201
UNDERSTANDING TV CAMERA RESOLUTION
Some manufacturers advertise that their camcorders have
I have heard these questions over and over, and I've seen
the answers in print --- and often the answers are wrong. It is
always a tough question deciding how much money should be spent
on camera sharpness if the resulting image will be distributed on
fuzzy old VHS.
There is a fairly straightforward direct answer to this
question, but first I'll string you along with a science lesson.
Resolution and pixels explained
First, resolution. Resolution means picture sharpness and
is measured in lines of horizontal resolution. The TV picture is
always made up of 525 horizontal scanning lines, as if it were
painted on a Venetian blind with 525 slats. For this reason, the
vertical resolution, the sharpness going from top to the bottom
of the picture, always remains the same.
If you looked through a window with a giant Venetian blind
and could observe a distant ladder and count 525 rungs on that
ladder, than you could say you had a vertical resolution of 525
lines. If you couldn't count the rungs, because they were fuzzy
or blocked by the slats of the Venetian blind, you would have
less than 525 lines of vertical resolution. You could have
someone bring the ladder closer and eventually you could count
all the rungs.
I have oversimplified the above explanation, and will now
come clean. Although American TV sets make their picture using
525 scan lines, 35 of those lines are black, comprising the sync
pulse, that black bar you see when you misadjust the vertical
hold on your TV set. Only about 483 scan lines are actually seen
on the screen.
It would seem that 483 scan lines would give you a vertical
resolution of 483 discernable lines (483 rungs on the
ladder). This is not really the case. If one scan line displayed one rung,
the next scan line would need to show the space between the rungs,
and the following line would show the next rung in order for the
rungs on the ladder not to merge together. Put another way, if
each scan line saw a rung, then the ladder would look like it was
made of solid rungs with no spaces. Thus, an image that goes
"rung-space-rung-space" is defined as 4 lines of vertical
resolution and it took four scan lines to do it. Thus, 483 scan
lines can show only 241 actual rungs on the ladder, but still the
TV industry still calls the vertical resolution 483 lines.
Again, I have oversimplified. The vertical resolution
available from 483 scan lines calculates to .7 x 483 = 338 lines of
resolution. Why the .7? Imagine for a moment that you looked
through your Venetian blind at the ladder and could see all the
rungs inbetween the slats. Now if you moved your head up just a
little bit, all of the rungs would be hidden behind the slats and
you would see only the spaces between the rungs, erroneously
coming to the conclusion that the ladder had no rungs. Since the
definition of resolution insisted that you be able to count the
rungs, and under certain circumstances you cannot count them, the
formula doesn't work. Physicists and mathematicians have come up
with the Kell factor of .7 which says that no matter how you look
through the window, on the average 70% of the rungs will be
visible all of the time.
Reviewing the above, we find that a TV image made of 525
scan lines has 483 active scan lines capable of revealing 338
vertical lines of resolution. If you made an image sensor chip
for such a camera, the sensor would need 483 pixels arranged
vertically to accommodate 483 scan lines. You'll see why this is
important in a minute.
Horizontal lines of resolution, measure how sharp a picture
is from left to right (in the horizontal direction). Imagine aiming your
camera at a picket fence. A sharp camera with 400 lines of resolution will
allow you to see 400 pickets. A fuzzier camera would see only mush
until you zoomed in and perhaps counted 300 pickets across the
screen. Note that although the pickets are vertical lines on the
screen, you are counting those lines across the screen,
horizontally, thus the term horizontal lines of resolution.
Although vertical lines of resolution always stay the same
because of the way the pictures are made, horizontal lines of
resolution vary with the quality of the equipment. The more
lines of resolution, the sharper the picture.
Just as before, I oversimplified the explanation and now
have to come clean. In order for you to see 300 pickets in a
fence, you must also see the 300 spaces between the pickets,
otherwise the pickets would all merge into one giant picket mush.
So, as you look at picket-space-picket-space, you are counting
four horizontal lines of resolution.
