I am considering buying either the Canon EF 28-135mm or the Tamron 28-75. I don't fully understand the Tamron in that it is f2.8 full time. I acknowledge this could be beneficial in low light but as there is no image stabilisation on this lens exposure times will be required to be fast. My question is this. If it is f2.8 full time does that mean that the focal length is permanently short so taking a shot for example of a group of people, or a landscape, the shot will have more out of focus areas than a lens that stops down? Does the camera compensate for this? I hope this doesn't sound elementary (it probably is to the informed) but I genuinely can't understand this point.
