Camera Body Characteristics
Mp and ISO
These two are big marketing tools. 7 megapixels is plenty, even for a print that is 2 feet in length. As a side note, 7 megapixels on a D-SLR has better quality than 7 megapixels on a point-and-shoot - it may even be better than 10 megapixels on a point-and-shoot.
Concerning ISO, you'll see numbers like 100, 200, 400, 800, etc. A higher number makes indoor (low light) photography easier. On point-and-shoots, things above ISO 400 begin to look noisy/grainy. On D-SLRs, ISO 1600 still appears usable for a letter size, or maybe even larger prints.
The rule of thumb is, use as low an ISO speed as possible. That, of course, will vary with the available light where you're shooting. ISO speeds are the same numbers you'll see when buying film. It is a rating of the film's...or in the case of D-SLRs, the sensor's sensitivity to light. A higher number means it takes less light to produce an image, which is why it can be handy indoors. On sunny days outside, ISO 100 or 200 is typical.
Lets examine the Canon EF-S 18-55mm IS lens...
18-55mm - These numbers tell you how much the lens can zoom. Normally, on point-and-shoot cameras, you'll see specs that say 3x optical zoom. With D-SLRs, you're usually only given focal lengths, but to figure out how much zoom it has, just divide the bigger number by the smaller number (55/18 is about 3).
To give you a feel of what the numbers mean, 31mm is about human eye perspective, so, if you look into the living room with just your eyes, then look through the camera, it will look about the same - not zoomed close, or far, but just like what your eyes see. 18-55mm is good for everyday shooting.
What does wide, telephoto, and normal mean? A wide angle lens lets you see a lot of a scene, without having to step back so far, like, when taking pictures of people inside a house, you can just take several steps back to photograph the whole person. Secondly, think "telescope" with telephoto lenses. It's like looking through a telescope, and you'll be able to see something that's very far away, up close and personal, in good detail...there's not a lot of peripheral vision. Normal lenses act like your eyeball. It looks the same looking through the camera as it does looking with your naked eye.
F3.5-5.6 - These numbers tell you how big the opening (the aperture) gets. In short, once you see something like F2.8 on a lens, be ready to drop $1000-$2000! A smaller number (which means larger opening) is better, because it will help with indoor (low light) situations. Typically, the next smallest from F3.5 is F2.8, and F3.5-5.6 is great for everyday shooting.
Why are there two numbers? The first is how large the aperture can be at the wide end...like 18mm (zoomed all the way out), and the second is how large the aperture can be at the telephoto end...55mm (zoomed all the way in). I didn't know that for the longest time.
IS - This stands for Image Stabilization. Oooooh. In short, it helps prevent motion blur in photos. It doesn't get rid of it, since motion blur is associated with shutter speed, but it does help, especially when you're having a hard time holding still while shooting.
Canon has a wide variety of lenses available. Third party manufacturers, such as Tokina, Tamron, and Sigma, make lenses to fit Canon cameras as well. I've heard great things about Tamron and Tokina, and not so great things with Sigma. I've used a Tamron lens before, and it produced high quality images.
The same goes with Nikon, and third party lens manufacturers. I don't know anything about Olympus, Pentax, Panasonic, or Sony's lens compatibility, and that's because most everyone I know shoots with either Canon or Nikon.
Here's the link to the site where you can compare cameras side by side.
More Bathroom Reading
I wrote this up for a friend to help with her picture taking. Eventually, it will all make sense. There are really only five things you control to produce an image - focus, aperture, shutter speed, ISO, and white balance. Or, simplify it to three things - focus, light, and color. Happy reading! A camera primer.