A camera image sensing chip (monochrome) with 400 pixels
across could register one white picket on one pixel and the black
space between the pickets on the second pixel, show the next
picket on the next pixel, and the next space on the following
pixel and so on. Thus, a CCD chip with 400 pixels across could
see 200 pickets and 200 spaces for 400 lines of resolution.
Well, almost. Remember the Kell factor. The above formula only
works when the pickets line up exactly with the pixels. We have
to multiply our theoretical number of 400 by .7 to get a more
realistic number that works most of the time. Thus, a row of 400
pixels gives us only 400 x .7 = 280 lines of horizontal
resolution.
Well, almost. TV people measure horizontal lines of
resolution in a funny way. They measure it per picture height.
The above calculation would be correct if TV pictures were
square. Since TV pictures are 1.33 times wider than they are
tall, we cannot count all those 280 lines. We are allowed to count only
3/4 of them, so multiplying 280 x .75 = 210 lines of horizontal
resolution. That's what is available from 400 horizontal pixels.
All of this mathematics is building up to something and it
is just around the corner. Hang on for a moment and we will
apply this knowledge to a real world situation.
A camera manufacturer advertisers an image sensor with
410,000 pixels. How sharp will the picture be? Let's make the
calculation. Of the 410,000 pixels, only about 380,000 (92%) fall within
the borders of the video image; the rest are off the edge of your
TV screen and don't show. The matrix of 380,000 active pixels
get divided into 483 active scan lines (in the NTSC television
system). Put another way, the pixels form a box that has 483
vertical pixels along the left edge. This leaves how many pixels
per horizontal line? 380,000 divided by 483 = 787 pixels per
line. Multiply that by .7 (the Kell factor) and we get 551
lines. Multiply that by .75 (because TV people don't count all
the lines, just count the lines that would fit in a square box)
and we get 413. Thus a 410,000 pixel image sensor yields 413
lines of resolution.
If you ever need to make a quick mental calculation, simply
take the number of pixels, divide by 1000, and call it lines of
horizontal resolution. Thus a chip with 300,000 pixels would
give a horizontal resolution of about 300 lines.
VCR AND CAMERA RESOLUTIONS COMPARED
Some VCRs can record sharper pictures than others. VHS,
8mm, and 3/4-inch U-Matic VCRs can record about 240 lines of
resolution. These, incidentally, are black-and-white lines or
luminance resolution. Color sharpness is much lower but is not
very noticeable to the eye. Since luminance resolution is
responsible for most of what we preceive as a sharp picture, we
will talk about luminance resolution. SVHS and Hi8 VCRs can
record and play back 400 lines of luminance resolution. This
higher resolution is only seen if you are using the Y/C
connectors. If you use the composite outputs of these machines,
the resolution is generally reduced to 330 lines. Broadcast
television also uses 330 lines as its maximum picture resolution.
Professional video machines and DV (Digital Video) recorders now
becoming popular with prosumers can record and play back higher
resolutions than this, (DV is often 500 lines) but the picture
sharpness is reduced to 330 lines when it is broadcast or
cablecast to your home.
Professional TV cameras can yield 500 to 700 lines of horizontal
resolution depending on their cost. Since the video recorders
can only register 240 to 500 of these lines, some of this
sharpness gets wasted. Wouldn't it make good sense to use a
camera with only 240 lines of resolution if you are going to
distribute your tapes on VHS capable of only 240 lines of
resolution? Logic would say yes, but the right answer is no. The
sharper your camera's image, the better the tape image will look.
Of course there is a law of diminishing returns; a VHS video tape
recorded from a 400 lines camera will look much better than an
image recorded from a 300 line camera. A 500 line camera will
yield a little better image still. A 700 line camera would yield
a slightly better image yet. Although the 700 line camera yields
a better than a 500 line camera, the small improvement might not
be worth the expense. Professionals who strive for the absolute
best they possibly can get, will use a 700 line camera. If
working on a budget, you may wish to buy the best you can afford,
but not go all the way.
Why does a 700 line camera yield a better picture than a
240 line camera if 240 lines is all the VHS VCR will reproduce?
The answer is in depth of modulation, a technical term that I'll
describe first in a general way, then in a technical way.
