I was trying to get a better understanding of depth of field, so I pulled out my 1946 printing of the Kodak Reference Handbook. As I was entering the formulas into an Excel spreadsheet, I found an error on page 34 of the "Kodak Lenses" chapter. Near the middle of the page, formulas are given for "Near limit of depth of field" and "Far limit of depth of field". Don't use them, use the formulas in the diagram instead. Sorry I'm 60 years late in reporting this error. If you want to try the spread sheet, it is at http://users.adelphia.net/~ashtongj, in the list of files for study or discussion section. It isn't very user friendly.