Orthophoto is an orthorectified image representing the area from above. DEM is needed to make a true orthophoto.
In the Orthophoto dialogue window, select the digital elevation map for the basis. To increase or decrease the resolution of the orthophoto, select the GSD value. The smaller the value, the higher the resolution.
Suggested value is 4 times smaller than the used digital elevation map. The value can be even smaller, but it might cause the longer processing.
It is no use going smaller than the photo GSD that is calculated during the flight planning stage, which is the maximum quality available. If there is need for better quality, a lower altitude flight must be performed.