, here only thin
drawn pixel outlines. Depending on the type of sensor such a blind
area then can reach and even surpass the area of the valid active
pixel. Mosaic filters offer a higher resolution than stripe
filters, the color interpolation, however, is more complex.
Therefore the pixel count is not the main issue. At best it is
as evident as the processor clock as sole criterion of PC
efficiency. What matters is also contrast, sensitivity, dynamics,
color separation and many more. A megapixel monster with slack
images just inflates memory demands with a lot of useless data in
Besides more pixels per square means smaller cell sizes paired
with reduced sensitivity and increased noise tendency.
In this context one can just check what camera and sensor
suppliers expect what number of defective pixels their customers
have to tolerate. Usually one does not see them, because they are
»mapped« by using the interpolated values of pixels in their
neighborhood. Concerning quality suppliers one has to search for
them, whereas others force sensors with dozens and dozens of faults
There are various color filter patterns and arrangements, not only
stripe and Bayer patterns.
Usually this coating is done by photo lithography. Also
one finds other color sets than RGB, e.g. magenta instead of red
and yellow instead of blue or two different shades of green. Also
additional uncoated pixels can be integrated in the matrix and so
In fact each electronic color camera gains its images by
interpolation, because there is no colored current. (Not even from
Yello ;-). Because the color algorithm knows the corresponding
filter color above each individual grayscale cell it is able to
calculate the color at this location. As a rule several neighboring
cells are used for the calculation.
Just to mention: Expensive three-chip cameras (also named 3 CCD)
as used in standard video applications, however, have a uniform
masked red, green and blue sensor for each color channel. The beam
split is done by a prism arrangement.
In the meantime for still cameras there are also sensors with
vertical instead of lateral structure, i.e. one uses the dependence
between penetration depth and wave-length of the light to separate
the colors, refer to Foveon (www.foveon.com).
If the application is not pressed for time one can use color
filters or color wheels similar to those which are common sight
with professional spotlights. Some studio still cameras shoot so
three successive photos each with a different color filter in front
of the sensor or lens.
Filter effects - moiré, aliasing and fix pattern noise
Effects of filters are responsible for color artifacts at
edges or lattices. If the image on the sensor is about the same
size as its filter grid (or its cell structure, resp.), moiré and
aliasing effects will possibly be the result. Good to visualize by
error colors in shots of venetian blinds or fan grills. Just make a
survey with your camera. Also the effect is well-known when
scanning rastered newspaper photos.
Here on principle the Nyquist-Shannon-Kotelnikov theorem is valid.
One can easily check it: When one shoots e.g. a white (or gray)
fence, the image of each slat will have to cover at least one row
of red, green and blue pixels otherwise the white tint cannot be
»mixed«. It is created when all color channels are illuminated with
the same intensity.
Due to the fix pixel arrangement even monocolor cameras can suffer
from similar errors. Due to different partial coverage/eclipse of
pixel, »beats«, ghost images, occur, simply structures are shown
where none of them really is - thus aliasing. (This effect often
occurs when scanning newspaper or magazine photos. Depending on
resolution and magnification suddenly harsh patterns appear due to
the rastered printing (~ grid) of the source material.)
Moiré will occurs, when two grids in the optical path are shifted
to each other, especially distorted. Then wide, format filling
streaks occur with their numbers and directions depending of the
angular position between them. This could even happen to human
eyes: Just lie two finely woven fly screens over each other and
slowly change rotation angle between them.
Because each pixel provides a different dark current (i.e. does
not necessarily show »0« when not exposed), below a certain
threshold level all pixels are compulsorily pulled down to black.
Otherwise one would find scattered colored pixels in a dark area.
That limits dynamic and leads to an effect called »drown in black«
- dark surfaces and shadow areas show no structures,
inhomogeneities are wiped-out. In the beginning a major part of
digital cameras suffer from this typical disease. Currently
especially (cheap) CMOS cameras and notably those with small pixel
sizes are affected.
One has even tried to use sensors with not regularly but randomly
arranged color pixels, which means without fix scheme for
positioning the single RGB color filters.
