Joined: 06 May 2005
Location: Richland, WA, USA
|Posted: Fri Dec 30, 2005 8:18 pm Post subject:
|Hi folks! Here is some more background you may find interesting.
I recently contacted Ted Clarke, author of the Modern Microscopy article that I referenced in my Dec 28 posting. I asked some questions about his article and asked him to check over what I had written. He graciously did that, and in our conversation also provided some additional information that I had not known.
It turns out that Gibson's 1960 result, showing larger DOF with larger film, was only partly due to his consideration that blur was added in the enlarging process. More importantly, the paper also contained a mathematical error that pushed the results much farther in the direction of favoring larger film. Based on his mathematical results and apparent experimental confirmation of them, Gibson was firmly in favor of using the largest available film for microscopy. (Shoot a huge negative and reduce it, if possible!) The erroneous results were propagated into Kodak publications N-12 and N-15, where they received wide distribution for many years. The error was not corrected until 1984 (24 years later!), when Clarke published an article containing conflicting formulas and images supporting them, showing that small film worked just fine. There followed an exchange of letters between Clarke and Gibson addressing both the mathematical analysis and experimental confirmation, and in 1986, Gibson published a corrected article with revised aperture recommendations for users of 35mm film.
As I wrote in previous postings, the situation has become even simpler with the advent of digital sensors that completely remove enlarging blur, so that now sensor size makes no apparent difference to DOF, if and only if you can set different f-numbers corresponding to the sensor size.
I have tweaked some of my earlier postings to clarify the wording. There were no substantive changes.
I have not yet gotten copies of the 1984 and later articles, but I'm looking forward to reading them. I'll let you know if anything else turns up that seems worth sharing.
Happy New Year!