Tilt/Swing and Depth of Field

[Rick Olson]

I understand that when I tilt or swing the lens plane, the plane of focus changes to form a new plane with the depth of field perpendicular to this plane in a type of "wedge." My question is when stopping down the lens, does the depth of field extend equally in both directions perpendicular to this focus plane?

Thanks,
Rick

 

[jbbooks]

http://www.largeformatphotography.info/how-to-focus.html

 

[BradS]

Take a book and hold it closed with the binding on a table top and the open end pointing up. Now imagine the book is really, really big. One of the pages somewhere in the middle is the plane of sharp focus and the thickness of the book represents the depth of field. As you stop down the lens, the book gets thicker. With me so far?

With all your camera's movements "zeroed" the plane of focus is parallel to the plane of the lens and the plane of the film. If you tilt the lens forward (so that it poitns down) a little, it moves the plane of focus - the book tilts forward too. Now, to your question...the depth of field. It gets thicker too but not uniformly. It's like opening the book. The depth of field "fans out as the distance from the lens increases. Does that help?

 

[rbarker]

There are a couple of interpretations of your question, Rick. The ratio of fore and aft DOF depends, in part, on the design of the lens. In most cases, the ratio will be somewhere between 50:50 and 2/3:1/3. Some lens designs, however, will place almost all of the added DOF on one side of the focus point. If you think of the wedge of DOF, with the actual plane of sharp focus lying somewhere in the middle, stopping down will fatten the wedge at the distant end, and my understanding is that the ratio of front/rear or fore/aft DOF will remain constant. So, assuming a forward-tilted plane of sharp focus, in addition to the increased DOF perpendicular to the plane of sharp focus, there will be a gain of DOF along the horizontal plane, too, but how much depends on the angle of the tilted plane of sharp focus. The wedge still comes to a zero point below the camera, however.