The portrait photographer strives to make an image that satisfies. Photography is an art form and there are no rules in art. However there are tried and true techniques. As to lens choice, the idea to mount a lens that helps creates an image without distortion. We are taking perspective. Actually the focal length of the lens is not the chief indigent; it is camera to subject distance.
Things close to the camera reproduce bigger than things further from the camera. If you work in too close when imaging the human face, you run the danger for distorting facial features. What happens is, the nose reproduces too big and the ears too small. To avoid we simply move the camera further from the subject.
As to focal length: If we use a “normal” or shorter than “normal” lens, we have a tendency to work in too close. This is because we naturally frame the subject in the viewfinder and we have abhorrence to leaving empty space around the principal subject. The remedy is to use a longer than “normal” lens. Such a lash-up will force us to back away. This is the act needed when capturing the human face for correct perspective.
First, what is “normal” as to focal length? A normal lens by industry standards is a lens with a focal length about equal to the diagonal measure of the format frame (digital sensor). Your Canon 500D sports a sensor that measures 14.9mm height by 22.3mm length. The diagonal measure of this rectangle is 27mm. typically we round this us to 30mm and declare this as “normal”. The angle of view delivered is about 45⁰ with the camera held in the horizontal position.
Most professional portrait photographers will gravitate to lens that is about 200% thru 250% longer, which is 60mm thru 75mm. Why is this? Most clients receive an 8x10 or larger print. These are mounted on the wall or mental and viewed from a distance. An image will display correct perspective if viewed from approximately a distance equal to the focal length of the taking lens multiplied by the magnification used to make the print. An 8x10 taken with the Canon 500D will require enlarging about 15× to make this size print. Say you mounted a 60mm, then the viewing distance for correct perspective is 60 × 15 = 900mm (35 inches). If you had mounted a 75mm then 75 × 15 = 1125mm = 44 inches.
Bottom line is: For most images we make, viewing them with correct perspective is unimportant. When it comes to portraiture, though, correct perspective makes or breaks the results. We choose a portrait lens 2× thru 2.5× of “normal” because we are anticipating the final image will enlarged and viewed from a distance.
Art has no rules — you are free to use any focal length and view from any distance.