Tamron SP Auto Focus 1.4x Teleconverter for Nikon Mount Lenses (Model 140FNS) 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.For use with Nikon Digital SLR cameras 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