UNDERSTANDING TV CAMERA RESOLUTION
Some manufacturers advertise that their camcorders have image sensors with 300,000 pixels. Others advertise 410,000 or 500,000 pixels. Once the image is recorded in VHS (capable of 240 lines of resolution), or DVD (capable of 540 lines) or appears on cable TV (capable of 330), do the extra pixels make a difference? Some professional videographers buy $8,000 TV cameras with 700 lines of resolution and feed the signals to DV (Digital Video) decks good for 400-500 lines of resolution. Once edited, the productions are distributed on VHS or cable or DVD. Would a cheaper camera have done as good a job?
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, DV or DVD.
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. Analog 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 and cable television also uses 330 lines as its maximum picture resolution. Professional video machines and DV (Digital Video) recorders popular with prosumers can record and play back higher resolutions than this, (DV is theoretically 500 lines) but the picture sharpness is reduced to 330 lines when it is broadcast or cablecast to your home, or runs through a video or antenna cable.
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. A 700 line camera will be appreciated more if fed to a DV recorder, hard disk, or DVD.
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 or DVD. In most cases, their image looks better than yours. In both cases, it was still a VHS videocassette or DVD 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. or DV or Digital8 or DVD. 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 copies. When the copies are made, even though they are VHS DV, or DVD, look far better than they would have if mastered from a low resolution camera.
Incidentally, when in the digital realm, we have to consider COMPRESSION as well as resolution. If a DVD recorder compresses the beegeebers out of a signal (so more fits on a disk), the picture becomes blocky and less sharp. Resolution may fall well below the 240 lines common for the antique analog VHS VCRs.
High Definition Television (HDTV) adds even more complexity to the issue, where we consider wider screens and whether the image is scanned interlaced 1080 times per second (1080i) or progressively at 720 times per second (720p). Both are sharper than common analog NTSC (525i), but that's a different story. See the HDTV articles.
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