Should you buy a teleconverter in 2021?
In wildlife photography, you never seem to get close enough to an animal and the focal length could still be a little longer. But the higher the focal length of the lens, the higher its price usually is. Teleconverters, however, promise to be able to multiply the focal length with relatively little financial effort.
A 300mm can be turned into a 420mm with a 1.4x teleconverter, and even a 600mm with a 2x teleconverter. Those usually 'only' cost a couple hundred francs, which is not a lot if you compare it to lenses with such focal lengths. But teleconverters also have disadvantages: you lose some sharpness and light with the teleconverter. The big question is whether the disadvantages outweigh the advantages or whether a teleconverter is worthwhile.
How does a teleconverter work?
A teleconverter is attached between the lens and the camera. The teleconverter contains additional lens groups that throw the light rays further out. As a result, only light rays from a narrower image area end up on the sensor. The image is enlarged and the magnification is that of a lens with a longer focal length. But the focal length is not actually changed. So the image compression remains the same as it would have been without the converter. So a teleconverter works in exactly the same way as cropping an image would.
Why a teleconverter
With a teleconverter, focal lengths can be enlarged very efficiently. For photographers who do not crop their images, this is the only way to change the frame with fixed focal lengths.
For photographers who crop their images, however, a teleconverter can also be interesting. Because of the higher magnification, you can work with more pixels. So if a bird is very far away, you have more pixels of the bird with the teleconverter than if you simply crop the picture on the PC.
This is especially advantageous if you want to take part in photography competitions. These often have minimum requirements regarding the resolution of the pictures. You will see later why this makes no sense. In principle, however, the teleconverter allows you to enlarge the image more.
In theory, this can be helpful not only in photo competitions. You should actually get a better resolution for printing. In practice, however, the advantages are only marginal, if they exist at all. This is because the teleconverter also has very big disadvantages.
Disadvantages of a teleconverter
Since the teleconverter only enlarges the final image, the image quality is highly dependent on the optical performance of the lens. If a lens renders an edge incorrectly, e.g. out of focus or with chromatic aberration, this can be amplified by the teleconverter.
The performance of the teleconverter also depends on the sensor of the camera. On a camera with a high resolution, image errors are much easier to detect than on a camera with a low resolution. So depending on the lens and camera, the performance of the teleconverter can vary greatly. In this article I refer exclusively to my experience with the TC1.4E III teleconverter from Nikon on my Nikon D850 and the 500mm f/4 FL ED.
The poorer image quality is not the only disadvantage of a teleconverter. You also loose light with a teleconverter. A certain amount of light no longer falls on the sensor and the amount of light for each pixel is correspondingly smaller.
With a teleconverter with a 1.4x magnification, you lose exactly one f-stop of light. The largest f-stop at 500mm f/4 is no longer f/4 but 5.6. With a 2x teleconverter you would even lose 2 f-stops. Instead of f/4, f/8 would then be the widest aperture.
To compensate for the loss of light, you either have to choose a slower the shutter speed or increase the ISO sensitivity. Especially with the increase of the ISO value, the image quality can again suffer significantly.
Another disadvantage of a teleconverter is the slower autofocus speed. Even though my lens is actually extremely fast, it had a lot more trouble with the teleconverter and was generally slower.
As you can see, the gain in focal length with the teleconverter is a big compromise. Not only do you lose image sharpness in general, you also lose image quality due to the lower widest aperture and therefore a higher ISO value. Finally, there is the slower autofocus, which in certain cases can also contribute to blurred photos. Despite these major disadvantages, it is not clear whether a teleconverter really makes sense. If you compare photos with and without a teleconverter, the photos are practically identical.
Converter vs cropping
During my first attempts with the teleconverter, I quickly noticed that the image quality suffers quite a bit under the TC. Details were no longer as sharp, contrasts were somewhat lower and, due to the loss of an aperture stop, the noise of the camera was of course somewhat stronger. So I was able to crop a lot less than I would have been able to without the teleconverter. But this was not absolutely necessary, because I already had a narrower image section due to the TC.
But since I never had the time to take off the converter when photographing birds in order to compare the image qualities, I show two test images here for comparison. The only difference is the teleconverter. I then cropped the picture without the teleconverter so that it had the same image detail as the photo with the converter.
Even after looking at it for a while, I couldn't see any big differences between the two photos. Although the image with the teleconverter has a much higher resolution, the images are almost identically sharp. At a very high magnification (about 300%), the combination with the TC is slightly ahead. But the differences remain minimal.
This should also make it clear that minimum resolutions do not necessarily make sense in a photo competition. The picture with the converter has 2000 pixels more on the longer side, but it is practically as sharp as the one without the teleconverter.
What could not be demonstrated in this test is the autofocus. This is much slower and somewhat less accurate with the TC. In these ideal conditions, the autofocus never had any problems focusing, even with the TC. When photographing birds or other moving subjects, the slower autofocus is quite noticeable and can lead to blurred photos. In the end, the photos taken in the field with the teleconverter are more negatively affected than in a static test.
500mm without 1.4x TC but cropped
500mm with 1.4x TC
500mm without 1.4x TC but cropped
500mm with 1.4x TC and cropped
Is a teleconverter worth its money?
Now that I have shown that the image quality hardly differs from the image without the teleconverter, but that the autofocus is much slower, the question arises whether a teleconverter is worthwhile at all.
For me personally, this is a pretty clear no. However, this is not necessarily due to the teleconverter, but may also be due to my equipment. I can well imagine that the same teleconverter would have worked much better with, for example, a Nikon Z6 or a D6. With 24MP or 20MP you can crop much less and the teleconverter gains in usefulness again.
Personally, the sacrifices in speed and reliability of autofocus are too great for me to justify the minimal advantage in image quality. I would rather have a picture of a bird where the eye is in focus than a picture where the tail feathers are in focus but the image quality is marginally better because the autofocus was too slow.
I would therefore only recommend a teleconverter to people who have a camera with a rather low resolution and a very good lens. A rather cheap lens may seem very good without a teleconverter, but with the converter the flaws become apparent. The same applies to cameras with high resolutions. Nevertheless, I recommend everyone to try out a teleconverter. It may well be that the converter achieves very good results with your equipment. I was also able to try out the teleconverter. I would like to thank Thomas Schüpbach from Digital Studio Schüpbach for this.