A few weeks ago I spent a considerable amount of time wrapping my head around the topic of Depth of Field in regards to my full frame camera. Specifically, how it appeared to have a more shallow DoF then my 40D which has a 1.6x crop sensor.
For those that don't know.. basic Photography theory 101:
The aperture setting not only is involved with exposure, but also DoF. The larger the f-stop the larger the DoF (and the smaller the actual opening). Sounds semi-simple right? Well it is actually more complected then that....
While the f-stop does change the DoF.. the actual *effective* DoF varies on the distance to the focus point, and the lens focal length. What varies is the zone of focus.. the distance in front, and behind the focus point that remain in focus.
As an example.. lets say you use f5.6 and you focus on the eyes of a subject standing maybe 15 feet from you at 50mm. In that case you may observe that the tip of the person's nose remains in focus. However if that person comes closer to you and you again focus the same way you may find that their nose is now out of focus... the same way that may have happened if they stayed where they were and you moved to f2.8.
So getting back to my full frame dilemma.. I did some shots in my studio and realized that my usual f5.6 was not going to cut it.. and started using f8.. which I also found had a slightly shallower DoF then I desired. My initial conclusion was that the full frame sensor had a shallower depth of field then the crop sensor.. but I didn't understand why. It made no sense. A google search directed me to several articles that confirmed that my conclusion was incorrect.. that both sensors have the exact same DoF.
So I sat there and tried to wrap my head around it.. I just kept thinking that if I took the same exact picture with both cameras the DoF on the FF camera was shallower. So WTF?
What I didn't realize was that I was forgetting something that WAS different between the two.. the Field of View.. because the FoV is larger on the FF camera I would have to either get closer, or increase my focal length to compose the same shot that I would on the smaller sensor camera.. this means that I was adjusting at least one of the two other parameters which determine the size of the ZoF! So inadvertently by just trying to recompose the same shot I was creating a smaller DoF. If I kept the distance and the focal length the same, and just cropped the picture smaller to create the same composure, the FoV would be the same as the crop camera.
It was one of those "how did I miss that?" moments that had me going for a few hours.. In the end I decided that because of this effect I was going to stick with using my 40D in the studio. Because of the large FoV with the FF 5DmkII I would have to zoom in or get closer to my subjects to get similar shots as I get with the 40D... and thus would need to raise my f-stop and strobe power.. which is ok for most things except for high-key.. So just to simplify I'll stick with my trusty 40D.