If you want a converter go with a high quality f1.4. To check if Sigma's converter will work with your 70-300 you will have to check it out on the Sigma website as it will only physically fit certain lenses without causing damage due to it's physical design. A very good converter which will fit is the Kenko 1.4 Pro 300. However you may lose autofocus with the combo which again you will have to check out on the Sony website this time as many "cheaper" cameras will not autofocus with a converter if the main lens is slower than an f4. For instance in the Canon line up you have to go to a Series 1 professional camera.
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