How Many Pixels Are Enough? A Guide to Choosing a Digital Camera

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Edit: Source: http://www.digitaloutput.net

The first thing people want to know about a digital camera is "how many pixels does it have?" Although this is not a bad question to ask, it's not the only factor to consider when choosing a camera. To help you decide how many pixels you need (and how much money you need to spend to get those pixels).

The first thing people want to know about a digital camera is "how many pixels does it have?" Although this is not a bad question to ask, it's not the only factor to consider when choosing a camera. Cameras with higher pixel counts generally create higher quality pictures, but they also create larger files that aren't appropriate for some uses.

For example, if you're purchasing a camera to use primarily for sending snapshots via e-mail or the Web, you have to resize your images to a smaller size to reduce upload and download times. You don't need a 3-megapixel camera if you'll always resize the images down to a megapixel or less.

Counting pixels

If you plan to print most of the picture you take with your digital camera, you want as many pixels as you can get. This is especially true if you'll be printing your images on a high-quality photo printer. The larger the print, the more pixels you need to get an acceptable picture. Table 3.1 shows how many pixels you need for several popular print sizes.

Approximate Number of Pixels Needed to Produce a High-Quality Print at Different Paper Sizes

Megapixels ----- Maximumm Print Size

1.3 --------------- 4 x 6''

2.0 --------------- 5 x 7''

3.3 --------------- 8 x 10''

5.0 --------------- 11 x 14''

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Many people-including camera manufacturers-often incorrectly use the term resolution to refer to the pixel count, or the number of pixels produced by a camera. Resolution refers to the camera's ability to capture small details. Pixel count is simply the number of pixels produced by the camera's image sensor. Although the two terms are related, they're not the same.

Is More Always Better?

The short answer is yes. All things being equal, a camera with more pixels produces better pictures than a camera with fewer pixels. The pixel count determines the overall size and quality of the images created by a digital camera. In general, the more pixels, the more detail a picture contains. Pictures with more details appear sharper than pictures with less detail.

Printer technology is improving almost as fast as camera technology. As discussed in Chapter 9, "Inkjet Printers," inkjet printers (the most popular type of printer for printing digital camera images) print images using tiny dots of ink. The more dots the printer can produce, the clearer the image appears.

Just a few years ago, 600 dot per inch (DPI) printers were the norm. Today, there are several inexpensive printers on the market that can print more than 2,000 DPI. These new printers take better advantage of the increased pixel counts and higher detail of newer digital cameras.

In a film camera, picture quality is a factor of the size of the film, the sharpness of the lens, and the resolving power of the film. In a digital camera, resolution is determined by the number of pixels in the image sensor, the sharpness of the lens, and the camera's ability to convert raw pixels into an electronic image. In the film world, the easiest way to get more detail is to use a larger piece of film. In the digital world, you get more detail by creating more pixels-up to a point, as you'll see in a moment.

Pixel count has become the main yardstick used to measure and compare cameras. Digital cameras produce images with millions of pixels, so the term megapixel is used as shorthand for "a million pixels."

To understand what pixels are and why they're important, it helps to understand how conventional film cameras work.

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Photographic negative film contains millions of tiny, light-sensitive silver halide crystals on the surface of the film. Each individual picture on a roll of film is recorded on a unique area on the film called a frame. As you take pictures and wind the film, the most recently exposed frame moves out of the area behind the camera's lens, and another, unexposed frame moves into place, until you get to the end of the roll of film.

When the film is developed, the crystals that were exposed to light remain on the film; those that weren't exposed to light are removed in the developing process. (The process works just the opposite for slide film, which produces a positive image instead of a negative.) As a result, dark areas on the film have more crystals; lighter areas have fewer.

Where Do Pixels Come From?

