Teleconverters are something I hear many people ask about and something that seems not to be properly understood by many. I think everyone hears about this magical accessory that extends your lens by about 40-100%, but their disadvantages are not as well known. Used correctly and within their limitations teleconverters are an excellent to any photographers kit bag.
Teleconverters are a small device that fits between the lens and the camera. When fitted they magnify the focal length of the lens but reduce light transmission and the performance of the lens. The great thing is they are effectively like carrying another longer lens but without the additional weight. Teleconverters were first designed for use with fast long prime lens like 300mm f2.8, 400mm f2.8, 500mm f4 and 600mm f4. The main camera manufacturer offer teleconverters for their lenses, third party manufacturers also offer teleconverters. The advantages and disadvantages of teleconverters can be summed up by:
Generally is accepted that it is best practice to use a Nikon teleconverter with a Nikon lens, Canon teleconverter with a Canon lens, a Sigma teleconverter with a Sigma lens. There is the exception of Kenko that fits most lenses but you still have to get the compatible version for your lens. I would recommend keeping to your manufacturers teleconverters to ensure future compatibility with existing and future lenses.
The main manufacturers offer 1.4x and 2x teleconverters, Nikon also offer a 1.7x teleconverter. As a Nikon user this blog will talk about Nikon teleconverters but most of what I say will also be relevant to other manufacturers as all teleconverters work using the same principles.
Nikon offers three different teleconverters (above reference image: Nikon USA), each of these versions I could recommend using with Nikon lenses . There are other older teleconverter versions that can be used but the four listed below I consider as the best performers:
The Nikon TC-14EII/III teleconverters will magnify the lens focal length by 40% (i.e. a 300mm lens effectively becomes a 420mm lens) but at the cost of losing 1 stop of light at its widest aperture (i.e. f2.8 to f4, f4 to f5.6).
The Nikon TC-17EII teleconverter will magnify the lens focal length by 70% (i.e. a 300mm lens effectively becomes a 500mm lens) but at the cost of losing 1.5 stops of light at its widest aperture (i.e. f2.8 to f4.5, f4 to f6.7).
The Nikon TC-20EIII teleconverter will magnify the lens focal length by 100% (i.e. a 300mm lens effectively becomes a 600mm lens) but at the cost of losing 2 stops of light at its widest aperture (i.e. f2.8 to f5.6, f4 to f8).
Teleconverters will degrade the image quality (contrast and sharpness), the 1.4x teleconverter (TC-14EII/III) will degrade image quality less than the 2x teleconverter (TC-20EIII). The same can be said for auto focus speed, the 1.4x teleconverter (TC-14EII/III) slows the auto focus speed less than the 2x teleconverter (TC-20EIII). The 1.7x teleconverter (TC-17EII) lies somewhere in between the 1.4x (TC-14EII/III) and 2x (TC-20EIII) teleconverters.
Generally the 1.4x teleconverter (TC-14EII/III) will affect the lens performance the least but the compromise is the lowest increase in focal length. The choice of which teleconverter to choose is dependant on what lens/lenses you own and how much of a compromise you want to make on light gathering to focal length increase. To see how teleconverters affect auto focus speed performance there is a good video on you tube that shows a comparison of the 1.4x and 2x teleconverters, it can be seen here. Its clear to see the 1.4x teleconverters affect auto focus speed the least and the 2x tele converter the most. What is interesting to see how much different lens are affected. The video compares the Nikon f4 PF VR lens and the 70-200 f2.8 VRII, its clear to see the Nikon 300mm f4 PF VR lens is affected more by teleconverters than the 70-200 f2.8 VRII. The 1.4x teleconverters don't seem to have as much affect on the 70-200 f2.8 VRII as the Nikon 300 f4 PF VR.
The conditions you normally can also affect performance. If shoot in good light the compromises of the 2x teleconverter may not be too much of an issue. If you are shooting in low light a 1.4x teleconverter may be a more suitable compromise when trying to keep the aperture wide open, shutter speed high and ISO low. The 1.7x teleconverter may be a 'goldilocks solution' where you are looking for the middle ground.
Which lenses you own also plays a part in which teleconverter you chose to use, generally the faster the lens the lens performance is affected. Teleconverters will affect a f2.8 lens less than a f4 lens, a f4 lens will be affected less than a f5.6 lens.
If you have a Nikon f2.8 long prime lens I would recommend using any of the Nikon teleconverters. The choice of which you decide on will likely depend on if you are going to purchase one or two teleconverters. If you are going for two the 1.4x and 2x teleconverters are a good pairing, I personally don't think there is enough difference between the 1.4x and 1.7x teleconverters or the 1.7x and 2x teleconverters as pairings. If you are going for only one teleconverter I would recommend either the 1.4x or 1.7x teleconverters depending on the compromise you are will to make. You could of course decide to go with a 2x teleconverter as it would give the most increase in apparent focal length but its use would likely be reduced as it would need better light compared to the 1.4x or 1.7x teleconverters. It all depends on the compromise you are willing to make.
