Crop Factor when calculating DOF?
Posted 14 December 2010 - 08:42 PM
Posted 14 December 2010 - 08:51 PM
Just wondering, when calculating depth of field, do you factor in the camera's crop factor? I'm using a 7D right now, and when compared to a 35mm frame of film, it has a crop factor of 1.6. If I'm using a 50mm prime at f2.8, should I calculate the depth of field using a focal length of 80 (50mm x 1.6) or just use 50mm? Thanks
When you say "35mm frame of film" you should specify that you mean the Full-Frame 35mm still camera format (8-perf horizontal, like VistaVision) not the 35mm cine / Super-35 format, which is similar in size to the APS-C sensor of the 7D.
You should use the DOF chart for a 50mm if that's what you are using. The factor that will affect the calculations is the Circle of Confusion figure you want to use, but for now, you can use the one that 35mm cine uses.
If you wanted to match the field of view of a 80mm on a FF35 camera, you'd have to use a 50mm on a S35 camera, hence why the smaller format has more depth of field, you end up using shorter focal lengths to achieve the same field of view.
The sensor size affects the field of view of the lens -- the crop factor -- and thus affects which focal length you end up choosing. But once you've decided that the 50mm gives you the field of view you want on the 7D or whatever, you use the DOF chart for a 50mm, same goes for a FF35 camera, if you use a 50mm on that camera, you'd use a 50mm depth of field chart. But just remember that the 50mm looks more wide-angle on the FF35 camera, and that you'd also use a different CoC figure if the final image was going to be presented at the same size as one shot on the 7D (in other words, the 7D image would have to be enlarged more.)
Posted 15 December 2010 - 12:32 PM