Digital images on your computer screen are composed of a series of colored squares called pixels. Each pixel is described by three or four numbers that define each pixel's color and brightness. In the RGB color space system most commonly used for consumer digital imaging, each picture has a red, green, and blue value, and each value ranges from zero (dark) to 255 (bright). Red, green, and blue light combine to make white, so a pixel with an RGB value of 255,255,255 displays as 100% white. Similarly, a pixel with a value of 0,0,0 displays as black, and a pixel with a value of 0,255,0 displays as bright green. There are other color space systems besides RGB. For example, the cyan, magenta, yellow, black (CMYK) system is often used for images that are to be printed via conventional four-color offset printing presses, which use cyan, magenta, yellow, and black inks.

What's a JPEG?

Digital images are stored in electronic files, and the most common of these is the Joint Photographic Experts Group, or JPEG, format. JPEG files can be stored with varying degrees of electronic compression, which make the files smaller and faster to work with. Information about file formats and compression is presented in more detail in Chapter 20, "Outsourcing Your Printing."

Digital cameras are basically small computers that convert live images into digital files. They record images by electronically detecting light (photons) striking the face of an electronic image sensor. The face of the image sensor contains millions of light-sensitive transistors called phototransistors or photosites. Each photosite represents one pixel, and the terms are often used interchangeably when discussing image sensors. When light strikes one of the photosites, it causes a change in the electrical charge flowing through the transistor. The stronger the light, the stronger the change.

The camera builds an image from the array of pixels by electronically scanning the contents of each pixel. Image sensors are monochrome; that is, they see light as black or white. To make a black-and-white sensor see color, each photosite on the sensor is covered with a layer of color filters called a color filter array, or CFA. Most cameras use red, green, and blue (called GRGB) CFAs, although some use a cyan, yellow, green, and magenta (CYGM) array. For clarity, I'll illustrate the more common GRGB arrangement, but the process is the same for CYGM sensors.

The dye layers effectively make each photosite sensitive to only a single color, depending on the color of the dye. The dye is applied in a pattern (called a Bayer pattern) such that each row has either alternating red and green or blue and green pixels. If you do the math, you'll see that in a GRGB sensor, there are twice as many green pixels are there are red or blue. That's because green provides much of the perceived detail in the picture, while red and blue contribute relatively little detail information. By using twice as many green pixels, camera designers can squeeze the most detail out of the image sensor.

When you take a picture, a chip inside the camera called an image processor reads the data collected by the image sensor. The processor mathematically combines the data from each pixel with the data from its neighboring pixels to produce an RGB value for each pixel. The RGB data is collected and saved as an image file on the cameras' storage media.

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The image sensors used in most digital cameras were originally developed for use in camcorders, so they are very small. Sensors are measured on the diagonal. The Charge Coupled Device (CCD) image sensors used in most digital cameras measure 1/1.8'', or .555'', or 13.7mm. For comparison, a 24 x 36mm (35mm) film frame measures 1.70'' or 43.2mm on the diagonal.

Smaller image sensors have some distinct advantages, but they pose a number of design problems, as well. Smaller sensors enable manufacturers to sell the same sensor to both video and still camera makers, reducing costs through economies of scale. They also require smaller lenses, which reduces the size and weight of cameras. But the pixels on a small sensor are very close together, which in turn requires that the lenses used with small-sensor cameras be of very high quality. In fact, a big part of the cost of a 3-megapixel P&S camera is in the lens.

"The CCD and CMOS image sensors used in most digital cameras are much smaller than a 35mm film frame."


When comparing camera specs, you'll often see numbers for "number of sensor pixels" and "effective pixels." The latter number is the actual number of pixels produced by the camera, and it is always lower than the first number. So where did those pixels go?

Some of the pixels at the edge of the sensor have an opaque black dye that blocks light from hitting those pixels. This is done so that the camera has a reference point for the darkest part of the image.

Some cameras lose some effective pixels because the camera's lens can't cover the entire sensor area. Canon's PRO90IS, for example, uses a 3.3-megapixel sensor, but the lens-borrowed from one of Canon's DV camcorders-is too small to cover the entire sensor. As a result, the camera only uses the central part of the sensor, giving an effective pixel count of 2.6 megapixels.

