Well, I have done some research on this topic, but have found no real answers---so here is the question: Are any of you familiar with a chart, graph or calculation that you can estimate minimum focus distance based on the size of an extension tube and lens? For example, I want to shoot humming birds with my 500mm F4L which has an MFD of 15 ft. What would the MFD be if I used it with a 24mm extension tube? or how much of a reduction in MFD could I expect with a 24 mm Extension tube? I know I can simply go out and do the trial and error thing----But I am looking for some formula, graph or chart that would get me in the ballpark based on the lens/extension tube combination that I am using. If you have any Ideas or refrences please share.


Thanks


Bob