I wrote the following in an e-mail to a friend about a year ago. Might be useful to some. He wanted to buy a DSLR for his GF but didn't know whether to get a Canon PowerShot S3 or Rebel XT...
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Well, first of all, the PowerShot S3 is not a DSLR. It's a P&S (point & shoot).
DSLRs = Interchangeable lenses as well as a shutter that opens and closes in front of a sensor. There is no live-preview from the screen. You look into the viewfinder and what you see is what you get. Sort of like old 35mm film cameras. These cameras are larger, more expensive, but much more versatile in terms of shooting speed and ability to focus and shoot in low-light.
P&S = 95% of your digital cameras on the market. They are smaller and cheaper with built-in zoom lenses. You rarely use the viewfinder and usually use the screen to see what the sensor sees, which is often different from what one would see in the viewfinder. These cameras are smaller, cheaper, very good in daylight, but suffer in low-light conditions and/or when speed is necessary.
People choose either portability and price (P&S) or optical quality, accuracy, and speed (DSLR).
When it comes to DSLRs, you basically have two choices, Canon and Nikon. Sort of like Intel and AMD, nVidia and ATI.
For around $1k, your Canon options are the Rebel XT or EOS-20D or its replacement, the EOS-30D. Since the 30D's introduction, one can get better deals on the 20D. The 30D and 20D have more or less the same internals. The 30D is an evolutionary product with a slightly different case and some firmware/software modifications. I personally have a 20D. The Canon Rebel XT has very similar internals as the EOS-20D/30D as well but a plastic case instead of alloy. The Rebel XT is actually a pretty small DSLR, maybe just perfect for smaller, feminine hands, but too small for mine. A hands-on analysis is recommended.
Your Nikon options are the D50 and D70S.
Now, what sets these "entry-level" DSLRs apart from the high-end models? Basically three things: 1) Sensor, 2) Case, 3) Shutter. 1) Canon's Rebel and EOS-20D/30D cameras, along with Nikon's D50/D70S, are APS-C cameras. An APS-C sensor (Crop) is about 40% the size of a Full-Frame (FF) or 35mm-equivalent-sized sensor like you would find on the Canon EOS-5D and higher models. A smaller sensor is cheaper to manufacture obviously. More on this later. 2) High-end models are weather-sealed with rubber gaskets and use stronger, lighter metals. 3) High-end models also have shutters capable of faster speeds with a longer shelf-life.
More on APS-C: Take a look at the image below. If you took a standard 50mm lens and put it on a FF camera, you would get the image in the red box. If you took that same lens and put it on an APS-C camera, because of its smaller sensor size, you would get the image in the blue box, everything else being equal. This is known as the 1.6x crop factor. Meaning, any lens you buy for an APS-C camera, you need to multiply its zoom rating by 1.6 to get its 35mm equivalency. For example, a 28mm lens on a FF camera is a 44.8mm lens on an APS-C. A 50mm lens on an FF camera is an 80mm lens on an APS-C and so on.
*Image Below*
This gets us into lenses. Because APS-C cameras have a smaller sensor/mirror box, manufacturers are making lenses that are smaller and lighter but unfortunately not cheaper. In Canon lingo, these are called EF-S lenses. EF-S lenses will work ONLY on cameras with an EF-S mount such as the Rebel, Rebel XT, EOS-20D, and EOS-30D. Canon currently makes only five EF-S lenses. The rest of the lenses out there are EF lenses, which have been the standard for decades. You can use EF lenses on APS-C cameras as well but cannot use EF-S lenses on FF camera bodies. Nikon uses the term AF-S.
Continuing with lenses: Lenses are the most important and most expensive part of owning a DSLR!!! You can spend $5K on a camera body, but if you spend $100 on a lens, you're gonna get ***** results. So the rule is to spend more on glass than on the body. Bodies come and go but lenses are forever as they say! Finding the right lens is a highly personal decision! What are you going to be shooting? Sports? Fast-action? Weddings? Portraits? Landscapes? Indoor? Outdoor? Unfortunately, there is no one lens that does it all perfectly. Much to the contrary. Hence why dropping $1300-$2000 on one lens is not uncommon. Many people will travel with three such lenses in their bag.
There are many types of lenses, but I will focus on two types and give a few examples of the Canon terminology.
1) Prime lenses (don't zoom): Canon EF 50mm F1.4 USM. (Canon is the manufacturer. It has an EF mount (for APS-C and FF). It has a fixed zoom range of 50mm on FF (80mm equivalent on APS-C). It's got a small aperture of 1.4. (Smaller the number, the wider the lens can open to allow more light inside, allowing for faster shutter speeds in low light situations.) USM is Ultra Sonic Motor; a faster, more accurate auto-focus mechanism for Canon lenses.
2) Zoom lenses: Canon EF-S 17-85 F3.5-4.5 IS USM. (Canon is the manufacturer. It has an EF-S mount (for APS-C only). It has a variable zoom range of 17 to 85mm (27.2mm to 136mm equivalent on APS-C). It's got a variable aperture: F3.5 on the wide end (17mm) and F4.5 on the telephoto end (85mm). It features IS or Image Stabilization which counteracts the movement of shaky hands when shutter speeds are quite low/slow. Again, it has USM auto-focus.
Another example would be the Canon EF 24-105 F4L IS USM. It has an EF mount. Variable zoom range of 24mm to 105mm (38.4mm to 168mm equivalent on APS-C). It has a fixed aperture of F4 at all ranges. It features IS. It features USM. And last but not least it's an "L" lens. L stands for Luxury. These are the really high-end lenses from Canon, denoted by a red stripe around the barrel.
Depending on what you want to shoot, you'll want to look for a certain type of lens. Sports/fast-action: You need low-aperture (F1.4, F1.8, F2.0, F2.8). Same with indoor/low-light. For landscapes you need a wide lens (10-22mm). If you want to shoot birds or animals, you want a good telephoto (70-200mm or 70-300mm). If you want to shoot portraits usually a good prime lens is best. Plan on going without a tripod? Then IS is recommended. Want to buy the best? Buy an "L" lens. Etc.