Camera Types and Parallax Error
When you're looking to purchase a camera, you'll need to know a little bit about different camera types. Some cameras will let you see exactly what your camera sees, and exactly what will appear on the film or digital image file. Other cameras will not.
The difference between what some cameras show you through the viewfinder and what the picture will actually look like is called parallax error. Some cameras that suffer from this problem will have marks in the viewfinder that will give you a good idea about how to adjust for this problem. Usually it's not a huge difference, but when you're doing up close (or macro) shooting, parallax error is an insurmountable hurdle.
Point-and-Shoot CamerasPoint-and-shoot (or PnS, P'NS, PNS, etc.) cameras are what your average person that's not serious about photography will have. They do everything automatically for you, and generally you can't override its automatic functions. They're popular because they're cheap, compact, and easy to use. But the viewfinder is a very simple piece of glass--and these cameras suffer from parallax error.
However, with a digital point-and-shoot camera, you will have the benefit of the LCD screen. This screen will show you exactly what your camera sees. In other words, it doesn't suffer from parallax error. So if you're using a digital point-and-shoot, remember to always use the LCD screen, not the viewfinder.
Twin Lens Reflex CamerasA twin lens reflex (or TLR) camera, like my new toy the Yashica MAT 124, also suffers from parallax error. These cameras have two lenses of identical focal length placed one above the other. The top lens is the one that shows you what you see in your viewfinder, and the bottom lens is the one that exposes the film. Focusing is not a problem because they are both connected to the same focusing mechanism, and are synched up by the manufacturer.
Aside from parallax error, TLRs also suffer from a reversal of the image in the viewfinder, although this is not a big problem once you get used to composing images with the camera.
A positive attribute of a TLR is that the viewfinder doesn't go blank when you're taking a picture. However, this advantage is limited in practice because of the image reversal and the fact that it's only really helpful when taking action shots. Some TLRs attempt to make up for this by including a "sports finder," which is essentially just the same thing as a PNS viewfinder (which generally won't hide your view while snapping a shot), so it doesn't take advantage of this factor.
Single Lens Reflex CamerasA single lens reflex (or SLR) camera is the weapon of choice for most professionals and serious amateurs. These cameras do not suffer from parallax error. This is because the same lens that exposes the film is also the lens that shows you what you see with the viewfinder. They do this because of a mirror that moves out of the way as the shutter is released. This can cause problems because it causes slightly more camera shake and it is quite noisy, but the advantages it gives are very much worth it.
SLRs are also favored because of their quality and adaptability (through various accessory lenses and filters, among other things).
Rangefinder CamerasA rangefinder camera is an unusual thing these days. They were popular particularly in the 1950's, and some higher end (esp. Leica) models are still popular today. Earlier models would tell you what distance the subject is from the camera and allow you to set the lens's focus to that distance. More recent models will show you two images in the viewfinder, and you turn the focusing ring until they match perfectly.
The problem is, though, that the viewfinder is separate from the camera's lens, and so these cameras also suffer from parallax error--and this factor contributed largely to their fall from popularity (superseded by SLRs). Most will have some mechanism for correcting parallax error, but these are not effective at close distances.