While for the color separation by penetration depth the angular
depending path length and stochastic effects cause errors. Even
three-chip cameras can show colored lines, speckles or wedges when
beam splitting prisms are not well adjusted or separated or under
intensive illumination. Rotating color wheels can cause unstable
brightness and wrong colors when transmission differs or
synchronicity is not perfect.
Spectral sensitivity (or spectral response)
Spectral sensitivity; comparison
human eye and CCD/CMOS sensors
Color separation of the CMOS sensor channels;
why an IR attenuation filter is necessary
Emitted color spectrum visible for human eyes (VIS) lies
best case between about 380 nanometers (violet) and 780 nanometers
(dark red). With the highest sensitivity in green-yellow at about
550 nanometers. CCD and CMOS sensors show a broader spectrum.
Especially they work in the near infrared region beyond 780
nanometers up to the so-called bandgap of the basic material
silicon at about 1 100 nanometers with a maximum sensitivity
between 600 and 900 nanometers. Their maximum sensitivity is more
pushed to the red compared with the human eye. (By the way: For
wave-lengths beyond the bandgap silicon turns to be
The figure on the upper right shows the spectral
sensitivity of the human eye in bright light mode in comparison
with that of uncoated monolithic silicon, the base material for CCD
and CMOS sensors in the visible and near infrared spectrum. (Each
curve is normalized on its maximum.)
The characteristics of CCD sensors are well given by the typical
bare silicon curve. CMOS sensors show a wider maximum pushed
expanded to shorter wave-length due to their flatter structures
supporting the reduced penetration depth of light with smaller
The figure below shows the spectral sensitivity of the three RGB
channels of a accordingly masked (= coated with red-green-blue
filter pattern) color CMOS sensor. One clearly perceives the
transparency of the polymer in the red and especially in the IR
region and caused by that the demand to attenuate or respectively
to block this part of the spectrum, in order to avoid overstress of
red and to reduce noise, wrong level and overdriving.
When using color sensors one can manipulate this within certain
margins by selecting transmission specifications of the color
filter pattern and the arithmetical weighting of the color
channels. Additional color conversion filters (reduction of
sensitivity in red; for color video cameras often made of Schott BG
39 or BG 40 glass) help here, too.
In the short wave-length region of the spectrum (UV, blue) the
sensors are comparatively insensitive. Besides here the glasses add
The spectral sensitivity of a camera, however, is not only limited
by the sensor or the film and the filters, but also by the optics,
especially lenses and the IR-cut filters mentioned above. Because
using the full spectral sensitivity can lead to dull/blurred
images. Due to focal length is a function of wave-length, the
single color images arise in different distances to the lens
(chromatic aberration). Achromats (lens systems in layer structure)
help to avoid this effect.
Often filter (and lenses) are additionally coated in order to
reduce reflection in the visible spectrum. The reflection loss at
each air/glass interface amounts to about 4% to 5%:
ΔR = (1 - n)² / (1 + n)²
with the refraction index n of the lens material in air
(nair ≡ 1).
Thus about 8% to 10% per transmission through a glass plane.
Sometimes one happens to discover that by the iridescent colored
surfaces when light incidents diagonally because ΔR is a weak
function of the angle of incidence.
Lenses of higher quality sometimes offer an IR switch for
operating exclusively with wave-lengths in the near infrared of
about 780 nanometers and more. Then the inscription on the distance
ring will fit again. (The IR image is displayed deeper inside the
camera, due to smaller refraction of longer wave-lengths.)
By the way: The transmission of standard lens glass badly
decreases with wave-lengths of 320 nanometers and less. In the long
wave section the drop lies far away from what a sensor made of
silicon perceives. Just to mention the coating layer of a sensor is
made of quartz glass (silicon di-oxide). It is covered by a thin
layer of silicon nitride.
If one wants to make shots in another region of the spectrum, one
has to use different sensor and even lens materials. For instance
IR cameras and especially thermographic cameras, which work in the
wave length region to 3 micrometer and above, partly use sensors
made of Indium-Gallium-Arsenide (InGaAs).
