The lens effect of high pressure optical systems involving glass or quartz windows has been studied and found to be due to four primary causes. (1) Pseudo-lens effects due to temperature gradients following changes of pressure. (2) Bulging of the outer surface of the pressure window. (3) Pseudo-lens effect due to unequal distribution of strain in the window. (4) The difference in index of refraction of the material in the cylinder and that of the window combined with the bulging of the inside surface of the window. These effects are measured and methods for their correction outlined.