Back in the day when a SLR was a camera that shot 35mm wide film, sold in a small metal canister with the end of the roll sticking out like a tongue, there was a "standard" for cameras that used lenses that focused on this 24 x 36mm image area and all was good. Then came the age when a camera with film was not desirable because a competing format, called digital, came to be. The new format offered benefits of "on-the-spot" image review and computer-based photo "developement". And so things where changed.
But the new digital format proved challenging because the camera had always been an instrument which had aided the imaging of a photo medium, or film. Though the new camera would also image the scene, it too would be the "film" and with a sensor where there once was film, capture the image. And that too was good.
Yet, the imaging of the sensor proved challenging because the size of the sensor made a big difference. If the sensor was still desired to image the film area of 24x36 it must be made large and this was a big challenge for the scientists. Eventually, such a sensor was made and became known as the "full frame" sensor. Full because it was sized make it image as the old 35mm film had imaged. And so the, the 50m lens attached to this camera, with its 35mm sensor, gave the same "normal" perspective as had been. And people where happy.
So it was and the digital camera existed with lenses that could be used with camera's designed to accept their fitting but whose performance (specifically their angle of acceptance) was determined by the size of the sensor which received the focused light.
The world of photography had changed forever but in this new world of photography the manufacturers continued to name the lenses with the "terms" that where from the age of 35mm film. In that age, the 50mm lens was generally considered a "normal" lens because it approximated the viewing angle that a human eye could see. A big "iris" combined with a short "back" focus gave big advantanges for collecting light where little exists. The comparision of the size of the opening and the "back" distance is described as a function using the initial "f". F1.0 is a wide open lens which loses nothing to the "human" eye. As a extraordinarily well evolved species, as far as light is concerned, a lens of f/1.0 is extremely rare. But a lens of f/1.8 is very "wide" open when used to its fullest potential. You, the photographer, can choose to use this capabililty but like most things in life, that wide opening comes with compromise. The bottom line, however, is that the greater the front opening was when compared to the distance to the back, the more light is was said to gather.
The 50mm Canon lens you mention , has a "normal" perspective for a "35mm" camera. However, your camera contains a sensor which is smaller than this "35mm" area . So instead of receiving the full image, it receives that portion of the image that the sensor covers. In this case, it is the center-most area with a size smaller than the 35mm standard. (Remember, it was the manufacturers that decided to continue to use terms that came from the age of 35mm film.)
If you want to imagine the size of the sensor in your Rebel XT, you must first image two frames. The larger is the 35mm frame which has outside dimensions of 36 (w) x 24 (h) in millimeters (24.5mm = 1 inch). The second box is located immeditately inside the larger box and it has a size of 22.2 (w) x 14.8 (h) in mm. This second box is known as APS-C for Canon. (APS-C = Advanced Photo System - Compact). Both have a similar shape because both 35mm and APS-C formats have the same relationship of width to height.
A "normal" lens is only normal when using the language that was created during the time of 35mm film photography. When you capture a smaller "frame", as described in the previous paragraph, the angle of view changes. Like an eye that has closed slightly to "focus" on a subject in the center, the smaller APS-C sensor captures a narrower field of vision and the difference between the narrower field and the 35mm standard can be said as a multiplier, or factor. The Canon APS-C factor is X1.6.
Instead of seeing the same range of "normal" vision that a 50mm lens sees when focusing its light in a image area 36mm x 24mm in size the 50mm lens gets most, but not all, of that image. It gets the center part which shows an area with a size that is equivalent to what a 80mm lens would see on that 35mm camera. (Really, it was the camera manufacturers that insisted that we continue to describe everything in terms of the 35mm film cameras!!!)
When you capture the center of something, you leave the outsides behind. If you look at this new image at the same size as you looked at the image before it was changed, it will look like everything got bigger. That is why a 50mm lens is described as being "equivalent" to a 80mm lens (50 x 1.6 = 80) when mounted to a Canon camera containing as APS-C sensor..
The lens you mention will photograph a scene to capture the equivalent of a 80mm lens, making objects larger than normal, and it will do it with the maximum "eye" opening that is a ratio of f/1.8. For capturing images in low light (a dimly lit room) it is best to use a camera lens that has a maximum opening that is described with a number less than 3 (2.8 to be exact). Your lens can do that and objects will appear larger than "normal" when captured with the combination of your APS-C sensor and lens.
Good luck.
But absolutely more than any 1.6x crop factor which was my intention.