CCD versus CMOS sensors
Each of both present solid state sensor technologies CCD
(Charge Coupled Device) and CMOS (Complementary Metal Oxide
Semiconductor) are designed with silicon as raw material. Their
advantages and disadvantages depend on their basic functions. They
use the inner photoelectric effect - in both the pixel work like an
array of solar cells panels on the rooftop or in a meadow: one
pixel = one solar cell, more light = more charge or current,
CCD and CMOS circuit principles
The image on the left shows the basic principles of
a CCD cell in comparison with a CMOS cell.
The charge generated in a CCD cell by incident light is directly
read out from each cell. This charges are moved step by step
outside of the photoactive area. Outside they are analogue/digital
converted and amplified, resp.
In the CMOS cell the incident light generates a photo current
proportional to its intensity in each cell and decreases the
reverse resistance of each photo diode. These reverse currents
through the photodiodes (i.e. the generated charges) are
The cells of a CCD sensor operate as exposition meters
accumulating charge and are read out in certain intervals. The
»fast« CCD sensors exist in three types: FT ((Full) Frame
Transfer), ILT (Interline Transfer) and FIT (Frame Interline
Transfer). For still photography the full frame principle is
sufficient enough: The charges remain in the active area until
read-out shaded by a central mechanical shutter.
In FTs the complete frame is moved through the photo active cells
in a separate light shielded area and then it is processed. Due to
the different structures often it is possible to identify both
regions by one's naked eyes. ILTs show alternating photosensitive
and read-out lines. The charge of each photoelectric cell is
directly pushed to the according light shielded neighboring cell
and this line is then read-out. FITs are a combination of both
FTs reach a fill factor of almost 100%, but suffer of a second
exposure during the read-out phase (so-called smear). ILTs and FITs
show a reduced fill factor, but are less sensitive in read-out
The expression fill factor gives the ratio between optical
sensitive area and area occupied by addressing circuits of each
cell. Thus it allows to compare sensitivity under the precondition,
however, using the same basic technology.
CMOS sensors continuously measure the light induced photo
current. One uses the proportional relation between reverse current
or induced charges, resp., and exposure of a photo diode.
(Expressed in correct terms one measures the current to re-charge
the reverse capacity at the pn junction inside the photo diode. One
can imagine a capacity parallel-connected to the diode being
discharged by incidence of light. There are different possibilities
for appropriate circuits.)
The cell structure and wiring are rather similar to the layout of
a DRAM, and so experimental shots were already made with opened
A PPS (Passive Pixel Sensor) operates like a CCD ILT. By
illumination caused charges generated in a photo sensitive cell,
mainly a photo diode, are read out cell by cell directly and are
amplified and analogue/digital converted outside the photoactive
CMOS APS cell with global shutter
Common nowadays is the APS principle.(Active Pixel Sensor. The
acronym has nothing to do with the APS-C format.) Here photo
sensitive cells just work with the indirect principle of a CCD
sensor: The exposure of a photo diode controls the connection to
the operating voltage Vcc using a CMOS transistor (amplifier). Each
cell just has its integrated amplifier circuit.
Because illumination reduces the charge over the photocell one can
compare it with the negative film inside a camera.
The figure on the left shows the structure of a
CMOS APS cell. Description of one cycle: Over the reset switch a
pre-charge is brought in. After that the switch opens again and
over the photo diode D related to illumination charge floats off.
With that the control voltage at the amplifier T changes and with
it the voltage V. Using the optional switch t shutter (if
available) one can interrupt the discharge process, then the
capacitor C controls the amplifier all alone.
The pre-charge (realized by the reset switch) each time during
exposure reduced by the photo diode is the reason for CMOS APS
being insensitive to saturation effects unlike CCD sensors. More
than complete discharge is not possible.
