The focus issue may need some clarification. When Nikon talks about AF working, they are referring to the lens wide open and not your aperture. The Nikon system uses a wide open aperture for AF, then stops down when the shutter is depressed. That's why the DOF preview button is important. So when you are talking about AF working at f/8 on new camera bodies, it is looking at the lens and teleconverter wide open regardless of your aperture setting.

With a variable aperture lens like a 70-300 f/4.5-5.6 VR the f/5.6 aperture applies to the long end of the zoom - the place where you want more reach. So losing a full stop with a 1.4 teleconverter puts you at f/8 - an area where focus is only reliable with a few of the newest cameras. So even if you chose the Kenko teleconverter which will physically fit, you are at the limit of practical use.

The other thing to keep in mind is that not all AF sensors are the same. The center sensor is the one that produces f/8 AF. The adjoining cross type sensors produce AF at f/5.6. And the remainder AF at f/4. So if you want to try a teleconverter, you are probably going to want to use the center AF sensor. That is one reason why many of us use the back button (AF-On) to focus - so we can focus and recompose quickly. Also keep in mind focus reliability is not as good as you move away from the center sensor. You will get more images with missed focus and a wider range of AF error.

Many of the zoom lenses are not as sharp at the maximum zoom or long distances as they are in the sweet spot. But when you are using a teleconverter, you are often at maximum zoom - so of course the images are going to be softer. Add the normal image degradation of a teleconverter, and you end up with soft images unless your technique and conditions are optimal.

Finally - a teleconverter is going to reduce aperture and extend effective focal length. The longer focal length means you need a faster shutter speed to freeze any vibration or movement, and you need to offset the smaller aperture. So in practice you need enough light to add two stops to whatever you had without a teleconverter. This is true for all lenses - even those that accept teleconverters - and is part of why teleconverters are not used more often.

So I agree completely with the other posts. The teleconverter does not physically fit, AF is not as good or non-existant, you need more light, and image quality drops. You can overcome the physical fit with the Kenko Pro teleconverter, but the other factors remain.

I have the Kenko teleconverter and almost never use it. It is the most useless piece of equipment I own not because it is a bad teleconverter - its just the occasions when I have used it produce sharp images less than 10% of the time. That's too unreliable to be useful. I'm better off living with the constraints of the situation and gear, and working on techniques to get closer.

If you are on a tight budget with medium consumer lenses, the 70-300 ED (not VR) version is available used for around $175. It's a pretty good lens that has been around a long time. While not as good as the newest version, it is better than trying to turn a shorter zoom into something its not.

to specify Garteth post - most current bodies - from d200 can (in good light) focus with f8 or smaller (officially only d4, d800 and d600), though the focusing speed (AF tracking) is slower. what does it mean to TC users. basically, all f4 lenses can be used with TC20. all f2.8 lenses can be coupled with TC20 and TC14 - this option is also not supported by Nikon and requires TC trimming (I would say it's advance use of TC's).

wow!this is a great info Gareth...& i am totally agree with you dude.

Anytime I've considered buying a particular TC to use with a particular lens, I always send Nikon Tech an email and give them the model numbers and ask them what they think; They have always been pretty good about answering me.

A very lengthy and informative article on this topic can be found on Lens Rentals site, written by in-house guru Roger. I can't put the link here because I can never remember whether they're a sponsor, so you'll have to Google it. Sevencrossing does a good job of summarizing the issue though.

I will warn against trying ones that aren't on the list...I would make certain they will work. You will scratch the rear element on your lens by say putting the 18-200 on a teleconverter.

So two factor...light loss as mentioned and space between the rear element. General rule...constant F2.8 to use any of them. Constant F4 is decent with 1.4 and 1.7, but too much loss at 2.0. I don't know of any variable aperture lenses that are compatible, but I haven't looked into it much.

Teleconverters reduce the aperture. A f2.8 lens with x 2 converter becomes f 5.6

The smaller the aperture the slower the auto focus

The auto focus does not work well below a certain aperture, so for example the x 2 only works well if the main lens is f2.8 or wider, this rules out all "consumer" zooms
leaving you with the 70-200 f 2.8

If you have the 200-400 f4 , Nikon, say auto focus will only work with x1.4 but people do use this lens with the x 2, although the focus may be sluggish

the latest Nikon D800 and D4 are ment to have better auto focus at smaller apertures

remember autofocus happens with the lens wide open ,

there needs to be enough space for it to fit for one thing. i.e. the back element is far enough forward in the lens for the tc to slot in behind it.

then there are issues of aperture for af, which nikon doesn't over commit itself to. they don't want people complaining when it doesn't af, they'll just say they never said it did. that doesn't necessarily mean it wont though.

What determines teleconverter - lens compatibility?

Looking at the "official" Nikon list, not that many lens are compatible. However, I keep seeing comments about various lens not on Nikon's list being used successfully with a TC.

Is Nikon's official list a marketing effort to keep people from using the TCs except with the most expensive lens, or is there something else going on here?

Intuitively at least, I would think that an IF lens would stand the most chance of compatibility, as nothing external moves...

Can someone please clarify?