Tamron SP Auto Focus 2x Pro Teleconverter for Canon Mount Lenses (Model 300F) Increases focal length from same camera position by 2x Superior performance for unsurpassed image quality and ease of use Fits easily between lens and camera body Includes carry case For use with Canon SLR cameras It is natural for the image quality to deteriorate when using a teleconverter since the lens becomes darker due to aperture loss: one F-stop (with 1.4X) or two F-stop (with 2X). Depending on the shooting conditions, the Auto Focus may hunt. In that case, please use MF (manual) mode. This teleconverter is not designed for digital use. We do not guarantee its performance when used on Digital SLR cameras. When using the teleconveter, it maybe difficult to achieve focus using AF because the depth of field becomes shallow. If this occurs, focus manually. If used with a variable F-number zoom lens (e.g. 2.8-3.5), AF may not operate at the telephoto side. If this occurs, focus manually.