The simplest cell is made of three transistors, namely the switch
for the initiation before starting the frame/exposure (reset), the
switch for read-out process (read) and the real amplifier (T). This
circuit offers a rolling shutter only. If one adds some kind of
sample and hold circuit (t shutter and capacitor C) in the
amplifier control one will get a global shutter. This is an
electronic shutter operating on the whole sensor at one given
With more transistors one receives additional opportunities for
controlling and improving the signal quality. Thus using a 5 or 6
transistor cell one can decouple recording and read-out, so already
start recording or exposure, resp. again during the read-out
process is still running.
provide good images by nature, otherwise one would sort out the
doubtful sensor and will not use it at all
can (as FT) reach a fill factor of almost 100%
are also sensible in near infrared (therefore especially many
color cameras are equipped with an IR-cut filter)
often need more than one supply voltage and show a high power
are several times more sensitive to light - in positive as in
negative manner. One could shoot with less illumination, but the
sensors are sensitive to overexposure. Thus very bright regions
show flare-shaped extinction (blooming) in the neighborhood which
can cover the complete frame; high irradiation during read-out
phase can lead to some kind of double exposure (smear).
are only well done by specialized semiconductor manufacturers.
With increasing number of pixels there is a dramatic yield
decrease, because due to the basic function, especially in FT
sensors, the optical active regions are used as wire during the
read out phase. A defect cell can paralyze a complete line or
column. Then the sensor is not to use.
6 inch wafer with 45 CMOS sensors on blue tape
are more noisy because of their not homogenous structure and
show similar problems as flat panel displays. Some error pixels may
occur which will be computed away then. One has to put emphasis on
the image processing (among others masking defective pixels
so-called mapping). Each pixel is adjusted individually
may reach a fill factor of about 50% and therefore in general
they need usually more illumination
are as APS almost completely insensitive to overexposure. PPS,
however, are a bit more critical
show about the same spectral sensitivity (i.e. color perception)
as the human eye
often need just one supply voltage and show a moderate power
come from the much more present technology. Assuming a
sufficient quantity they are significant cheaper. Besides read out
is made with additional lines, thus a defective pixel can be
interpolated by its neighbor pixels and does not ruin the
offer the possibility to integrate additional circuits or signal
pre-processing on the same chip in order to shrink the camera and
to increase flexibility; the more or less random access to the
single cells makes possible e.g. the preselection of a window, so
called sub sampling, windowing, ROI (region of interest) or AOI
(area of interest). Also image processing for trigger events (in
frame trigger) or a second exposure run in each frame are
comparatively easy to realize.
Color filter in diagonal arrangement (magn. 1 000 x)
State of the art
Evidently still many of the photo and surveillance
cameras use CCD sensors while it is not possible to imagine
up-to date photo and video cameras without CMOS sensors. In mobile
phones with camera function CMOS sensors are used almost
exclusively. The trend moves to CMOS technology, to the
In high-end cameras yet one sometimes tends towards low
integration to optimize each part like analogue, digital
electronics, control, power supply, ... individually. In standard
use hardly one technique shows an advantage. In noise critical
special applications, however, the CCD sensors lie in front.
Cooled, if necessary. For shots against illumination sources and
possible overexposure effects (shiny surfaces, ...) CMOS sensors
may be the better choice.
By the way: The often made assertion CCD sensors are in principle
slower than CMOS sensors is not true so far, because there are CCD
high-speed camera sensors available as well, refer to the very
upper photo on the right.
Images on the upper right: Silicon die without housing, about
20 mm x 15 mm x 0.5 mm in 0.5 micron technology. The
greenish area is the mosaic filter. In the framing dark border one
recognizes 137 small squares, so-called bond lands or pads, for the
later electrical contacting with bond wires.
This die is directly, thus without housing, glued on the printed
circuit board and is bonded then chip on board (COB). After that
for protection reasons a cover with glass bottom is put over and is
glued to the board. (Of course, traditionally housed chips are
available as well.)
