For more than one year I am now carrying Nikon’s 2x teleconverter TC-20E III in my camera bag. I bought it from a local store in good used condition, with intent to get more reach with my Nikon D300 (which is APS-C sized) and the Nikon AF-S 70-200mm f/2.8 VR lens. Since this lens is very fast and its image quality superb, the 2x teleconverter would still allow for high shutter speeds at f/5.6 on bright summer days when doing wildlife, e.g. bird photography.
So far the theory, but after taking my first shots with the 2x converter attached to Nikon’s 70-200mm f/2.8 VR, I was really disappointed with the results. Images taken at the widest aperture through the TC are of poor quality and very smooth, not sharp at all. Stopping down improves the quality, but still not to a level I would be satisfied with.
Now comes the surprise! Just recently, I got hold of a very nice and sharp Nikkor AF-S 200-400mm f/4 ED VR lens, which came together with the teleconverter TC-17E II, both in very good used condition. When using the 1.7x teleconverter on that lens for the first time, I was really “shocked”, because the image quality was only slightly degraded and very sharp. Next, I attached the 2x teleconverter TC-20E III to the Nikkor AF-S 200-400mm f/4 ED VR as well and was likewise astonished by the image quality, which was still good and reasonably sharp.
Remark: Teleconverters and Autofocus performance
Autofocus is getting much slower with the TCs attached. However, although the D300 is not explicitly mentioned on Nikon’s TC compatibility chart, apparently the camera supports f/8 autofocus and the Nikkor AF-S 200-400mm f/4 ED VR will autofocus when either of the TCs under consideration is attached.
TC Image Quality Comparison using SpyderLensCal
In order to make a fair comparison, I decided to setup a typical lens calibration session with SpyderLensCal (the distance was 5m, so that enough focus path was left on both lenses). That way, I would get a fair image quality comparison of the lenses and the TCs, and would at the same time calibrate all my camera+lens+TC combinations. Both, SpyderLensCal and my camera were mounted on a tripod. Shots were made with my D300 using different AF finetuning settings. Vibration Reduction (VR) was turned off, ISO set to 200 and the largest aperture was chosen using aperture priority mode. The resulting shutter speed was always faster than 1/500s. SpyderLensCal and my D300 were brought onto the same optical axis through leveling SpyderLensCal with its integrated bullseye bubble level and the camera using the hot-shoe to level with a common level meter.
The distance between the camera chip and the calibration device was always 5m, but since the focal length changed with each lens+TC combination, I decided to scale down each frame to a focal length of 200mm and then make equal crops around SpyderLensCal’s ruler and save a JPG file. That way, all images can be compared on a pixel-by-pixel basis and more easily displayed here. However, down-scaling and cropping does not have any effect on the results and all images shown below are very good representations of the true RAW images I have taken.
Nikkor 70-200mm f/2.8 ED VR @ 200mm f/2.8
Nikkor 70-200mm f/2.8 ED VR + TC-17E II @ 340mm f/4.8
Nikkor 70-200mm f/2.8 ED VR + TC-20E III @ 400mm f/5.6
Nikkor 200-400mm f/4 ED VR @ 400mm f/4
Nikkor 200-400mm f/4 ED VR + TC-17E II @ 680mm f/6.8
Nikkor 200-400mm f/4 ED VR + TC-20E III @ 800mm f/8