Flashcards in Lecture 3 Deck (30)
what gives the iris an opaque appearance?
due to the melanin granules in the epithelium and sometimes the stroma
does the iris have optical power?
what are the 4 important optical roles of the iris?
defines pupil by blocking undesirable light, regulates retinal illuminance, controls depth of field, and aides in controlling retinal blur
what makes a brown iris different from blue/green?
brown = melanin granules in stroma and epithelium
blue/green = granules only in epithelium
what limits the brown iris transparency?
absorption by pigmentation
what inhibits the transparency in the blue iris?
scatter (Rayleigh) by molecules in iris
what is the luminance range for the human visual system?
how does the pupil size affect the brightness of the retinal image?
image brightness is proportional to the pupil area (not diameter)
how much brighter is an image in an 8mm pupil vs. 2mm pupil?
8mm pupil will pass 16x more light = (8mm)^2/(2mm)^2
how much does the pupil contribute to the 10-log luminance range of the eye?
only 1.2 log (rest comes from dark adaptation and photoreceptors)
what determines depth of field and depth of focus?
maximum size of acceptable blur at the retina
what 3 things does the depth of field ultimately set?
refractive state of the eye measured, focusing optical instruments, accuracy of accommodation system
what happens to the depth of field and focus with a small pupil?
they both increases (larger pupil = decreased)
what is the depth of field most textbooks use?
1/2 to 1 diopter
what is the equation to find depth of field?
what is the clinical significance for depth of field for presbyopes?
increased depth of field with increased ambient light
what is a downside for small pupils?
more sensitive to nuclear cataracts
what 2 things are compromised by small pupils?
accommodative amplitude and subjective refraction
can a pinhole lens correct a refractive error?
no but it ca help make the image more clear
what happens to the blur size on the retina with a change in pupil diameter?
a decrease in pupil diameter = decrease in geometric blur size at retina
what happens to aberrations with an increase in pupil size?
larger pupil = more aberrations (more retinal blur)
will a large slit or a small slit cause more diffraction to occur?
a small slit generates more diffraction
what is the image blur size for a point object?
1.22 (wavelength) f/d
what happens at a 3mm pupil to diffraction and aberrations?
less than 3mm = diffractions have a larger role
more than 3mm = aberrations have a larger role
what is a real pupil?
defines the bundle of light rays that pass from object to retinal image (we cannot see this)
what is an entrance pupil?
image of the real pupil formed by cornea (erect, virtual image that resides 0.5mm in front of real pupil)
what is an exit pupil?
image of real pupil formed by lens (erect, virtual image that resides 0.1mm behind the real pupil)
how much bigger are the entrance and exit pupils compared to real pupil?
entrance pupil = 13% bigger
exit pupil = 3% bigger
what type of magnification is given with Snell's law?