Originally Posted by Roy C
I am very seriously considering a 500mm f4 lens instead of my current 400mm as a means of getting more 'reach' with my 40D.
Would you say I am better off sticking with the 400 and upgrading to a 50D? If so that would be great as I could save myself over £3k.
You don't actually gain all that much extra reach with a 500/4 as compared with a 400. A bit, yes, but nowhere near enough to justify the expense (huge) or the weight (massive). Of course, you can add a 1.4 converter for 700mm f/5.6, and that becomes significant, but look at it this way: you will get more detail with a 50D & 400mm than you get with a 40D and 500mm.
But the 500/4 isn't really about extra reach. Most of the time, the reach you gain is lost before you get it because you simply can't move as quietly and unobtrusively with the big white as you can with a 100-400 or any other 400/5.6-clas lens. It's big, it's heavy, and you can't hold it up for long so you wave it around a lot and, in the end, you don't get as close to the bird.
What you do
gain, above all else, is that vital extra stop. The difference between having f/4 available and being limited to f/5.6 is massive
and a lot of the advantage applies even if you are shooting at f/8. With an f/4 lens your camera AF system (any camera) is faster and more accurate, and this really does make a big difference. With an f/4 lens, your ISO 800 shots are taken at ISO 400, and your ISO 1600 shots at ISO 800 - and your can't-take-any-shots-it's-too-dark shots becoume tough but do-able.
Perhaps a good way of putting it overall is to say that with a 500/4 every hour you spend walking around looking for birds and trying to get into position for a shot is pretty damn painful, and every moment when you actually have a bird in frame and are ready to take pictures in unalloyed joy.
I've posted a number of times here to say how impressed I am with the 50D, and I am. But neverteless, the difference between a 50D and a 40D (or a 50D and a 350D) is one only of degree. Stepping up to a 500/4 is a difference of kind
Sooner or later, if you are keen, you will get a 500/4 (or something else in that general class). It will last you for many years, much longer than any camera. You might as well do it now while you are still young enough and strong enough to carry the damn thing for a few more years!