OK, this is going to get a little long and complicated but please stick with me till the end! I will say straight away though that I am not 100% sure why the Kenko does not cause as much hunting, nor have I fully considered all the details of tcs and AF system interaction before now but I will try to explain my thoughts.
Lets start with a fundamental and often misunderstood concept: hunting is caused by the camera's AF system
not the lens itself - the camera can't find the correct focus point so tells the lens to 'scan' so it can try and find it - notice how the lens AF motors work more slowly when you have little light or 'low contrast' situations? The AF sensor needs more time to find focus.
(D)SLR AF basically works using a phase detection system. Keeping it simple, a pair of sensors measure the light received from each side of the lens. The system then 'measures' the light and dark areas and compares the readings from both sensors – i.e. each side of the lens. If they are the same ('in phase'), correct focus has been achieved (it works in fundamentally the same way as a split focus screen on a manual focusing SLR). You can imagine that each of the focus points you see in the view-finder is a pair of those sensors, so when the light is 'in phase' for that focus point, the camera has achieved focus.
The AF system will work better the greater the distance between the sides of the lens (just trust me on that - this is getting too complicated to explain without pictures). So the widest aperture of a lens determines how far apart the sides of the lens are (that is a
huge over-simplification but you get the idea). Most AF systems require an aperture of f5.6 or bigger because any less and the AF sensors won't get enough 'separation' to allow a good comparison. This means that, as well as greater amounts of light (more light means easier determination of light/dark areas), lenses with wider maximum apertures have a greater distance between the lens sides, so faster/better AF operation.
OK, assuming that made any sense at all, let’s look at using teleconvertors (tc). I am going to assume we are using a 1.4x tc, which reduces the aperture by one stop. Take one f5.6 lens, add a tc and strap it to a 30D…the AF system is disabled because the lens/tc combination is reporting a maximum aperture of f8 to the camera. The AF system is not ‘designed’ to work with that small a distance between the lens sides and the light may miss the sensors all together!
When you ‘tape the pins’ on a tc, the lens/tc reports that the combo is f5.6 and the camera leaves the AF system switched on. As luck would have it, the AF sensors on the 30D seem to be able to handle an f8 lens (makes you wonder if the next generation of the 30D will support AF at f8 as a ‘new’ feature…). From memory, the 350D didn’t handle the f8 combo as well – I can only assume that the AF sensors are even less designed for that small a gap between the lens sides.
Finally I can try to answer Roy’s question. For lenses faster than f5.6 the AF sensors are ‘adjusted’ to handle the extra light and greater distance between lens sides. A slight side note - one of the ‘breakthroughs’ on the EOS 3 was that the AF sensors could work even faster for f2.8 lenses – which has now been carried on in the 1 series bodies and even the 30D (I think).
Why does this matter with taped pins? When using an f4 lens (like the 70-200/4) Canon’s AF system will ‘adjust’ the sensors for the light and width from the f4 lens. By adding the tc you are making the lens/tc combo f5.6 but by taping the pins on the tc, the AF system is still expecting an f4 lens…so the sensors are going to have a tough time with less light and less distance between the lens sides - it is going to cause the sensors to hunt.
Now, why does the lens hunt less with the Kenko and taped pins? I don’t honestly know. I can only assume that the Canon tcs send extra data or manipulate the data from the lens in some way whereas the Kenko one doesn’t (they will make the insides of their tcs the same irrelevant of the camera manufacturer the mount is for). Does this mean that the Canon AF system is designed to work fastest with Canon branded tcs and lenses? Probably – it gives them a competitive advantage! Following that thought through, having the Kenko tc on the camera may mean that the AF system is operating in a generic ‘safe’ mode that allows AF at any aperture of f5.6 or wider…so taping the pins has little effect on the faster lenses.
Whew…I went on longer than I intended and congratulations to anyone who read through to the end and I hope it made some sense to someone.
BTW, before any ‘well actually’ responses, I know that ‘distance between the sides of the lens’ is not a strictly accurate description, but it is a simple way to think/write about it without diagrams…
