The traditional depth of field figures are based on viewing a print of a given size at a given distance. The calculation involves focal length, subject distance, aperture and image format. If all those factors are the same, and long as you view at the same size, the DoF will be the same whether the camera is 35mm film or FX digital.
Sun 22-Sep-13 03:14 PM | edited Sun 22-Sep-13 03:17 PM by aolander
An image is only focused at one plane. All other areas in front of and behind that plane are not, but are "acceptably" in focus depending upon the DOF. Each point of light becomes a small circle (circle of confusion) if not at the focus plane. When the circle becomes large enough, we perceive it as being out of focus. If you enlarge an image, those circles become larger, but if you view the image from further away, the circles become less apparent.
Sun 22-Sep-13 03:22 PM | edited Sun 22-Sep-13 03:23 PM by briantilley
>Brian, are you saying that if I view my photos from different >distances, the DOF changes? Or if I print a different sizes my DOF >changes?
Yes, that's correct. Counter-intuitive, perhaps, but correct all the same
DoF figures were always based on a definition of what is considered acceptably sharp in an image, and this varies with print/viewing size and distance. The original "standard" was something like a 10x8 inch print viewed from a foot or so, I think.
The focal plane position has always been the same for all Nikon F mount lenses: 46.5 mm between the lens mounting flange and the film or sensor image plane. So of course your FX lens will have have the same DOF at a given focal length and aperture with an FX d800 sensor as will using an F5 with the same FX focal length lens and aperture with 35mm film loaded. Nothing counterintuitive here
The register distance is not a factor here - after all, Nikon's DX DSLR's share exactly the same lens mounting surface to sensor plane distance, but their DoF will differ from FX because of the sensor size (all other things being equal).
What might be considered counter-intuitive is that the apparent DoF changes if you view a smaller or larger print, or change the viewing distance. That's what reply #2 was questioning.
Getting further or printing smaller does wonders for rescuing a less than sharply focused image.
When shooting events, where someone will always want all the photos, even if I would be embarrassed to release one due to technical faults, no one seems to notice that the DOF was too shallow to get other people in the scene in-focus if I release the files that are already scaled to how they will probably be used, small prints, Facebook avatars, or web viewing. The perception of DOF certainly does change with scale and distance for the reasons cited about involving human eye/brain versus circles of confusion. Stan St Petersburg Russia
DOF calculations take into account a variable factor known as the "circle of confusion." That is the degree of sharpness that you use as a criteria. It was also defined as the maximum size of a theoretical point source of light that was acceptable. The "COC" in turn, often varied by film format, which would be analogous to sensor size today. Since you didn't often blow an 8x10 inch negative up to the same degree as a 35mm frame, the "COC" was less critical.
This is difficult to explain -- recommend looking up the circle of confusion on the internet until you find an explanation that makes sense to you! Pictures will help!