  The familiar tools of Fourier analysis and Fisher matrices are applied to
derive the uncertainties on photometric, astrometric, and weak-lensing
measurements of stars and galaxies in real astronomical images. Many effects or
functions that are ignored in basic exposure-time calculators can be included
in this framework: pixels of size comparable to the stellar image; undersampled
and dithered exposures; cosmic-ray hits; intrapixel sensitivity variations;
positional and ellipticity errors as well as photometric errors. I present a
formalism and a C++ implementation of these methods. As examples of their use,
I answer some commonly arising questions about imaging strategies: What amount
of dithering is ideal? What pixel size optimizes the productivity of a camera?
Which is more efficient---space-based or ground-based observing?