If you have a f4 lens I would recommend the 1.4x teleconverter as although Nikon's newer cameras now allow auto focus up to f8 I find I would rather not be restricted to a maximum aperture of f8 if light levels drop. Some Nikon f5.6 lenses, like the new Nikon 200-500 f5.6, allow the use of Nikon teleconverters. Personally I would only use the 1.4x teleconverter with this lens only if the light was very good and I really needed the extra apparent focal length. The 1.7x and 2x teleconverters wouldn't be able to guarantee working and accurate auto focus on a f5.6 lens as they would take the lens beyond the maximum f8 compatibility.
One question I regularly see being asked is which teleconverter would be suitable for use with a variable aperture zoom lens like a 70-300mm f4.5-5.6 VR or 55-200mm f4.5-5.6 VR. Its likely any teleconverter would badly affected the lens auto focus performance and image quality. There is also the possibility a teleconverter may not fit on a variable aperture lens. The lens rear elements protrude too far back and would cause the lens rear element to come into contact with the teleconverter glass element. I wouldn't recommend using a teleconverter on any variable aperture lens like these, its best to save the money to put towards a faster lens.
Although teleconverter and lens compatibility may at first seem to be a nightmare to work out which would work with the lens/lenses you own, thankfully Nikon have made this easier by producing a teleconverter compatibility list, this list can be found here. You may notice the list of compatible lenses is not very long. Another thing to notice is the newer TC-14EIII teleconverter is not compatible with some of the older Nikon lenses the older TC-14EII is compatible with. This could be important if you have some of the older type lenses.
With modern high mega pixel cameras like 24MP or 36MP where it is possible crop in camera or in post-processing are teleconverters really needed? I have previously wrote a blog post about comparing using a teleconverter and cropping in post-processing, you can read it here. If image size is the most important factor a teleconverter may be useful. If you are only display images on the internet where image size isn't so important you may be able to crop the images to a similar size as a teleconverter would give you and still have a suitably sized image for website display. The added benefit of cropping is there is no loss of light so you can still shoot at the maximum aperture of the lens. The use of in-camera cropping can also give the benefit of improving the frames per second rate and increasing the number of images that can be stored on a memory card.
Over time I have at different points owned the Nikon TC-14EII, TC-17EII and TC-20EIII teleconverters and used them on a range of Nikon lenses. I have found all of these to be very good if used correctly and within their limitations. I would also recommend any of these if they used in the right situations on the right lenses. The Nikon TC-14EII and Nikon TC-20EIII are the teleconverters I have used the most, the Nikon TC-14EII hardly ever came off my 300mm f2.8 lens. I did try the Nikon TC-17EII for a short time but I rarely used it as I found I could easily crop the same field of view when using the Nikon TC-14EII. The Nikon TC-20EIII was a nice teleconverter when used with the 300mm f2.8 when I wanted extra focal length.
I have ended up keeping only the TC-14EII (the first teleconverter I purchased), this is because of reasons stated above. I find this teleconverter suits my needs, the lenses I now own and the way I shoot the best. Friends of mine have decided on the TC-17EII as they feel that it suits their needs the best as a one teleconverter set up using f2.8 lenses. It is after all a personal choice to suit your photography style. I found a teleconverter is a great addition to my photography backpack. It gives me additional versatility to extend the range of a 70-200mm lens if I don't want the added weight of a fast long telephoto lens, or the ability to extend the range of a fast long telephoto lens if I occasionally need that little reach without needing to spend thousand on a fast 500mm or 600m telephoto lens.
Below are some example images I have taken with Nikon teleconverters.
D7100, 300mm f2.8 lens, 420mm, f6.3, 1/640 sec, ISO 400.
D7100, 300mm f2.8 lens, 420mm, f5.6, 1/500 sec, ISO 2800.
D750 70-200mm f2,8 lens, 280mm, f8, 1/1000 sec, ISO 800.
D7100, 300mm f2.8 lens, 420mm, f8, 1/1600 sec, ISO 1800.
D7100, 300mm f4 lens, 500mm, f8, 1/1000 sec, ISO 640.
D7100, 300mm f2.8 lens, 600mm, f8, 1/640 sec, ISO 360.
D7100, 300mm f2.8 lens, 600mm, f5.6, 1/640 sec, ISO 280.
D7100, 300mm f2.8 lens, 600mm, f5.6, 1/160 sec, ISO 1400.