if i divide the sensor output i will get between 0.22-1.25v.
so still loosing resolution.
without replacing the arduino. still looking for cheap alt to that ADC ic.
Dividers don't have to be 2:1, but you don't need neither dividers nor changes to the Arduinos Aref.
I haven't tested this myself, but according to Dave Cook over at RobotRoom, the Sharp PSD's apparently have a "[...] limited resolution of approximately 0.019V. [per step]"
If we accept this as true and hold it up against the 10 bit A/D-C of the Arduino, which have a step size (with an Aref of 5V) of 4.88mV, leaving 4 steps in between each possible reading from the PSD - without any changes.
Tailoring the output of the PSD to the A/D-C range of the controller, whether with Aref adjustment or an op-amp,will only leave you with even more values possible with each step from the PSD.
The photo array in a Sharp PSD has got a digital output followed by a D/A-C to give you a pseudo analog output