DSLR/Lenses and Laptop for vacation? - Calling all LiquidRetro's

Quote from APhamX : I really don't see why people would buy the 20mm or so lens when this is a rang eof 17-50mm wide angle. I mean it's same /f and everything. Is it just because of focusing/size differences? Anywho I'm researching on it right now, I have three months left to look for a lens.

The 17-50mm lens that you mention is for crop cameras only. If you were to mount it to a 35mm camera or full frame camera, like a 5D, you'll probably see mechanical vignetting, where the light is physically blocked by the lens itself.

The Rebel cameras are what are known as a crop cameras. This means that the sensor is smaller than 35mm film and is equivalent to cropping film down. Cropping film effectively shrinks the angle of view. The angle of view determines how far to the left/right/up/down/diagonal you can see in your image. A large angle of view shows you more from left to right and up to down, whereas a smaller angle of view shows you less (also sometimes referred to as magnification or "zooming in"). Focal length determines the angle of view.

The "crop factor" is an approximate way of determining the equivalent focal lengths between full frame and crop cameras. Canon crop cameras have a crop factor of 1.6. All of the dimensions of the crop sensor are basically x/1.6 of 35mm film. Multiplying a focal length by 1.6 gives you an idea of the equivalent focal length to get the same angle of view on a full frame camera.

For instance, the 17-50mm lens is equivalent to a 27-80mm lens on a full frame camera (of which there are tons of varieties).

That being said, a little history on the matter should be discussed. Wide angle lenses were still rather exotic back in the 1980s. 20mm was considered the most affordable wide angle focal length before you started getting into exotic focal lengths. As you got smaller and smaller, the price jumped up incredibly. Canon had a 20mm lens for ~ $500 and a 14mm lens for ~ $1400. Needless to say, that's quite a jump. Nikon also had several lenses <18mm that became increasingly expensive, including a 13mm lens that is worth several tens of thousands of dollars.

When the EF 20mm f/2.8 was released, in JUne 1992, there were only a few other wide angle lenses: Canon EF 20-35 f/2.8, Canon EF 14mm f/2.8 and Canon EF 15mm f/2.8 fisheye. The 14mm f/2.8 was several times more expensive than the 20mm f/2.8 and probably more expensive than the 20-35 f/2.8 zoom. It wasn't until the mid-to-late 90s that focal lengths less than 20mm become common and affordable.

To go back to the crop factor, a 20mm focal length on a full frame camera is equivalent to a 12.5mm focal length on a crop camera, like a Rebel. Ultimately, 20mm isn't particularly useful on your rebel, but the 20mm lens wasn't designed for the crop format either. However, that doesn't preclude it from being useful on a full frame or 35mm camera. The same can be said of the Canon EF 17-40 f/4L. It has a smaller focal length range, smaller aperture and is probably larger and heavier. It's easily overshadowed by the 17-50 and even the 18-55 f/3.5-5.6. However, on a 35mm camera or 5D, it's a completely different look. It would be similar to how the EF-S 10-22mm looks on a crop camera.