The EDS detector does not have any inherent focusing properties. However, there is an optimal working distance that allows the maximum number of X-rays to enter the EDS detector. This is entirely a geometric effect and reflects the limitation of solid angle entering the EDS detector electron trap.
In this graph Cu Kα signal was measured from Cu foil as a function of working distance. The count rates per nA are quite small given that the EDS detector was operated at only 10% dead time. There is an obvious plateau around 15 mm. Previously the EDS detector was optimized for a working distance of 10 mm. This optimal working distance was lowered slightly to allow for larger EDS elemental maps. Clearly there is little significant change in signal over a large range of working distance from 12 mm to 18 mm.
It should be noted that this does slightly change the effective take-off angle, ψ, of X-rays leaving the sample-- by about 1.5°. This does have a slight impact on quantitation where the A or absorption correction is a function of the cscψ. The differential effect on the A-correction factor is no more than 3.5%. In general, in standardless quant this is of little concern, but for those doing quant with standards, working at a working distance of 10-12 mm is recommended.