Hi Guys,
Les, you hit the nail right on the head, after forking out 71/2K for a lens there is no way that I would expect the poor thing to works to its best when I hung a $600 extender on the back of it. Turns out that the lens alone does work superbly on its own and needs a -6 correction because it was front focussing with the extender added.
VR was not a problem because it wasn't turned on and never is unless I'm shooting below 1/640 shutter speed.

The real proof of the pudding will be when I take it back into the field this week and see how we perform as a team, handheld??
More to follow. . .
Bruce

Hi Bruce and Les. Glad you've made some progress. Some more background on AF microadjustment might be useful. The reason it is needed, as Ian Wilson once explained to me, is that the AF sensor in the camera has it's own small lens in front of it. The sensor sits in the bottom of the camera. AF does not use the main imaging sensor. If it did I think there would be no need for AF microadjustment.. The separate optical path for AF means that what the AF sensor sees is not what the imaging sensor sees. AF can be spot on but that does not guarantee a sharp image on the imaging sensor. So that's why we have AF microadjust. It will vary from lens to lens (and different copies of the same lens). It will vary with a teleconverter added. The need for AF microadjustment is a result of a less than perfect bit of optics in front of the AF sensor. It should not be assumed to reflect a problem with the main lens you are using although it can still contribute.

As for using image stablisation, the only comment I can add is that it is certainly critical at lower shutter speeds. I found out the hard way one day when I accidentally switched it off on my bird lens. Not a good outcome. I know less about positive or negative effects at higher shutter speeds but would be surprised if it did degrade image quality in that situation. I'd be interested to know more about that though.

Geez, thanks Glenn,
I'm learning more and more about this stuff every day. And from the experts too!
Glenn, the only reason that I have the VR turned off is the time factor. I use rear button focus and I haven't yet found out whether pushing the rear button also starts the VR system, everyone that I have asked asumes so, but no-one can catagorically state yes or no? Now if the VR does come on with the push of the rear button, well and good but if it comes on when I push the shutter button and suddenly swings into action as the image is being recorded, is this going to affect the focus that was already locked in?
If someone can give me a definitive answer on that, well I will leave the VR system turned on all the time.
Cheers Guys,
Bruce

VR does work when using the back button. As regards why turn VR off if you are shooting at speeds above 1/500th of a second, this was Thom Hogan's explanation some years ago, when asked about it:
"Don't you get some effect from VR even if your shutter speed is above 1/500? After all, the VR elements are probably moving between samples." Yes, sometimes you get a VR-like effect above 1/500, and it's probably because the elements are in near constant motion and the designers have picked a movement frequency and smoothing curve that takes advantage of the known sampling frequency. But the problem with using VR above 1/500 is that you will get clear image degradation often enough that you'll get burned by it. And I believe you get burned by it more often than you'd get burned by having VR off. Again, Nyquist tells us that when we sample something, we can only be "precise" about our data at one-half the frequency. Above that you don't get useful data, and a mechanical system can induce ringing effects as it tries to adjust. Let's see if I can explain it simply (a very gross generalization and simplification coming up): Pretend we're moving enough to impart a different motion that needs new correction ten times a second. Assume we're sampling five times a second. So what if we move Left, Right, Down, Left, Up, Right, Right, Left, Down, Up? The sampling sees Left/Right, Down/Left, Up/Right, Right/Left!, Down/Up! See the problems? If we take images at ten frames a second, the system is lagging us in the first few samples and may settle down while we're still moving in the last two. The problem with Nyquist is that there's a strong chance that the system is going the opposite direction you want it to when you exceed the sampling frequency. But, yes, there's a chance that it's going the right direction, too. Not a good enough chance to use VR, in my opinion. Moreover, I don't know of a working sports or wildlife pro using the long lenses that hasn't discovered the same thing by practice: VR tends to degrade shots above 1/500. "
I'm not clear if the sampling frequency of has been changed to address this problem in newer cameras. If you're shooting at 10 frames a second, designers may have had to change things to accommodate this.
The newest cameras like the D850 are now auto fine tuning and correcting their auto focus systems, so the problem of focus with differing lenses will probably be gone from all cameras in a few years. A good thing too.

Hi Guys,

,

Thanks Les.

Bruce.

Bruce is asking for help re Nikon gear about which I do not have hands-on experience. The advice he has been given on VR is not necessarily relevant to users of Canon telephoto lenses. Canon users please be aware that the best advice re image stabilization (IS) is different to what I am reading in this thread for Nikon telephotos. In particular, for avian flight photography, where exposure times are typically 1/2000-1/4000 sec, it is recommended to use the panning mode (Mode 2) image stabilization. Mode 2 works in the vertical plane only and helps with smooth tracking of the bird by ameliorating up and down movement of the target in the viewfinder while panning. More generally, Mode 2 can be used for most situations, including perched birds, irrespective of exposure time. The exception is when one rotates the camera to portrait orientation, in which case the plane of action of Mode 2 will have been rotated to horizontal. For portrait orientation Mode 1 should be used.