From One Medical Student to Another
Ophthalmoscope: Think of looking through a dirty window when using a direct ophthalmoscope to examine a patient with a cataract. If a patient has told you his or her vision was blurred or yellowed when looking out, and those symptoms are caused by a cataract, you will probably have the same experience looking in. A cataract will dull the red reflex but should not cause a relative afferent pupillary defect. Dulling of the red reflex in this situation is caused by light scatter, not blockage of light. So the same amount of light is getting to the retina, but it is not focused on the fovea, so this eye should not have a relative afferent pupillary defect if cataract is the only reason for visual decline.
Slit Lamp: If you adjust the angle of lighting for optimal viewing, the slit lamp will always give the best view of any abnormality in the lens.