I would be more than surprised if that were true.
From other reports from other users who have had both TC's (I have never had the TCII, only the TCIII), they have said that the new TCIII does allow for faster focus. I can only assume that the extra contrast afforded by the new design is responsible. In my experience using the TCIII, I have found little difference in focus speed when using or not using it. There is a very slight difference in normal light and this is slightly worse again in lower light, but overall the difference is minimal.
That's true, and there are sound engineering reasons why AF operation must slow down when a TC is added, however I should point out that the degree of change may be camera-model dependent. Some cameras, such as the D90, already slow their AF down so much at the 200mm zoom setting, that adding a TC requires little change. Higher-level bodies such as D3 will be better optimized over a range of lenses and lens/TC combinations, so you should always see a significant change in AF speed when adding a TC to the D3.
The D3 should perform better than this, even when using the 400mm setting. One of my high-use D3 bodies developed an inability to focus with this lens/TC combination, and it was due to mechanical wear-out affecting the AF mirror position. You may want to have Nikon examine your camera with the lens/TC combination attached.
This sounds very strange. If the image isn't in focus, the camera should at least go into focus search. No focus movement at all, and no focus search, would imply that the AF system thinks focus has already been achieved.
You're not reducing the light to the AF sensors, until you make the lens slower than f/5.6 - and in that case, it can vanish completely. The AF sensors only accept light from the f/5.6 circle of the lens exit pupil. Lenses faster than f/5.6 create a brighter image for the main sensor, or the viewfinder, by adding light which arrives from the area outside of the f/5.6 circle; the AF sensors do not use this light.
This time, I will agree with you. There would generally not be any reason for one 2x TC to perform better with AF than another.
This AF mirror is in a fixed position inside the body, isn't it ? Mechanical wear-out implies movement. How is this possible ?
There is a fixed mirror below the mirror box, just in front of the AF sensor module, but that's not the one I'm referring to. It's the mirror attached to the back of the reflex mirror which moves, and its alignment depends upon the reflex mirror alignment. The reflex mirror pivots wear down quite a bit; if you're interested, I can provide some photos since I have saved the original parts from my D3.
I've never heard anyone make the claim that it doesn't reduce the light to the AF sensor.
Hmm, my D700 and to a lesser extent my D7000, focus in lok ow light with slight slowness, depending on how low the light is and what the subject matter is like. Sounds like an issue with your camera?
True.
A TC takes the centre part of the image and magnifies it, it thus uses only part of the light and discards the other part. But this naturally is not a uniform reduction in light (as a ND filter would do) but a 'geometric' cut of some of the light. Depending on whether the light needed for PD AF operation (henceforth abbreviated as AF) is cut, is a question of the original lens f-stop and converter magnification, ie, either the TC does not cut any of the light needed for AF or it cuts all, notwithstanding naturally smaller effects like transmission losses and secondary effects like increased vignetting.
Could be some slight differences in optical or mechanical alignment, could be converter firmware having an influence.
Two things to explain:
No, and I would be interested to know what Nikon did as the repair action. Other than mechanical wear-out, I haven't had any AF malfunctions.
Incorrect. As I already commented in my previous post, lenses much slower than f/5.6 will project no light onto the AF sensors. This is because their exit pupil is smaller than the f/5.6 ring, where the AF sensors are aimed. This is the same reason why the split-prism focusing aids found on MF viewscreens go black when a slow lens is used.
Again, as I already posted elsewhere on this thread, the behavior you describe suggests an AF alignment problem with your camera. See the post below by RGT5.
They are not "at" the f/5.6 ring, they use the f/5.6 ring of the exit pupil. This is done so that AF can remain operational with lenses as slow as f/5.6. Nikon AF systems do not include additional sensors aimed at wider rings, such as the f/2.8 ring, so they cannot benefit from the extra light gathered by lenses faster than f/5.6.
Here is something else you can try at home:
