For some time now, I've been pondering the concept of 'stacking', with regards to teleconverters for camera lenses. Recently I had the opportunity to borrow a 2x converter to supplement my 1.4x converter, and make this experiment possible.
Other people who had done similar experiments (usually bird photographers shooting smaller targets at longer distances) had reported that the loss of sharpness with the 2 converters stacked, made the exercise almost useless.
This experiment was conducted using a Canon 70-200 f/2.8L (non-IS version) fitted to a Canon EOS 1D Mk III, with a Canon 1.4x TC II and a Canon 2.0x TC II fitted between lens and body. You can only fit the converters one way round, as the rear of the 1.4x is flat, and both the converters have a short nose that fits inside the rear of the lens.
The following photo was not resized at all (it is resized for display here, but I've provided a link to the original image), only cropped down from the 1D Mk III's 10.1 megapixels. As a side note, using 2 converters does screw with the camera's brain a little, the lens seems slightly slower to focus, but still quicker than the Sigma 50-500 that I was comparing the results with. Also, the EXIF info suggests this photo was shot with only the 2x TC fitted to the 70-200L, focal length as a result is reported at 400mm, when it was actually around 560mm.
For comparison, here's a similarly treated shot, again, only cropped, not resized (except for display in this post), out of my Sigma 50-500mm (aka The Bigma).
My conclusion? Previous photographic evidence by other photographers has shown that stacking converters is probably not a good idea for bird photography, and it's probably something to be avoided where possible, as there is definatley some loss of sharpness occuring, however, when shooting motorsports, which typically involves larger targets moving at speed, the sharpness of the image is sometimes not that critical anyway, as there tends to be a bit of motion blur even at higher shutter speeds.
Would I do it again? Probably, if I had the gear at my disposal.
Would I use stacked converters over a Canon 100-400L with a 1.4x TC? Probably not, but I don't have a 100-400L or a 2x TC, so this is a bit of an irrelevant question.
Would I use stacked converters over my Sigma 50-500? I don't know. The results are close, but the winner for me has always been the Sigma's ability to zoom all the way from 50mm to 500mm, and perform pretty capably all the way through. However, the reason I'm thinking about this at all, is because my Sigma is failing, and I'm not sure it's worth repairing or replacing with another Sigma lens.
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