That depends GREATLY on the purpose of the pictures. For widest coverage, they fly high, and with wide-angle lenses. The detail of such flights is comparable to commercial photo satellites.
Now, if the purpose of the pictures is to capture a particular feature, then the flight will be low, with a longer lens. If it's sufficiently low, and the lens sufficiently long, you can not only read a watch face, but read the date window on it.
But just so people can do some resolution calculations for themselves: Some of the best general-purpose aerial film out there that balances light sensitivity for detail has a resolution of about 7 microns. (That's around 3600dpi.) The area of exposure is about 230mm x 230mm. One of the most commonly-used cameras for aerial photography is the Leica RC30, which has three available lenses: 1/4x, 1/2x, and 1x. In technical terms, that's the ratio of focal length to objective lens width. In practical terms, it means that the distance covered across the diagonal of the exposed area is either 4 times, 2 times, or equal to the distance from the objective lens.
So, if they're flying high and wide, say at 25,000ft with a 1/4x lens, then the diagonal of the exposure is going to be 100,000ft. That means that the area of the exposure is going to be 70721ft x 70721ft, or 21556m x 21556m. That's 21556m spread across 230mm, so it's 94m to the mm. Resolution of 7 microns is about 147dpmm. Therefore, the smallest resolvable feature is going to be 0.64m. I don't know how "much smaller" that is... |
