First off, I'm sorry for the slide-show, but I've had this lens a long time now and I'm fed up with it. A while back I tried adjusting this ring to either extreme and couldn't see a difference.
No matter what I do I cannot get SHARP pictures with this lens. The view-finder will confirm I'm in focus. Even if I use my camera's live-view I cannot get very sharp pictures with it.
As I said I've had this lens a while and in that time I can now just "lift the camera" to my face and look at stuff, I've also gained quite a steady hand, I can usually go as low as 1/400 with 1500mm and 1/160 for 500mm before my movement becomes a problem. To eliminate that though, the 1500mm pictures were shot at 1/4000th and the 500mm at 1/1000 (the day wasn't so bright)
First picture, here's what I think to be some sort of calibration ring, when I first tried it (according to a guide I found online) I tried either extreme, middle, you name it but couldn't really see a difference. However it seems to move back and forth – so probably calibrates something!
The ring I am referring to here moves if the screw just to the left of "FEET" is loosened, not sure about the collar behind that.
500mm pictures 1/1000th shutter This is probably the best one, in the view-finder I could see the circles that indicate a good focus, the rangefinder dot was on, however if you zoom there's not much detail there!
1/4000th shutter With both of these, I took a series of pictures moving the focus ring ever so slightly, these are the best ones.
1500mm pictures All 1/4000th
As you can see, they're VERY fuzzy, especially the bottom one (for some reason) – I simply cannot get better than this!
I'd really like to get nice pictures of the moon, but I really cannot get past this. I am sorry for the slide-show, I want to convey "look, it isn't just out of focus!"
Note: I had to scale these pictures quite a lot, if you want a link to a full-size one, just tell me.