Modulation transfer functions and contrast sensitivity through low vision telescopes

Telescopes are coherently coupled to the eye. Because their wavefront aberrations may be altered by the optics of the eye, especially by accommodation, some researchers question the ability of their modulation transfer functions (MTF) to predict changes in contrast sensitivity functions (CSF) through them. We measured the CSFs of visually normal and aphakic subjects through telescopes. We found that MTF appears to be useful for ranking the telescopes, and accommodation appears to improve focus and partly balance wavefront errors. Our results suggest that cascading (multiplying the contrast of the instrumental MTF with the unaided CSF at each spatial frequency) is useful for predicting visually aided CSFs to within 4 dB.