I am reading Bryan Peterson's book Understanding Exposure. On page 38 and 39 of the book, he shows two pictures, both shot with a zoom 20-35 set at 20, f/22 for 1/30 sec. One is much sharper than the other in the foreground and far background. He said one could get maximum depth of field byaligning the distance above the distance-setting mark on the lens. (quoted from the top right hand side of page 39). I am wondering what setting he is talking about and what do I set it at? The difference in the two pictures is amazing.