Originally posted by jfischer My basic question was just will that Sigma 70-200 HSM OS lens, not the bare bones HSM model work well at all with the TRUE Pentax 1.7x TC. Just wondering if any had tried the combo together.
The Pentax 1.7x AF-A will work with
any lens. It doesn't matter if it's screw-driven, SDM, HSM or manual focus. All of the auto focusing takes place inside the 1.7x AF-A itself. You will have to manually focuses in the ballpark, and then you can engage the 1.7x AF-A to take you the rest of the way and lock focus. If your target it pretty much at a constant distance away, you won't need to adjust the focus ring of the lens very much. The AF-A focuses pretty quickly, often quicker than the lens alone.
One downside (in addition to needing to pre-focus) is that the AF-A doesn't transmit focal length (then again, no Pentax TC's do as far as I know), so you won't be able to
optimally use SR. You'll have to enter a focal length manually when you attached the lens, and depending on if you plan on zooming a lot or using it at lens at it's maximum focal length will dictate what focal length you enter when you attach the lens.