A teleconverter is a small piece of glass that extends the focal length. You can put it between the lens and the camera body. Most teleconverters have a zoom factor of 1.4 or 2. The disadvantage is that the aperture decreases by the same factor. Teleconverters also have a bad reputation, because many photographers associate them with poorer image quality. But is a teleconverter good enough for an advanced photographer?
The short answer is yes. Teleconverters are an ideal extension to long lenses with a big aperture like a 70-200mm f2.8.
I personally love my teleconverter and would never give it back. Especially when I travel it gives me more flexibility and I would have definitely missed some shots if I did not had it.
What is a teleconverter
A teleconverter is like an additional lens that you can put between the normal lens and the camera body. If you use a 70-200mm f2.8 lens with a 2x teleconverter you will get a 140-400mm f5.6 lens. A teleconverter increase the focal length and decrease the aperture by its zoom factor. It is always a choice between a longer focal length and a larger aperture.
The usual zoom factor for a teleconverter is 1.4 or 2 but there are also teleconverter up to a factor of 3. Nearly every lens manufacturer does produce teleconverters. This means for nearly every camera with a changeable lens there is a teleconverter available. Some expansive lenses have even a build in teleconverter. The Canon EF 200-400 f4 has a build in 1.4x teleconverter, which you can activate with a switch at the lens. Teleconverter can also be stacked to a get a longer focal length but you always loose apertures. Nevertheless you can read sometimes on petapixel, that some photographers have stacked some teleconverters to super long tele lenses like a 1120mm, a 9600mm and even a 25600mm.
If you plan to buy a teleconverter you should check if the lens that you want to use with it is compatible with a teleconverter. Most of the Lenses with a shorter focal length like the Tamron 24-70 f2.8 do not fit on teleconverter.
If you talk with photographers about teleconverters you often hear the drawbacks an the problems caused by them.
Loss of Quality
Often photographers avoid use teleconverters because they are scared by the decrease of the image quality. This is true in some cases. If the lens quality is poor it this is amplified by the converter. If the lens has strong chromatic aberrations you will see them better with a teleconverter. If you push a lens to its limits with a converter, of course you will get less quality. With most of the newer lenses the decrease of quality is not a problem anymore.
Loss of f-stops
With a teleconverter you always loose f-stops. If you have a f2.8 lens with a 2x converter you will get a f5.6 Lens. If you have an aperture of f6.3 with a 1.4x teleconverter you will get 8.82. This is maybe the biggest drawback of a converter.
If you use a teleconverter you can get focusing problems especially on cameras with mirrors. If you take a photo with a normal lens the aperture is always wide open until you take the photo. To find the right focus the camera needs as much light as possible. If you use a 2x converter on a f6.3 lens you will get a aperture of f12.6. Most of the cameras with phase detection will have focusing problems if you are not photographing on a really bright day.
Beside the cons there are also a lot of really good features which most of the photographers do not see when they think of teleconverters.
More focal length
The best thing on teleconvertes is that you have more focal length. A 2x converter makes your 70-200mm lens to a 140-400mm lens. You get an extra lens with a simple piece of glass. You have more options you are more flexible if you need more focal length. You do not have to carry around an extra lens with more focal length you can use a converter.
Small and light
Another point that is a big pro of a teleconverter is that the converters are small and light. A 1.4x converter has the size of a pancake lens, a 2x converter is as big as a 50mm lens. They are a bit heavier than the lenses, especially than the nifty-fifty, because converters are made out of metal and have more glass in it. A 2x teleconverter from Sigma is about 290g (10.6oz). Compared to a lens like the 70-200 f2.8 Sigma for Canon which is 1430g (3.15lb), a teleconverter is extremely light. If you have to fly a converter is a really good option to save weight.
Compared to a new lens a teleconverter is cheap. You can get a new 1.4x teleconverter for about 350 $ on Amazon. Compared to a new telelens which cost about 1200 $ new this is really good price. If you want to have more focal length, just got started in photography or want to try to photograph with a longer lens this is a really good and cheap option.
No effect on minimal focus distance
You may think the minimal focus distance of a lens changes with a teleconverter, like on extension tubes. But this is not a problem. A converter does not have any effects on the minimum focus distance.
If you are photographing with a teleconverter you may get some problems. You are pushing your gear to its limits. This will not damage your gear but some functions can be limited. This can be focus problems or some problems with the image quality. Here are some of the most common problems and a solution to them.
As I mentioned earlier you can get focus problems with a teleconverter because, they reduce the light that is coming to the sensor. Especially with phase detection focus sensors like in cameras with mirrors you can have this problem. When you are pushing the gear to the limits, like when you put a 2x converter on a f6.3 lens and get a f12.6 lens. Of course you can use the manual focus but nobody wants to do this when you have auto focus. In this case it can help, if you turn on the live view mode. The contrast detection that is used in live view is most time better when you photographing in low light situation.
The longer the lens gets the more problem you will have to hold it still to get a sharp photo. If you have a build in stabiliser in your lens you are pushing it to its limits. If you are using a 2x converter the stabiliser has to compensate a lot more movement. To prevent unsharp photos caused by camera shanking you can use a tripod. If you need more flexibility you can also use a monopod.
Another trick to get sharp photos with low shutter speed is to take a lot of photos. Even at very low shutter speeds, you have a certain chance that a picture will be sharp. If you take a lot of pictures, the chance that one or two pictures are sharp is higher.
If you are using a teleconverter you probably have to use a higher ISO than in normal situations. Sometimes you just have to accept this for the long focal length. You can try to push the shutter speed a bit, but you will reach a limit. You can find a list of minimal shutter speeds in this article on Improve Photography.
Lens does not fit with the converter
Some lenses do not fit on a teleconverter. Teleconverter have an outstanding part on the site where you attach the lens. With y telephoto lens, you will not have a problem because the last glass element is a bit inside of the lens. On shorter focal lengths like a 24-70mm the last glass element is directly at the end of the lens. In this case a converter is not compatible with the lens. If you are not sure if your lens fit with a teleconverter go to a camera store or borrow one to try it gently, that you do not damage anything.
To give you a personal opinion and a hand on review of a teleconverter I will write down some of my experiences. As I mentioned earlier I do not want to give my teleconverter back. I have a 2x converter from Sigma for Canon. My camera is a Canon D760/Rebel T6s. I use the teleconverter with a Sigma 70-200mm f2.8 and sometimes with a Sigma 150-600mm f5-6.3.
In combination with the 70-200mm lens I have a really good combination. Especially when I travel and I have to fly or if I am not sure if I need a longer lens than 200mm the teleconverter is really handy. Even on a cloudy day I have an aperture of f5.6 which is still pretty good for a focal length of 400mm. This combination has proven itself. The image quality is also right for me.
If I use the teleconverter with the 150-600mm lens I get some of the problems I mentioned earlier in the article. On cloudy days the autofocus get really slow and I have to use live view or manual focus. On days like this these image quality is not very good because the low light capability of my camera is not really good. The reason I use the teleconverter despite of all drawbacks is, that I rather want to have a low quality image for my memory than to have no image or an image that is better and the subject is too small to recognise it. On a bright day I have no problems with the 2x teleconverter on the long lens. Than I can have fun shooting with a 300-1200mm lens.
A teleconverter is a really good option for photographers. The price is not high, and if you have a lens with a big aperture you can get something like a second lens that is not a big investment or heavy to carry. For travel or long hikes a teleconverter is a good option to have not too much weight with you. In low light situations you have to decide what is more important to you better image quality or being closer to the subject.