I am new to teleconverters. I have a Pentax K-7 and I occasionally use a Sigma 18-300mm 1:3.5-6.3 DC Ø72 lens. From what I read, the Tamron-F AF Teleconverter 1.4X Pz-AF MC4 may be an appropriate converter with little downside. Looking for input or an improvement on my choice. Many thanks.