Razr
11-04-2007, 10:58 PM
Focal Length 'Equivalents'
There is a lot of misunderstanding out in the world of photography about focal length, aperture, field-of-view (FOV), and depth of field (DOF). If there's one thing I hate arguing about, writing about, and educating people about, it's these topics—especially DOF. Misunderstanding of DOF is ineradicable. Arguments always follow assertions about it. It was that way in 1910, and 1940, and 1980, and it's that way now, and it will be that way in 2030 and 2050—regardless of what I or anybody else might say about it, now or ever. I edited a technical magazine about photography for six years; take my word for this.
When "translating" a focal length, all we're doing is bringing back a FOV crop to a context that is more or less readily understandable to people because it's familiar and/or standardized. With digital's multiplicity of format sizes, it's become conventional to "translate" FOV crops into 35mm equivalents—and it amazes me that anyone still questions this, since it's a widespread practice with digicams, many of which use the 35mm equivalent focal lengths in their menu systems or even engraved on their lens barrels.
When I write that a 150mm ƒ/2 lens on 4/3rds format is "equivalent" to a 300mm ƒ/2 lens in 35mm, no, the actual focal length of the lens hasn't changed—it's still 150mm. And no, the aperture doesn't change either. You don't have to "translate" that ƒ/2 to ƒ/4 or ƒ/5.6 to "be consistent." There's one thing that does change, and that's DOF, and yes, you would have to stop down the lens on the larger format to get the same DOF for identical scene FOVs—but aperture is not primarily a control of, or a measure of, DOF; it's primarily a measure of exposure.
(Continued) http://theonlinephotographer.typepad.com/the_online_photographer/2007/08/there-is-a-lot-.html
Quite interesting.
There is a lot of misunderstanding out in the world of photography about focal length, aperture, field-of-view (FOV), and depth of field (DOF). If there's one thing I hate arguing about, writing about, and educating people about, it's these topics—especially DOF. Misunderstanding of DOF is ineradicable. Arguments always follow assertions about it. It was that way in 1910, and 1940, and 1980, and it's that way now, and it will be that way in 2030 and 2050—regardless of what I or anybody else might say about it, now or ever. I edited a technical magazine about photography for six years; take my word for this.
When "translating" a focal length, all we're doing is bringing back a FOV crop to a context that is more or less readily understandable to people because it's familiar and/or standardized. With digital's multiplicity of format sizes, it's become conventional to "translate" FOV crops into 35mm equivalents—and it amazes me that anyone still questions this, since it's a widespread practice with digicams, many of which use the 35mm equivalent focal lengths in their menu systems or even engraved on their lens barrels.
When I write that a 150mm ƒ/2 lens on 4/3rds format is "equivalent" to a 300mm ƒ/2 lens in 35mm, no, the actual focal length of the lens hasn't changed—it's still 150mm. And no, the aperture doesn't change either. You don't have to "translate" that ƒ/2 to ƒ/4 or ƒ/5.6 to "be consistent." There's one thing that does change, and that's DOF, and yes, you would have to stop down the lens on the larger format to get the same DOF for identical scene FOVs—but aperture is not primarily a control of, or a measure of, DOF; it's primarily a measure of exposure.
(Continued) http://theonlinephotographer.typepad.com/the_online_photographer/2007/08/there-is-a-lot-.html
Quite interesting.