Avid photographers tend to be gear heads, taking pleasure discussing highly technical subjects such as varying focal lengths and their inherent influence on magnification factors. The snapshot happy public, on the other hand, seems satisfied remaining blissfully unaware of what goes on underneath the hoods of their high-tech picture poppers. In explaining how a digital camera works, we're going to shoot for a happy medium between the two. Without getting too technical we'll take quick peek at some of the primary elements at work, including the image sensor, image processor, lens, image compression methods, and LCD viewfinders.

The image sensor; a marvel of microscopic proportions
The primary difference between traditional cameras and digital cameras is the means by which they capture images.

Film cameras use strips of plastic covered in granular silver halide crystals that collect light. If you were to look closely at an enlarged image captured by a film camera, you would see millions of tiny, irregular grains of color or shadow. Each of these grains roughly represents an individual silver halide crystal.

Digital cameras replace crystal-coated plastic with image sensors. An image sensor is a seemingly flat surface composed of millions of photodiodes. These photodiodes are essentially substitutes for the individual silver halide grains found on film. Each diode collects light that will end up representing an individual pixel, or dot, in the final image.

The number of pixels necessary to approximate the amount of detail found in an image captured on standard 35mm film is up for debate. Sources for this story suggested anywhere from four to 20 million pixels -- or four to 20 "megapixels" -- would do the trick. Canon Canada's Neil Stephenson seemed to have the most definitive evidence to back up his claim. He said his company performed a test in which they compared the grains on an enlarged image captured with 35mm film with the pixels of the same image shot with a 16 megapixel digital camera and found that the two photographs were virtually indistinguishable from one another. That said, a 16 megapixel camera is, nonetheless, overkill for most consumers -- unless they plan on making poster size prints.

So image sensors capture lots of tiny dots of light. But on their own they would simply register the intensity of the light, not its color. Hence, a color filter is placed on top of the sensor. While there are many kinds of color filters in use, the most common is the Bayer filter, which is essentially a grid composed of squares representing the three primary colors -- with twice as many green photo sensors than red or blue because the human eye is more sensitive to green light. The filter assigns a color to each pixel in the image as light passes through. With both light and color captured, the camera now has a set of data that can be assembled to create an image.

Image processing: a vendor's "value add"
The information captured by an image sensor is digital -- just a series of ones and zeros. This data is converted into a viewable picture via image processors, which consist of circuit boards and software (hence the "computers with lenses" anology). Image processing not only decodes digital information to create a picture; it also manipulates the image by activating various algorithms to achieve, for example, balanced colors and lighting.

While many camera manufacturers purchase image sensors from other companies, image processors are largely unique to each vendor -- their so-called "value add." Different image processors employ varying algorithms, which can deliver vastly different image results.

As an example, most major camera manufacturers have their own unique demosaicing algorithm. So while the color filter that sits in front of the image sensor and provides a basic value for the light collected by each photodiode, without demosaicing the resulting picture would look like... well, like a mosaic -- an image composed of bits of basic color. A good demosaicing algorithm will look at those simple reds, blues, and greens and interpolate nuanced, lifelike shades to create a much more natural looking image. Poor demosaicing might result in a picture in which the colors seem slightly out of whack. In extreme cases a faint, grid-like pattern can emerge.

Good Demosaicing...        ...not so much             

It's all about the perpendicular
In order to understand the difference between traditional and digital lenses, we need to dig a bit more deeply into the way both types of cameras collect light. The silver halide crystals on film have a three-dimensional quality that allows them to gather light from many angles. Lenses designed for film cameras exploit this property by using concave, convex, or aspheric elements to direct light towards the film at an angle.

A digital camera's image sensor, on the other hand, is essentially a flat surface and not well equipped to receive light at an angle. If light arrives at the sensor at a slant, the resulting image could suffer from noise or loss of clarity. "Think of several shot glasses lined up in a grid," explains Olympus America's Sally Smith. "They create what appears to be a flat surface -- you can set a piece of paper on top of them. But, each glass has depth. The same is true of the photodiodes on an image sensor. Light needs to be guided directly into them."

That means the light passing through the primary lens of a digital camera needs to be redirected to hit the image sensor at a perpendicular angle, allowing it to funnel straight down into the photodiodes. This requirement has led to new lens optics designed specifically for digital cameras that direct light in a manner conducive to an image sensor's method of collection. Without getting into too much detail, a digital camera's lens system is made up of varying pieces of glass that redirect light to ensure it smacks the image sensor head on.

   Le Smack...       ...Smackless

(diagram courtesy of Olympus America)

Compression: a semi-necessary evil
Image compression is a procedure carried out by the camera's processor to reduce picture file size. Smaller files require less memory to store, can be transferred from one device to another more quickly, and are easier to work with in photo editing applications. Flash memory cards used to store images have increased in size to such a degree that users can now store a thousand or more photos on a card that costs less than $50. Consequently, these days, the purpose of image compression isn't so much to store more photos as it is to create files that are diminutive enough to be easily managed.

Compression essentially deletes pixels that are of similar but not exactly the same color and replaces them with a single color, resulting in less information and smaller file sizes. Most cameras use JPEG compression, which, when properly applied, results in a relatively minor amount of visual detail lost. However, there are varying levels of compression available within the JPEG format, hence the selectable file sizes in your camera's settings menu.

Kodak's Brian Fox says a simple way to understand varying levels of compression is to start by thinking of an image with a clear blue sky. "If you were to choose a very high level of compression, all of the subtle hues of blue in the sky might simply be replaced by a single shade repeated over and over again," said Fox. A low level of compression, on the other hand, or what Olympus' Smith refers to as "intelligent compression," would retain far more of the sky's blue nuances, replacing smaller groups of nearly identical pixels here and there with a single color.

Still, some detail is always lost in compression. And that's why most avid photographers prefer to shoot in uncompressed formats such as RAW and TIFF, which preserve every pixel of data captured by an image sensor. The files are larger, but images are unadulterated and quality is left completely intact.

"Intelligently Compressed"         ...Dunce Cap please           

View screen vs. viewfinder
The LCD screens on the backs of digital cameras have fundamentally changed the way people take pictures. Rather than forcing photographers to position an optical viewfinder directly in front of their eyes, digital photographers can position the camera well away from their faces, which makes framing a picture above a crowd, for example, much easier.

And, thanks to the fact that the picture on the display reflects exactly what is seen by the image sensor, LCDs provide 100 per cent frame accuracy. In other words, what you see before you press the shutter button is what you get in the final image. By contrast, the viewfinders on most non-professional film cameras, which provide previews using either a simple window above the lens with a direct view of the subject or a mirror assembly that directs light from the lens up to a closed window, typically provide a frame accuracy of 95 per cent or less. This loss of preview accuracy could potentially result in, say, in a portrait with the top of the subject's head cut off (hate it when that happens... except maybe at a guillotine revival).

However, many serious digital photographers still prefer viewfinders to LCDs. One reason is that a viewfinder image is optical as opposed to digital. An optical image delivers maximum detail, consequently giving the shooter an extremely clear look at the subject being framed, including tiny visual details that may not be viewable on the 200,000 or so pixels that make up most camera LCDs. But in the end it could simply be a rare case of old school camera technology being more comfortable and intuitive than digital. All of the sources interviewed for this story -- each one an avid photographer -- said that they prefer using a viewfinder to an LCD. As Canon's Stephenson puts it, "It just feels like you have more control. It sounds funny, but when you bring the camera up to your face you become one with it. That feeling doesn't happen with an LCD."