Adaptive Optics for Vision Science

Chapter 10.2.1 - Resolution Limits of Conventional Imaging Systems

10.2.1   Resolution Limits of Conventional Imaging Systems

Conventional ophthalmoscopes operate in cascade with the optics of the eye
(cornea + crystalline lens). The maximum pupil and numerical aperture
(NA) of the ophthalmoscope and eye combination is dictated by that of
the eye itself. For example, the maximum physiological pupil size and
numerical aperture of the human eye are about 8 mm and 0.23, respectively.
Assuming a well-designed diffraction-limited ophthalmoscope, the finite
pupil size of the eye imposes a fundamental limit on image resolution, with
the width of the diffraction-limited point spread function (PSF) governed
by:

 

where θdl is the angle subtended between the peak of the Airy disk and the
first minimum, λ0 is the wavelength of light in a vacuum, n is the index of
refraction of the medium, and d is the diameter of the eye’s pupil. An increase
in pupil size or a decrease in wavelength narrows the PSF and improves image
quality. For example, an 8-mm pupil at a wavelength of 0.55 μm produces a
θdl in the eye (n ~ 1.33) of 0.22 min of arc (or equivalently 1.4 μm for a 22.2-
mm focal length reduced eye). This is narrower than the smallest photoreceptor
apertures in the human eye.

The human eye, however, also suffers from aberrations that become sig-
nificant at large pupil sizes (see also Chapter 2 or Fig. 11.1). This further blurs
the retinal image and reduces resolution to, at best, about 1 min of arc (depending
on the subject and the imaging wavelength) as long as defocus and astigmatism
are well corrected. In principle, compensation of the eye’s aberrations
with AO allows image quality to be limited solely by diffraction. In practice,
diffraction-limited imaging through large pupils (>6 mm) has not been
reported with the bottleneck likely to be technological limitations of current
AO systems (see also Chapter 4).