The individual pixels on a small sensor have less surface area than the pixels on a larger sensor. As a result, they capture fewer photons, which makes them less sensitive to light than large image sensors. Many CCD sensors employ a grid of tiny lenses-one per pixel-called a microlens array. The microlenses are larger at the top (outside) than they are at the bottom (the side facing the CCD chip), so they act as light magnifiers and work to increase the sensitivity of the CCD.

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"A CCD image sensor contains three main parts: the microlens, the Color Filter Array, and the photosites."

Most digital SLRs currently on the market employ sensors that are larger than 1/2'' but still significantly smaller than a 35mm negative. As a result, the effective focal length of SLR lenses is multiplied, usually by a factor of 1.5 or so, when used on a digital SLR. The next generation of digital SLRs will likely use full-frame 24 x 36mm sensors, providing better light sensitivity and eliminating the multiplication factor.

Aspect Ratio

Because many CCD image sensors were originally designed for use in video cameras, they have the same 4:3 horizontal-to-vertical size ratio (called the aspect ratio) as a television screen. Unfortunately, 35mm film has a 3:2 aspect ratio, which is proportionally much wider than a TV screen. Some digital cameras allow you to shoot in either 4:3 or 3:2 mode. The 3:2 mode is very convenient for producing prints on 4 x 6'' paper, because the entire image fits perfectly.

5 x 7'' and 8 x 10'' are also very popular print sizes, even though neither of them match the 3:2 aspect ratio of a 35mm negative. And they don't match a 4:3 sensor, either-although a 5 x 7'' print is a very close match. Keep this in mind when you're shooting an important photo-a group picture at a wedding or other event, for example-that may wind up as an 8 x 10'' print.

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It's a safe bet that next year's cameras will have more pixels and more features at lower prices than this year's crop. The image sensor is one of the most expensive components in a digital camera. As with all semiconductor devices, they are cheaper to make in large quantities, so prices should come down as digital cameras become more popular.

So far, I've only mentioned CCD image sensors, but there's another type of sensor that's coming on strong. Complimentary Metal-Oxide Semiconductor (CMOS) image sensors are making a comeback. If the term CMOS sounds familiar, it's because you've heard it before. Much of the circuitry used to build PCs and other computer devices is made with CMOS technology. CMOS image sensors are very inexpensive to produce compared to CCD sensors. Early CMOS sensors suffered from poor light sensitivity, high noise levels, and awful image quality. They were typically used in low-cost, low-resolution applications like Web cameras, security cameras, and even toy cameras.

Advances in CMOS technology have led to the development of much higher quality CMOS image sensors. Canon's D30 digital SLR, for example, uses a CMOS image sensor that produces very high quality images with very low noise. CMOS sensors have some other advantages besides their low cost. CMOS sensors are made using the same process as microprocessors, RAM memory, and Digital Signal Processor (DSP) chips, so CMOS image sensors can contain additional circuitry directly on the sensor chip. This reduces the parts count, which decreases costs and increases reliability. CMOS sensors also use less power than their CCD counterparts, resulting in longer battery life.

At some point in the future, we'll reach a point where camera and printer technology reaches an equilibrium-a point where more pixels doesn't add more quality. Experts' opinions differ on where that point is. A high-quality 35mm film frame contains the equivalent of about 12-18 million pixels. But the vast majority of prints produced by commercial photo labs are 4 x 6''. Even a 1-megapixel camera can produce an acceptable 4 x 6'' print, and a 3-megapixel camera produces a 4 x 6'' print that is nearly indistinguishable from film.

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I want to buy a digital camera, but I'm afraid that by the time I learn how to use it, it will be replaced with a better model.

Welcome to the world of digital photography, where equipment obsolescence is a fact of life. Just remember, a good camera still takes good pictures, even after a newer, better model comes on the market.

I want to get into digital imaging, but I don't want to buy a digital camera just yet. Can I use my film camera to make digital images?

