I've had my 50D for awhile and have had some amazing results so far. I have a 50mm f/1.8 II which is awesome for the price, hope to get a 1.4 soon. I've been taking lots of portraits of my son and love the background blur. I've also had some shots ruined by not stopping down when I should have for group shots. Obviously with a fixed focal length and similar aperture the DOF is also affected by distance to subject.


Is there a quick metal formula for estimating DOF? Like, at 50mm and f/1.8 with the subject 48 inches from the camera DOF will equal x inches but at 20 feet from the camera DOF will equal y inches. Are there any other good tricks for knowing how wide open is too much?