The cheapo effective 1,650 mm 5.6 tele (Q + adapter + Tamron 70-300 and other lenses)

Actually you're not limited to wide open.  I just set my aperture to what I want with my canon 10D and remove the lens while holding the DOF preview button.  Stopping down to f4 make my pics with the Canon 50mm 1.8 much sharper.  With diffraction causing problems though above f4, it probably doesn't help with slower lenses.