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In the old days, if your camera (35mm of course) had a battery die on you in the middle of an important shoot, you still had options. I mean after all; the only thing you needed the battery for was the light meter, it's not like your whole camera will shut down if you have no battery. Oh, I'm sorry . . . if you are using all digital that might have stung just a little bit.

The point was if your battery died, you still had a way to get a good picture. If you were using 100 speed film you could set your shutter speed to whatever the flash sync speed was (usually 1/60th or 1/125th of a second), set your f-stop to f-16, and bracket every shot. Besides the obvious dependency on batteries, there are other differences between film and digital cameras.

If you have ever shopped for a digital camera you have no doubt heard the phrase: '35mm equivalent'. This means that the optics are not exactly the same in a digital camera. Since the 35mm SLR camera has been the standard for so long, that's what newer cameras are compared to. The difference between the two is a ratio of 1:1.4. Simply put, a 35-200 zoom on a digital camera would be like having a 49-280 zoom lens on a traditional 35mm camera.

The main reason the optics are different is because the sensor (the device that actually reads the light) is also a different size. Film cameras use film (sensitive to light) that is placed directly behind the lens. When the correct exposure is calculated, that image is literately burned into the film. Digital cameras use a sensor; that also sits behind the lens. This sensor is made up of millions of individual points that each represents 1 pixel. Once the sensor has gathered the information for each pixel it transfers that data to a digital media card (which can be used repeatedly.)

The sensor is the physical device that gathers information about the quality of light coming into the camera. The process of 'how' the sensor goes about gathering information is referred to as 'metering'. The human eye can distinguish a range of about 16 different f-stops; camera meters only have a range of about 5 f-stops. This is why camera meters are calibrated for a 'mid range' exposure of 18% gray, because 90% of the time that is as close as they can get to the human eye. It's not the camera's fault that it can not see as well as you do, it's simply a fact of life.

There are basically only three types of metering systems.

A) Spot Metering
B) Center-Weighted Metering
C) Matrix Metering

Spot metering as the name indicates only reads a small spot or portion of the overall image (usually 1% to 3%). This type of metering is useful in any situation where the lighting is extreme. Backlit subjects, macro shots, or even pictures of the moon can benefit from this type of metering. This type of metering is usually found on the more costly upper end cameras.

Center-Weighted metering averages the overall scene with an emphasis on the center area of the frame. Usually this type of meter bases its reading with 75% of the light hitting center frame and 25% for everything else. It assumes that the subject is dead center, most of the time. It is worth noting that most center weighted systems have a greater sensitivity in the bottom half of the frame; to avoid an overly contrasting sky from throwing off the readings. This type of system is the most common used in both digital and traditional cameras today.

Matrix Metering splits your image up into anywhere from 3 to 16 metering zones and evaluates the different zones to come up with one over all reading. In this process of evaluation it takes into account factors like: subject size, position, distance, point of focus, over all lighting, color and more. This system uses a microchip which has been exposed to thousands of different picture-taking situations. Currently this is the most complex and the most accurate metering system to date. This system is usually found on the higher end Digital SLR's.

I used the word 'digital' several times, but these are the same types of metering systems used in traditional film cameras as well. The only other way of reading light has to do with 'reflected light' verses 'surface light'. Most meters in the camera are reading reflected light (light reflected off the main subject that goes back toward the camera.) Every so often you might see someone with a hand held light meter that will go right up to the subject and read the light that falls on the surface of that subject. Some photographers still debate which way is more accurate. In my opinion; 'Spot Metering' does basically the same thing.

All photographers have their favorites; Canon, Nikon, Kodak. Some choose digital, some choose film. The thing to remember is what we actually need to get a great photo. Things like composition, leading lines, framing, and the rule of thirds are much more important to our success as great photographers than the physical tools we use. On the other hand, knowing what your camera can or can not do, let's you know if you have the right tool for the job.