Imagine a TV camera looking at a white dot on a black background.
A 300 line camera will make a fuzzy dot; a 700 line camera will
make a stencil sharp dot. When the dot gets recorded, the VCR
makes the dot fuzzy, depending upon the format and quality of the
VCR. In the case of the 300 line dot, the dot was already fuzzy.
The VCR made it worse. When a VCR
recorded a 700 line dot, it was sharp to start with and
even though the VCR made it fuzzy, it had only one layer of
fuzziness. When played back, the 300 line dot had two layers of
fuzziness.
Thinking of it another way, try photocopying a dollar bill.
Then try photocopying a photocopy of a dollar bill. The
first generation is fairly sharp, but the second generation,
made from a fuzzy copy, is not very sharp at all.
The lesson, in short, is the sharper the original image, the sharper
the copy will be. The same is true in TV.
Now for the technical explanation. Picture resolution is
technically measured electronically rather than "by eye". A test
pattern with diverging black lines on it is placed before the
camera. The image is viewed on an oscilloscope called a waveform
monitor. By adjusting the controls on the waveform monitor, you
can view a thin slice of the TV picture. You can view the slice
of the picture that is showing you the part of the test pattern
with 300 lines of resolution or you can display the slice that
shows the part of the pattern with 400 lines of resolution. If
your camera can resolve 300 lines of resolution, then your
waveform monitor will show a graph of mountains and valleys. The
valleys will represent the black lines of your test pattern and
the mountains will represent the white spaces between the lines.
Thus the black and white lines of the test pattern become a wavy
graph on the scope. Next, the scope is adjusted to view the 400
line resolution part of the test pattern. The 300 line
camera mushes the black lines in with the white spaces,
displaying gray on the TV screen. On the waveform monitor, the graph
looks different. Instead of deep valleys representing black
lines, the monitor shows little dips representing dark gray
lines. Instead of mountain peaks representing white spaces, the
graph shows tiny hills representing light gray areas.
Technicians measure the height of the mountains compared
with the valleys to determine how many lines of resolution were
discernible. If the mountain peaks reach 100 percent on the
waveform scale and the valleys reach nearly 0 percent on the
scale, then technicians certify that the camera could "see" that
part of the test chart, which was maybe 300 lines of resolution.
If they move their dials to examine the 400 line part of the test
chart and find that the mountain peaks are at 60 percent and the
valleys are at 40 percent, they say that there is not enough
depth of modulation, to display that part of the picture
correctly. Thus the camera does not have 400 lines of
resolution.
By adjusting their controls, they move their "slicer" up
the screen to some point on the test pattern where they can see a
50% depth of modulation. Perhaps the mountain peaks reach 75% on
the scale and the valleys reach 25%, making a difference of 50%.
This is the point that represents the maximum horizontal
resolution one can say they got from the camera image.
Now back to comparing cameras and VCRs. If you feed a high
resolution camera image into a common VCR, you will get a better
depth of modulation than you would if you fed a cheap fuzzy
camera image into the VCR. The fuzzy image may have started at
60% depth of modulation. If the VCR damaged this just a little
bit, it would easily fall below the 50% depth of modulation
minimum. A good camera, on the other hand, started with 100%
depth of modulation leaving the VCR with lots of room for damage
before the image broke down to the 50% depth of modulation
minimum.
In short, sharper camera images give VCRs more room to work
with. Marginal cameras damage the image before it gets to the
VCR that adds another round of damage.
OTHER FEATURES ON GOOD CAMERAS
Three chip cameras can use a technique called offset which
allows them to record a higher resolution than normally possible.
Although the number of pixels on each chip may allow 500 lines of
resolution, the combination of the three chips together can yield
700 lines of resolution. Thus you get extra sharpness without
paying extra for super high quality chips.
Better cameras give more than sharper pictures. When you
buy a more expensive car, you get more than just a smoother ride.
Higher quality cameras have better signal-to-noise ratios.
