How does the resolution setting affect accuracy on an OSA?

The resolution setting is basically a reference for the actual resolution you can achieve. In the resolution menu you will notice that there is a number in parentheses under the resolution setting. This number in parentheses is the actual resolution based on the center wavelength that is selected. Resolution varies at different wavelengths on the spectrum due to the grating monochomator design we use. The resolution refers to how wide or narrow the exit slit is which lets light pass through the internal photodetector.
 
A high resolution (narrow exit slit) in combination with a narrow core input fiber is important to resolve fine spectral detail.
 
A low resolution (wide exit slit) is important for accurate measurements of the optical power inside a broad spectral peak (e.g. a modulated signal). The complete peak falls through the wide exit slit, meaning that one sample (small measurement error) is sufficient for measurement.
 
The accuracy specification of our OSAs is specified at the highest resolution setting, so to achieve maximum accuracy we do recommend using the highest available resolution setting.