To do good sub-microsecond photodiode work, you really want the diode reverse biased (so the photocurrent is cleared quickly and the junction capacitance is small), and you want it returned into as low an impedance as possible -- you can use a very low value resistor, but the signal is very weak. (For a freaking flash, this is probably an advantage -- it's probably more than bright enough to saturate the diode, so the current will be relatively large.)
For ordinary signals, you use a TIA (transimpedance amplifier), which has (ideally) zero input impedance, and converts input current to output voltage (thus, a gain of V/I = impedance, but gain, so it's 'transimpedance'..).
Tim