Originally posted by little laker What size is the lens cap. From what I understand a tube will make the object appear larger, however up won't be able to get any closer to the object.
A teleconverter will make an object appear larger at a given distance from the lens by increasing the focal length of the lens. Very often it will also increase the minimum focusing distance though. As a result TCs aren't seen in macro work very often (except for the speciality Vivitar Macro TC) because they add extra optics and slow down the lens.
An extension tube will allow you to focus closer in, fundamentally allowing the object to appear larger.
The extension tubes may or may not allow you to focus in to within 5 CM, but keep in mind that due to the longer focal length of the original poster's lens, the subject will be the same size at 10-15 CM as his friends' lens at 5.
As another person mentioned, you will get 1:1 magnification with the lens focused to infinity when the extension tube length equals the lens focal length (50mm for a 50mm lens), but you can of course move the lens from infinity focus to get even closer.
For macro photography, most people tend to prefer longer lenses (50mm and longer) because they allow for greater working distances. Many of the extension tubes will produce undesirable effects with shorter lenses in addition to the issues with "too close" working distances.