Carl D wrote:
I respectfully disagree, all tc's degrade the image compared to a native lens. One is better off saving up the money and buying a good lens. Any time you add more glass to the lens you already have, the image becomes worse. Case in point: take several pieces of window glass and keep stacking one in front of the other, the image gets worse and distorted. Granted, window glass is nowhere near the quality of lens optics but the results are the same, just to a lesser degree.
Well, as imagemeister stated, it depends on the lens and TC combination being used. I'm using the Sigma TC-1401 with the Sigma 150-600 Sport and have been getting excellent results. This particular TC was designed specifically to mate with this lens, so image quality is quite excellent.