Well, it's f;11...... BUT it's a long lens, so the corners don't darken as with a wide angle, and the long focal length means you won't have much DOF even at f;11. As long as you have a good dark cloth and give your eyes time to adapt a bit, you should have no problems viewing and focussing with it. By all accounts it's a very good lens although it's not a true tele - you'll need 350mm~ of bellows at infinity.