Teleconverters and Long Lenses
A teleconvertors is a small device that goes between the lens and camera, it contains a lens that spreads out the light so that only the centre of the lens image will reach the sensor or film. This has two effects, it reduces the camera angle, similar to having a longer telephoto lens and as the light is spread out the image is dimmer.
In theory they can be combined together, so adding a 2x and a 3x equals a x6.
Teleconvertors come in a range of strengths. Most often x1.4, x1.7, x2 and x3, and are produced by a range of manufacturers.
The advantages that teleconvertors provide is:-
The largest problem this presents is that the falling off, of the light is such that the camera cannot reliably auto focus. It said to need F8. In practice on a bright day you can get away with f11 on some models, for example the D300, but it stops working on a duller day. You can still use it but then need to manually focus.
Some say they don't feel the picture quality is as good, but when you work at longer lengths, and there is more air, dust etc the image tends to be less contrast, on many days in any event. If you compare it with a long lens often a cheaper make than your usual lenses, the results are at least as good.
Like any long lens we have to remember to stabilise it or use a sufficiently high shutter speed, as well as remembering that our depth of field is extremely limited. All long lens photography is a challenge and the means we use to get the long focal length is only part of this.
So lets look at some examples
In the tables below the green rows will auto focus while the reddish either wont or may not not reliably.
The effect of adding a range of teleconvertors to a 18-200mm F3.5-5.6 VR lens, with the lens at 200mm.
The effect of adding a range of teleconvertors to a 300mm F2.8 lens
The effect of adding a range of teleconvertors to a 80-400mm F4.5-f5.6 VR lens, set at 400mm
Old cheap Tamron manual focus screw fitting and screw to Nikon converter 200-500mm f6.9, set at 500mm
We have a couple of old screw fitting teleconvertors x2 and x3 and can also of course use the Nikon fit x2 as well.
From this series you can see that if you start with a f2.8 lens adding a teleconvertor is a possibility, and you still have a fully workable system. You can add a x1.4 to nearly any situation and it will normally still work, while a x1.7 is border line working on brighter days, and x2 hardly ever being able to autofocus.
There is some variation in what you will see written about the effects of light loss, and I would expect that there would be some variation in efficiency between different models.
Overcoming the auto focus problem sometimes
Most zoom lenses have a 'f' range, for example the 18-200mm F3.5-5.6 VR lens, the difference between f3.5 and f5.6 is about 1.5 stops. With a x1.7 teleconverter at 18mm gives the same amount of light through the lens for focusing the lens at 400mm without it. Therefore if my lens will not focus, all I have to do is zoom wider until it will, then switch off the autofocus and zoom back in.
Another form of teleconvertor
There is also a device, I have seen, that is able to be screwed onto the filter thread of some lenses that is also a form of x2 teleconvertor, and again has the same effect on light levels. This appears to be available in a 52mm or 58mm filter size, so will only fit a restricted number of lens, and won't fit most DSLR lenses. From putting close up lenses onto the front of lenses, it would appear that the quality of the front elements of the lens has a far greater impact on quality to the back end, so non Nikon teleconvertors at the back end I would expect to produce a superior image to a non Nikon screw on front element. Although promoted as being available for DSLRS, its likely that they were developed for non DSLR models where this method is widely used. We are not looking at this type further at this time.
Both Sigma and Kenko have a rage of teleconvertors that will fit different makes. I think Kenco have two ranges one of higher quality than the other.
Nikon teleconvertors and Nikon lenses
As Nikon wanted to avoid problems of people not being able to focus when attaching Nikon teleconvertors to Nikon lenses they have configured them so that if they might not work they will not go together. See the information on lenses, to know which is comparable. This means that the majority of us have Nikon lenses that will not connect to Nikon teleconvertors so we have to look at teleconvertors made by others.
If you don't require extremely large poster sized prints you can section your images, and this is the approach that I would normally use, although I do have lenses available to me that cover all lengths from 10.5mm to 1300mm, plus the teleconvertors on top of this taking me to silly numbers, and the 200-500mm with a further range of teleconvertors, in practice I don't carry them all with me, especially the longest one, that goes from 800-1300 and the 200-500.
Instead if I am going somewhere where I may need a long range my choice is to concentrate on taking my 18-200VR, 80-400VR and take a long a 1000mm mirror lens perhaps and a teleconvertor or two and probably a couple of wide angles and a macro lens. With this and the application of sectioning there is not much that I cannot take on. Of course I am still carrying far more than most would want to.
So how effective are teleconvertors
See also an article that is a practical comparison, with examples, between a 18-200mm lens, a 80- 400mm lens, a 80-400 with a x1.7 teleconvertor added and a 1000mm lens. Click here to get to this article.