Image on the lower right: 1 000 times detail magnification
out of the filter matrix of the CMOS sensor. The light sensitive
area of the photo cell, marked yellow in the image, shows with 11
microns half of the cell size of 22 microns. The chessboard pattern
tilt by 45° built with »honeycomb« cells like Fuji and Sony
successfully use it in their »Super-CCD« and »ClearVid« arrangement
resp. allows a higher resolution especially of vertical and
horizontal structures compared with the standard Bayer pattern.
Also according high-speed cameras this change was obvious.
Whereas typical systems of the 1990s still used modified CCD sensors
in general, systems since the end of the 1990s mostly operated with
CMOS sensors. Nevertheless CCD sensor systems exist further
Well known high-speed camera sensor sources are Dalsa/Teledyne
(Canada), EG&G Reticon (USA), Fillfactory/Cypress (Belgium),
Photobit/Micron/Aptina (USA) or CSEM (Switzerland).
Everything counts in large amounts. Because of a fill
factor, i.e. the ratio photoactive area to control electronics
area, smaller than one area and therefore sensitivity is
The structure of CMOS sensors makes it worth to try microlenses in
order to compensate this lost at least partly. The lenses should
transport the light falling on the »blind« sticks to the light
sensitive parts of each cell. Especially one like to use them for
very small cell structures and fill factors (10 micrometers per
square cell size and 30% fill factor and less). The lenses are
directly deposit on the sensor surface by photo lithography. Due to
the small size of the lenses, diameter a few hundredths millimeters
each, and the large quantity - one lens for each cell (= pixel) -
it is a certain endeavor and the optical qualities and the
uniformity are not quite improved. This causes an underproportional
increase of sensitivity. The price is higher expense and possibly
reduced image quality due to optical faults and other parasitic
Back-side illuminated CMOS (BSI)
Eyes and especially CMOS sensors show a quite similar
layer structure. In both the light must pass a shading supply layer
- blood vessels or electric circuits, resp. - before received in
the photoactive layer. Fill factor and image quality are reduced.
Moreover with further reduced structures in semiconductor design a
trade-off between what is good for electronics and what is
necessary for optics. Shrinking and higher clock rates and
reduction of power consumption as well cause noisy small
photocells. For instance standard design rules (structure size,
width of conductor paths, gate width) of present DRAM and CPU
techniques do not match CMOS sensor demands by a factor 50 or more.
(Present structure width in 2013 is 28 nm!) Besides the
transistors in logic circuits are trimmed for speed and digitally
switch with (super-) saturation (bang-bang switch), while
photoelectric elements should be read out analogue and a saturation
should be preferably avoided.)
Therefore the idea to grind the sensor bottom and then to put the
sensor die upside down. The supply electronics are at the bottom,
the middle is filled by photocells and on the upper side color
filter pattern and perhaps microlenses are placed.
The gap between CMOS standard designs and sensor demands can be
reduced by that, what makes the sensors cheaper especially by
integrating further electronics and conceals deficiencies like e.g.
the meanwhile not deep enough p/n junction layer position for the
penetration depth of light beams.
Not really a quite simple semiconductor process (among other
massive grinding of the wafer is necessary), but similar technique
was used with special CCD sensors, too.
Fore more info refer to:
Often, however, the sensors are produced in technology families
with bigger structure widths than that of present DRAMs or
processors. This is especially valid concerning high quality
Existing lenses throw their images on a flat area, the
image plane. The mathematical correct projection, however, takes
place on a slightly curved area the so called Petzval Panel,
especially with simple lenses (astigmatism and image field
curvature). In order to improve among others depth of field
presently circular arc shaped sensors are in the market.
Maybe that the sensor is really curved or a location-dependent
path extending coating or according optics are added.
Lens mount, flange-back (back focus), focal length and crop
Elementary criterion for selecting lenses is to check
which mount is given by the camera body. The mount determines the
flange-back or back focus distance. It is the distance measured
between the last cylindrical landing of the lens housing before the
thread and the film or sensor plane. Thus the distance between
optics and image plane is mount specific. With a lens out of a
different mount family possibly the focal length inscription of the
lens is not correct, as far as one can adjust its focus and fix it
at all. The flange-back e.g. with C-mount is 17.562 mm and
with CS-mount it is 12.5 mm. When one happens to get a more or
less sharp image of the iris aperture and nothing else, one has
screwed a CS-mount lens on a C-mount camera. (With a 5 mm
spacer a C-mount lens fits onto a CS-mount camera.)
