« on: July 21, 2013, 02:24:18 AM »
DOF is subjective? Hmm. If my DOF is 8 feet in a photo, that is, 8 real-life feet out in the field, how in the world does that ever change after I take the photo?? 8 feet is 8 feet isn't it?
Actually, I wouldn't even need to take the photo. The DOF is still 8 feet.
Are you suggesting that by being subjective, it could be 8 feet, or 6 feet, or 10 feet, or 7.23838383 feet? How silly.
It seems you think that based on your equipment, there's a 'slice' of the photo that's in perfect focus, say 3.8 feet in front of where you focused, and 4.2 feet behind it, then WHAM like magic at 4.3 feet behind the focal plane, everything gets blurry. That's not how it works.
Light from the plane of focus (which is best approximated by a plane in the geometric sense - 2D and infinitely thin) is focused on the image sensor (we're ignoring field curvature, of course). Everything outside that plane, even a few millimeters, is blurry...and the further from the focal plane, the blurrier it gets. That's optical physics. Whether it looks blurry to you depends on viewing size and distance and your visual acuity.
Tell me - how do you know your hypothetical shot has that 'real' 8 foot DoF? Did you use a DoF calculator? That calculator determines the 8 foot DoF based on an assumed specific print size and viewing distance (commonly 8x10" viewed at 1 foot). Change those assumptions, you change the calculated DoF.
So here we go. Once that optical physics hits the sensor it's no longer optical, it's information. The sensor cannot capture the infinitely thin plane of focus made of sharp points. Instead, it captures everything between the two distances where "a circle" has the size of a pixel or smaller, so everything in that range is same sharp, because there can't be anything sharper than a pixel. At that level, enlarging the image isn't going to decrease the DoF, only soften it, because there is no hidden information.
How do we call THAT THING? The sharpest area between the two distances where "a circle" meets the pixel?