I recently acquired a Vivitar 1.5x teleconverter and awhile back I had also acquired a Vivitar 2x teleconverter. It got me thinking with everything people have always said about teleconverters of do they impact image quality and if so how much. I ran a roll through my camera using several different lenses with no teleconverter, the 1.5x and the 2x teleconverter on there. My results aren’t exactly hard science, especially since to really get the most out of the test I need to be using a good film scanner instead of scanning the prints using an older Epson flatbed.
What I found was that the 1.5x teleconverter reduces the IQ only slightly and the 2x reduces IQ some, but not a major hit. I did also find that with a long lens the 1.5x and especially the 2x reduce IQ significantly, but it appears to be more related to issues with poor long lens holding techinque as well as limited shutter speed (a 600mm f/8.2 or 800mm f/11 len at a maximum of 1/1000s with no IS and no tripod is not generally going to yeild sharp images).
In my 400mm test images the 1.5x doesn’t seem to really impact the image quality all that noticably, the 2x does. The test would have been better with a remote release and a solid tripod mount though, so I’ll ignore that set of test data. With a 135mm f/3.5 I can’t see a difference between the 135mm at f/4, f/5.6 and f/8 and the same with the 1.5x on there stopped down the same 1/3rd, 1 1/3rd and 2 1/3rd stops. The 2x is very slightly softer. Neither TC seems to impact CA noticably.
Using it for macros the IQ difference does appear more noticable. My final test was with my 35-70mm f/3.5 macro Tamron lens at f/4 and also with the 1.5x TC and the 2x TC at about the same relative image size for each picture. I then took 100% crops of a 350dpi scan of the print. The results are interesting, the 1.5x actually looks a bit sharper, but I think it might be more of a case of bad lighting for that one exposure, if you look at the Tomato plants filaments they are about as sharp between the 1.5x and no TC, the 2x is noticably softer.