Image circle, format and crop factor
Within a mount family there are not such problems. All adequate
lenses show the same flange-back, thus each image is displayed at
the same location. The only difference is the varied size of the
circular images the lenses offer at the image plane. Given is the
format of the sensor that is illuminated sufficient enough and
without too strong optical distortions. If the sensor completely
lies inside this image circle, everything will be all right. If it
is larger, the shot images will be considerable sharp, but their
corners tend to be underexposed (keyhole effect, vignetting). Thus
especially one can usually select without problems C-mount lenses
of larger formats for a C-mount camera with a smaller one, e.g. an
1 inch lens on a 2/3 inch camera. In the other way, however, with
restrictions only, refer to sensor C in the figure on the left.
The figure on the left shows the crop factor, also
named focal length extension. This is in fact a pseudo effect,
because focal length is an invariable attribute of each lens. The
different image impression, especially the angle of view (also
field of view), is only caused by showing the image of a smaller
sensor (B) with a greater magnification on a screen (or a print).
Its pixels lie closer to each other than those of a bigger one (A)
and are displayed somehow stretched, nevertheless without negative
effects on quality. Adequate resolution (pixel count) assumed.
At least the sensor must lie inside the image circle of the lens,
given as »format«, otherwise vignetting takes place (C).
If one compares two format filling images of an object, e.g. one
of a C-mount camera with a 2/3 inch sensor and one with an 1 inch
sensor, usually one will not see a difference. Displayed on the
monitor screen they are of the same size.
But due to the fact that they come from areas of different sensor
sizes, the magnification must be smaller on the smaller sensor
(here B), in order to make the image fit to it. Therefore the same
»XY Millimeter« lens in front of a larger sensor (here A) will work
at a camera with smaller sensor like one with a larger focal length
and vice versa. If, however, one compares the shots based on the
angles of view (= what fits onto the respective sensor using the
same lens) only. So far focal length and image format belong
together and give the imaging scale. The image circle, however, is
a lens parameter only. And the focal length, too.
Perhaps one could introduce the term »effective focal length«, but
related to what? Often in common usage this dummy effect of focal
length extension, when screwing the same lens on a camera with
smaller sensor, is called (digital) crop factor:
crop factor = 43.3 mm / sensor
The value (about 1.5 ... 2 comparing typical digital SLR cameras
with miniature 35 mm; format cameras of 24 mm x
36 mm and thus 43.3 mm diagonal; therefore sometimes the
expression »XY mm 35 mm format equivalent«) states the
apparent increase of the focal length finally due to the decreased
sensor diagonal. Thus the expression »crop« meets the facts better.
Much of the image just misses the smaller sensor. One will see the
center only and will interpret this as tele/zoom effect, when the
monitor is displaying the sensor full screen, refer to the figure
Hooked on high quality demands one has to take into consideration,
however, that the area with optimal corrected lens errors is
smaller than the maximum illuminated one and that smaller formats
claim lenses of better qualities because of the higher pixel
density. Besides one needn't spoil the image quality of a good
camera by using just a better bottom of a bottle instead of a lens.
Then the image remains sharp to the corners.
In professional still imaging widespread use of the according
possibilities is made.
If the focal length of a lens is of about the same value as the
diameter of the format, one will often call it the »normal lens«
for this format. Concerning miniature or 35 mm cameras this
would be about 50 mm ±5 mm, refer to next chapter
and [SloMo Tips]. Then
the (sharp) angle of view equals that of humans roundabout.
Elementary feature, too: Usually the format or film gate,
i.e. the size of the optical active area on the CCD or CMOS sensor,
is given for CCTV cameras in inches. The somehow arbitrary
dimensions are from the Vidicon valve era. The data derive from the
size of their (socket!) dimensions. As base for the inch class of a
sensor the 1 inch sensor with a diagonal of about 16 mm
Inch format ≈ sensor diagonal [mm] /
or when using inch measurement:
inch format ≈ sensor diagonal [inch] / 1.5875
although in real one inch measures 25.4 mm.
