Tuesday, August 12, 2008

The Depth Of Field Myth

Everyone thinks the longer the focal length and the wider the aperture the less depth of field. Well, this is true. But let's say you first photograph your subject with a 150mm lens and then take a second shot at 24mm keeping the subject the same size in the viewfinder for both shots - here's the question - which image has the greater depth of field? The answer, much to my surprise was that both have the same depth of field!

I know, you think I'm crazy here, but over the weekend I had that same conversation with a good friend of mine, Mr. Frank Dispensa, from Wappingers Falls, NY. Frank is a great teacher and really knows his stuff. He is a regular lecturer at WPPI and does his own series of seminars too. Here is the link to Frank's site right here.

Frank watched an earlier Technique Tuesday episode entitled, "I Can't See The Forest For All The Trees" and raised the question to me about my remark about depth of field. Well, we were on the phone for 30 minutes discussing the finer points of depth of field. Most everybody thinks that there is much less depth of field with long lenses at wider apertures and that is true. BUT, when the subject is the same size, photographed at varying focal lengths, the depth of field is the same. What happens is that the longer lenses show the background as larger and hence shows the apparent softness better. In other words the perspective of the background changes and that creates an illusion that the depth of field changes. In reality it does not.

Frank sent me a list of links that illustrate the point. Check out BlueSky-Web right here for a thorough, well illustrated discussion of the facts. DVInfo.com right here has a more geeky discussion also on this same topic. DOFMaster.com right here has a great discussion on the subject, especially when the subject's size is the same as the focal length changes. It's full of charts and graphs and is a good geek read. Anyway, like I said, Frank knows his stuff - but he is so much older than me too:~) Hey Frank, thanks for the clarification and heads up info.

Hey gang that's it for me today. Be sure to tune in for tomorrow's podcast entitled, "How to Run A Million Dollar Studio." I'm interviewing my good friends Kent and Sarah Smith about their ultra successful studio in the Columbus, Ohio area. I have to say, their insights and advice truly hit this podcast "out of the park!" It is a "must listen" show. Everyone at my studio has orders to listen to every word. I hope you enjoy it. So, until tomorrow, have a great one, -David

1 comment:

  1. I fully agree! I have the same experience using for instance my Tokina 10-17mm fish eye. When using at f/4.0 and almost filling the frame with a head, then the rest of the image is blurred (also when using at 10mm).
    I feel that focusing well with a fish eye or wide angle is as important as with telephoto lenses.