Zoomed boat, 500mm, 100% crop
Non-zoomed boat, 50mm, no crop

I don't know if all of you have thought about the actual difference between framing a shot with the zoom ring on the lens or just cropping afterwards - or how much a relatively inexpensive modern telephoto lens can magnify. I shot a few pictures while out bathing with the kids that I think will illustrate this.

The image with all the sea and sky has a red boat a bit to the left of the middle. See it? You'll probably have to look at the medium or original size (which I actually have set the camera to scale down to 6 megapixels for the jpegs, the RAWs are fullsize) to see it. That boat is the same boat as in the other image. The difference is that in one of the images I had the DA* 16-50 set at 50mm, in the other image I had the sigma 170-500 at 500mm plus I have cropped the image. But that's the kind of magnification and quality you can attain with consumer-level hardware today. I could have gotten tabloid-quality shots of Paris Hilton without her even seeing me. If she had been on that boat, that is. ;-)

And, I might add that I was standing up for both shots with no support except the camera's anti-shake mechanism.