Yes, you can. You can purchase a film scanner or have your images commercially scanned. Scanner prices range from about $400 to $2000, and they can produce stunning digital images from 35mm negative or slide film. If you'd rather not spend that much money, many photofinishers can scan your images for you; they deliver your scanned images on a CD-ROM.

Should I buy a 3-megapixel camera now or wait for the next generation of 4 or 5-megapixel cameras?

This is always a tough question to answer, because no one knows where the mexapixel race will stop. Many industry experts think that 3 megapixels are more than enough for most non-commercial applications.

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There?s been a tremendous explosion of digital technology in photography. Higher resolutions, more sophisticated metering and exposure capabilities and the introduction of digital SLR cameras are prompting professional photographers to make the move towards digital.

Last year, more than 100 million image sensors of all types were sold and the demand for imaging sensors is continuing to climb. Sizes of sensors vary greatly. Some pro medium and large format sensors go as high as 20 megapixels (MP). Kodak has built a line of 16MP sensors that are included in a number of leading medium format digital backs.

The highest resolution digital SLR, the Kodak 14n, has a 14MP sensor, with the Canon 1Ds being close behind at 11MP. The next level for professional SLRs is around 6MP, with a number of models from different manufacturers in that category.

All sensors initially capture their images in a continuous analog signal, through anywhere from hundreds of thousands to millions of picture element positions. Technically they?re not quite pixels at this point, since pixels are digital, but they?re still frequently referred to as such. The values of the individual picture elements of those sensors are then converted to an equivalent digital pixel value.

The CCD

The most common type of sensor is the CCD (charge-coupled device). With a CCD, light is captured with individual photo-diode sensors. The photons that strike the sensor are converted to an equal number of electrons stored at individual sensor positions. Those electrons are then read electronically and stepped off of the charge transfer register. Once off of the CCD array, they are converted to their relative digital value.

CCDs require a specialized chip construction process. Rather than having all the electronics on one chip, a separate chip set is required to handle support functions. There has been some progress made in integrating other electronics functions into the CCD, but for the most part CCD digital cameras require a considerable amount of supporting electronics. Depending upon the camera design, sets of anywhere from three to eight chips are incorporated in the camera?s image capture and conversion process.

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Sensor Tricks

With most CCDs, each individual sensor position provides one pixel of digital data. But with some specialized CCDs, such as Fuji?s Super FinePix CCD, additional pixels are added to the image through the electronic conversion process. The end result is that there are more pixels in the final image than there are sensors on the CCD. Fuji has developed very sophisticated electronics to increase the image resolution beyond what the CCD can capture.

Nikon is also going with a type of interpolation to increase the effective resolution of its D1X digital SLRs. Trying to compete with the 14MP Kodak 14n, which is equipped with a Nikon lens mount, and Canon, which has the 11MP EOS 1Ds, Nikon has developed enhanced in-camera firmware and related software to intelligently interpolate the 6MP images that the D1X captures up to 10MP. With both Fuji and Nikon, the resulting image quality is very good, but they?re still forms of interpolation.

All too often, the capture resolution and the effective image resolution are used interchangeably. In most cases, the effective resolution is less than the actual CCD dimensions. That?s because the camera lens doesn?t quite cover the entire sensor, so not all pixels on the CCD are sampled. With interpolation schemes, however, the effective image resolution is higher, sometimes considerably higher, than the capture resolution.

CMOS Arrives

There?s another type of sensor besides CCD that?s becoming popular in digital cameras, and that?s the CMOS (complementary metal oxide semiconductor) sensor. Over the last few years, CMOS sensors have become increasingly common. They are being used in medium and large format digital backs, in professional digital SLRs, as well as some consumer cameras.

Both CMOS and CCD sensors are constructed from silicon. They have similar light sensitivity over the visible and near-IR spectrum. At the most basic level, both convert incident light into electronic charge by the same photo-conversion process.
However, CMOS sensors can be made of the same silicon material as other computer chips. That means all the electronics can be incorporated onto one chip, reducing production costs, space requirements and power usage. With CMOS, it?s possible to produce entire digital cameras on a single chip.

