I need a new zoom lens & I am trying to figure out what the increase in the size of a subject would be with a longer lens. For example, I was trying to photograph a woodpecker at 75 feet with a Canon 24-105. Subject much too small. Would the subject appear twice as large in relation to the outside measurements of the photo, in doubling the length of the lens? I am not looking for a suggestion for a lens specific to bird watching -- I just want to be able to compare several different lens lengths, with some understanding. Is there an algorithm? A chart?