This means that the images are less grainy. An image that is
sharp but grainy is no fun to look at. A smooth silky image
(strong signal with very little noise) not only looks sharper but
records better. Just as the evils of fuzziness add up in the
camera/recording process, the evils of noise add up each stage of
the way. Each machine in the video chain, the camera, the VCR,
editing equipment, video processors if used, and even the TV set
add some element of noise to the image. Reducing the noise from
the camera will make the final picture look cleaner.
Better cameras have better image sensors. The better chips
will have fewer bad pixels (called blemishes) and will display
less "fixed pattern noise" (a stationary graininess that results
from some pixels being more sensitive than others). Industrial
and professional camera chips use technologies that are less
sensitive to smear, the vertical white streaks you see through
bright spots on the image. The better TV cameras, for instance,
can view the headlights of a car at night or the sparkle of a
welding torch with ease.
The better TV cameras handle color better. Most split the
lens image into separate colored images. Each image is sent to a
separate chip, one for each color. The process avoids the color
stripe filter used in single chip color cameras, and results in
sharper, purer colors.
The lenses on industrial and professional cameras are much
better than their consumer counterparts. Chroma aberration is a
cheap lens phenomenon where different colors focus differently,
resulting in slight colored ridges around objects and fuzziness
to certain colors. Higher quality lenses contain extra elements
to combat this aberration.
Inexpensive zoom lenses tend to distort the image when
zoomed in or zoomed out all the way. Barrel distortion makes
rectangles look like wooden barrels while pincushion distortion
does the opposite. Straight lines and rectangles should look
straight and orthogonal under all conditions, whether zoomed or
not, whether they are in the center or near the edge of your
image. The better lenses counteract against these evils.
The better lenses have greater speed and transparency.
This means they let more light into the camera allowing you to
shoot in darker environments. By properly coating the lenses,
another phenomenon called flare can be reduced. Flare results
when light enters the lens and bounces between the glass layers
inside the lens. These internal reflections cause halos and
glowing geometric shapes to appear when you aim the camera near
the sun or other bright objects.
The better lenses often have a better zoom range; they'll
zoom in closer or zoom out wider than the cheaper lenses.
Although most lenses have a macro mode allowing you to
focus on very close objects, a few allow you
to zoom your lens when focused on very close objects. Common
consumer lenses will either zoom or macro but not both
simultaneously.
Professional and industrial cameras often have other
features that allow you to shoot under adverse conditions or
enhance the image under perfect conditions. In all, the pro
cameras give you more than just a sharper picture, they give you
a nicer picture.
You've seen what happens when you record something at home
with your camcorder and display it on your TV. You've also seen
what happens when professionals record something and sell you or
rent you a VHS video cassette. In most cases, their image looks
better than yours. In both cases, it was still a VHS
videocassette that you were playing into your TV. What must have
been the difference? Yes, excellent lighting, makeup, and prop
selection played a role. But the camera also played a role. The
better cameras used professionally made better images, ones that
held up to editing and duplication and still yielded a good
picture when they reached your home in VHS. This is proof that a
better camera will make a real difference. The tough question
is, how much can you afford to spend on that camera and will it
be worth the trouble to you.
If you are a home hobbyist who occasionally records
everyone sitting at the Thanksgiving dinner table, you'll
probably be happy with a 300,000 pixel camera. If you are the
kind of hobbyist who lugs your camcorder everywhere on vacation,
you may want 400,000 pixels and perhaps SVHS or Hi8. This will
give you a little extra sharpness when you edit and consolidate
your vacation into a shorter more palatable version to share with
your friends. Part-time wedding videographers definitely want
400,000 pixel cameras and the super formats; they always edit,
and need to please their clients. Full time wedding
videographers and industrial videographers will most likely
choose top-of-the-line camcorders employing three chips.
Their careers are measured by their pictures
and their pictures need to be visibly better than their Uncle
Charlie's. The real professionals take a double jump; they
team a high quality camera with a high quality format such as
Betacam or DV. Attend one of the video expositions and you will
clearly see the difference between these cameras and camcorders
and the consumer models. The image, even after several layers of
editing, is still smooth and sharp, an excellent foundation on
which to make VHS copies. When the copies are made, even though
they are VHS, look far better than they would have if mastered
from a low resolution camera.