Focus and f/stops.. A brief introduction

To many photographers, even advanced amateurs and knowledgeable photo enthusiasts, digital cameras look, feel, and shoot almost exactly like conventional film cameras. That's because both systems have seemingly identical parts (lens, viewfinder, light sensor, and shutter button, among others), and they also share many common photographic terms (f-stop, shutter speed, ISO sensitivity, and so on). 

The main difference, of course, is that digital cameras capture and save images electronically, while conventional cameras imprint images on light-sensitive silver halide film. In reality, however, this apples-to-apples comparison of digital and film cameras is a misleading oversimplification. There are numerous, important differences between the two technologies, and what you don't know could very well affect overall image quality.

Here's a crash course on some of the most important differences. Are focal lengths equivalent on film and digital cameras? Take a look at the lens on your digital camera. You will probably see the focal length measurement (in millimeters) there, defining whether the lens is wide angle, telephoto, or normal. For instance, you might have a Casio QV-3000EX digital camera with a 7-to-21-mm zoom lens. But take a look at Casio's documentation and you'll see that the lens focal length is also listed as 33 mm to 100 mm. The latter figures refer to the equivalent wide-angle-to-telephoto coverage of a lens on a 35 mm film camera. Same thing, right? Wrong. Digital camera lenses have significantly shorter focal lengths because the CCD (or charge-coupled device, the light-sensitive semiconductor chip that captures the image) is physically smaller than a frame of 35 mm film and, therefore, doesn't need all that extra glass. 

While the amount of coverage (how much of a scene is captured) may be the same, the laws of optics say that the shorter focal length lens will always have greater depth of field (the area in which the subject is in sharp focus). Unlike 35 mm camera lenses, where selecting a specific focal length causes the background or foreground to be either in sharp focus or out of focus, digital camera zoom lenses automatically capture in-focus images at almost every focal length, whether you want that or not. (Fortunately though, some digital cameras feature a special portrait mode that electronically throws the foreground and background out of focus.)

The F-stop demystified

This brings up another important difference: f-stops. As every shutterbug knows, f-stop refers to the amount of light that passes through the lens to the film. There is a mechanical leaf diaphragm on 35 mm lenses that can be stopped down or opened up, to either increase or cut down on the amount of light; more light translates into less depth of field, while less allows greater depth of field. The rule of thumb in conventional photography is that the larger the opening, the faster the shutter speed can be. With few exceptions, most digital cameras don't have mechanical leaf diaphragms regulating the amount of light that falls on their CCDs. Instead, it's all done electronically, which means that manually selecting f-stops (on those cameras that even offer the option) won't affect depth of field in the slightest. 

This will, however, affect the camera's shutter speed. However, most digital cameras don't have shutters, either; the speed at which the image is captured is regulated electronically, not mechanically. In fact, the crude mechanical shutter found on better digital cameras is there only to protect the CCD from being blinded by bright light, rather than to precisely regulate the fractions-of-a-second image-capture time. Having an electronic shutter that does not have to open and close frees your digital camera from some of the limitations of mechanical shutters and also explains why many still digital cameras can capture short video clips.

ISO equals gain in digital speak

Another significant corresponding feature is light sensitivity. In film, the speed of the emulsion (the amount of light needed to correctly expose the image so that it's neither too light nor too dark) is rated by assigning it an ISO (International Standards Organization) number. For instance, an average film sensitivity would be that of ISO 100. Fast film, used for shooting in low light or for sports when a fast shutter speed is required, might have an ISO of 800 or 1600, while slow film, often used for high-quality enlargements and ultra-detailed prints, has ISOs between 15 and 50. There are trade-offs, however; using slow film generally means slower shutter speeds and more limited depth of field, while shooting with fast film can produce grainy, high-contrast images. 

Digital cameras, of course, don't have film's ISO speeds. But to make things easier to understand, ISO equivalency is often listed instead of gain, the correct electronic term that describes boosting or lowering the CCD's sensitivity to light. The problem with gain is that as the sensitivity level is raised, it produces noise (unwanted electronic artifacts in the picture, such as random white splotches or streaks), just as turning up the volume on a radio increases the amount of static you can hear in the background. Therefore, digital camera users should view gain in the same way that photographers look at ISO speed: turn it up when you need faster speeds or must shoot in low light, but be prepared for some image degradation. By better understanding your digital camera's similarities to and differences from film-based cameras--that is, the advantages as well as the limitations, you may be able to produce better pictures.