(Legend: fps = frames per second of some selected high-speed
camera matrix sensors at full resolution I have worked with.)
autonomous CCD camera for medical »in corpore« use (to swallow
like a pill)
CMOS cameras for special applications
CMOS cameras for Pads, Smart and Mobile Phones, ...
35 mm (photo film) cameras and »full format« digital
Entrance for light: The aperture or f-stop reduces the
incident light intensity falling on the sensor or film. The f-stop
number is defined as k = focal length / effective opening diameter.
On the f-stop adjustment ring one finds the f-stops inscription as
multiples of the square root of 2 (1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11
- 16 - 22 - 32 - 64). With increasing f-stops the intensity let
through is reduced in a way that one receives half of the intensity
from step to step.
The reciprocal of the smallest f-stop is often called
»transmission/light strength«. But if one wants two lenses offering
the same brightness, one will have to take two with identical
t-stops, defined as t = 10 × f-stop / √ transmission [in %],
as it is usually provided by lenses for movie cameras.
When using a sensor with the same size a smaller aperture diameter
(= high f-stop number) enlarges the depth of view, refer to [SloMo Tips].
Zoom lenses, digital lenses and resolution
Expression zoom stands for lenses, which permit to tear
closer an object by using a stepless focal length increase within a
certain range of given focal length. Usually they are large, heavy,
expensive and of poor light sensitivity. One should prefer to use
exchangeable lenses of different but fixed focal length. There are
available real zoom lenses where the image stays sharp during
zooming and vario lenses where one can change the focal length,
too, but has to adjust focus as well.
»Digital zoom« or »electronic zoom« stands for an enlargement of a
part of an image by means of a (simple) imaging program. High zoom
levels cause pixeled images. Normally electronic zoom is not a
worthy substitute of an appropriate zoom lens, even if marketing
likes to claim it so.
By the way, similar case: »5x zoom« does not say anything about
the magnification of a lens, but merely that its smallest
adjustable focal length is five times smaller than its
In the meantime »digital« lenses or sometimes also called
telecentric lenses are offered. They are designed so that the beams
at the image side hit the sensor in an angle as perpendicular as
possible. This is done to accommodate the shaft-like structure,
which especially has a bad influence on small sensors with high
pixel density causing faults like diffraction, shadows, color
errors, flattening, ... (Just the small, cheap sensors need complex
and expensive lenses.)
To call such lenses telecentric, however, is wrong. This type of
special lenses images an object in the same size disregarding its
distance. These optics are especially used in image processing for
measurement applications. For instance when one views in a tube it
does not become narrower with increasing distance, but it looks
like a ring washer and it is no problem to measure the inner
diameter and the thickness of the wall. Used with an usual
landscape scenery strange effect may occur, e.g. roads running to
the horizon do not get smaller.
When talking about resolution it is highly recommended to get an
idea of the MTF modulation transfer function, find an excellent
explanation e.g. at Optics for Digital Photography of
Glass delay (= z-axis image shift)
Exotic in most cases: When one screws a thin filter in
front of the lens, this will rarely have any effects on the imaging
position of the optical system. But this will drastically change,
if one adds a protection glass, a filter or a LC shutter behind the
lens. These plane-parallel plates cause a kind of glass delay
Δd = (1 - 1/n) × d
with the refraction index n and the thickness d of the
plate (nair ≡ 1).
With a glass plate the image would appear about 1/3 of its
thickness behind the sensor, due to the increase of the effective
flange-back. A mechanical solution must be designed in order to
adjust this certain additional distance between lens and camera
body, exact the sensor.
Therefore the threaded pins or special mechanics, resp. in the
lens adapters e.g. for cameras prepared for optionally mounting an
internal LC shutter or an additional internal (IR-cut) filter or
which are designed for IR shots.