The evolution of the optical and microscopic properties of highly strained In0.5Ga0.5As/GaAs heterostructures is studied as a function of the (InGa)As layer thickness, L, for different GaAs substrate orientations, (100) and (311). Optical and microscopic properties have been investigated by means of low temperature photoluminescence (PL) and atomic force microscopy (AFM), respectively. Samples grown on (100) show a clear transition in their photoluminescence (PL) spectra at a critical value of L due to the self-assembling of quantum dots. On the other hand, the same structures grown on (311) show a quite smooth evolution of the PL with L. The microscopic measurements indicate that these differences are associated with the morphology of dots formed on the (311) substrates. A study of the (311) heterostructure photoluminescence as a function of applied magnetic field is also reported.