CMOS sensors also have individual picture elements, but, unlike a CCD, the conversion of the electronic signal to a digital value is completed within the individual photo sensor. That makes it possible to read-out the values of the individual sensors in a single step, rather than having to step the electronic signal off of the register, as is the case with CCDs.

CMOS sensors have been around as long as CCDs have. A type of CMOS sensor, called NMOS (n-channel metal oxide semiconductor) was used in the early 1970?s in video cameras. They worked, but image quality was marginal, at best. An unacceptably low signal-to-noise ratio has always been one of the problems with CMOS.

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It?s only been in the last few years that the limitations of the technology have sufficiently been overcome to make CMOS a viable alternative to CCD. The quality gap between images that are being captured with CCD sensors and images being taken with CMOS sensors is narrowing rapidly.

That is especially true as digital camera resolutions climb. CMOS sensors don?t suffer from the decrease in the signal-to-noise ratio as resolutions increase. That means higher resolution digital cameras can be produced without having to significantly increase the supporting electronics.

One of the reasons that CMOS is finally taking off is that there are a large number of corporations, educational institutions and governmental agencies working on the technology. With more than 60 organizations working on CMOS development, the size and quality of the images that those sensors can capture are increasing rapidly.

X3 Marks the Spot

One of the companies on the cutting edge of CMOS development is Foveon, which developed the X3 sensor chip. In some respects, the X3 is revolutionary. It was the first full-color image sensor that captured red, green and blue light at each individual pixel position. Instead of using color filtration to capture RGB color values, the X3 is able to capture all three primary colors simultaneously.

It can do that because it has three photo-detectors at every sensor location, making it possible to capture full color images, without having to use a color mosaic filter. CMOS sensors are able to do that without the complexity, and cost, of some CCD systems.

Foveon was able to achieve the multi-color capture capabilities through the specific properties of silicon, which absorbs different light waves at different depths. Each X3 sensor consists of three photo-detectors located at different depths. Each detects the absorption of the red, green and blue light that has penetrated the silicon to that specific depth. Blue light is absorbed near the surface, green light is absorbed farther down and red light is absorbed even deeper.

The individual photo-detectors convert the absorbed light into three signals. Those signals are converted to digital data, which is then optimized through software. According to the company, the X3 CMOS image capture and optimization process results in higher quality and sharper images, as well as better color. It also eliminates the color artifacts that can be a problem with CCD sensors.

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A number of digital camera manufactures are incorporating CMOS sensors into their models. Canon, in particular, has been aggressively implementing CMOS in its digital SLR design. Both the 1Ds, which, at a street price of around $8,000 is one of the most expensive digital SLRs on the market, and the 10D, which, with a street price of $1,600, is one of the least expensive to utilize CMOS sensors.

CMOS sensors are also being used in many of the miniature cameras that are part of space missions. For example, some of these small cameras, which can be the size of a quarter, are used on the rover vehicles that NASA is planning to send to Mars.
To further increase the quality of the images that these tiny CMOS-based cameras can capture, NASA is working on what?s called hybrid imaging technology (HIT).

Theoretically, HIT merges the best of CCD and CMOS technology, in hopes of coming up with a new technology that?s better than either. Once implemented, the resulting technology should have higher resolution, better scalability and reduced power consumption.

NASA is also working on another type of sensor altogether. Under contract to the space agency, the Jet Propulsion Laboratory in Pasadena, California, is working on what?s being called an SOI (silicon on insulator) sensor. SOI sensors are extremely thin, just 1 micron, and could be applied to just about any flat surface.

Because of their light weight and low power consumption, they could be used for a wide range of applications. These sensors should be available commercially by the end of the decade. This could very well be another revolutionary step in digital imaging.

[McKinley]

What the heck??? You are banned?
nice effort there

[Karim]

nice effort
Copy/paste without mentioning source :eyes: