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Compare how a patient with early presbyopia (amplitude = 3.00 D) and a young adult (amplitude = 10.00 D) would perceive blur when viewing an object at 25 cm.What optical corrections would each need for comfortable reading?

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Compare how a patient with early presbyopia (amplitude = 3.00 D) and a young adult (amplitude = 10.00 D) would perceive blur when viewing an object at 25 cm.What optical corrections would each need for comfortable reading?

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A 50-year-old presbyopic patient claims they can still read fine print without glasses if they hold the text farther away. Explain why this might be possible, and calculate the approximate distance they would need to hold the text if their amplitude of accommodation is 1.50 D

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A 50-year-old presbyopic patient claims they can still read fine print without glasses if they hold the text farther away. Explain why this might be possible, and calculate the approximate distance they would need to hold the text if their amplitude of accommodation is 1.50 D

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Presbyopia is the age-related loss of the eye’s ability to focus on near objects. It's caused

by:

• Hardening of the lens

• Decreased elasticity of the ciliary muscles

When a person reads, they need to focus light rays from the object (like fine print) onto the

retina. For close objects, the lens must increase its power through accommodation.

But in presbyopia:

• The eye cannot increase lens power sufficiently

• So, the light rays from near objects are focused behind the retina, causing blur.

When the patient moves the print farther away, the light rays entering the eye become

less divergent (more parallel), meaning the eye lens doesn’t have to work as hard to focus

them and does not need to change shape as much

So, they’re compensating for lost focusing power by:

• Using distance instead of accommodation

• Positioning the text at their near point—the closest distance they can focus on

without blur

• So, the amount of focusing power required is lower

• This is a great example of how the body naturally adapts to physiological changes.

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In this patient’s case, their amplitude of accommodation is only 1.50 dioptres.

That means the closest point at which they can still see clearly—called the near point can be

calculated using the formula above.

So, their near point is around 66.7 cm Anything closer than that will appear blurry their eye can no longer focus on short distances.

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Young Adult (Amplitude of Accommodation = 10.00 D)

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Young Adult (Amplitude of Accommodation = 10.00 D)

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A young adult with an amplitude of accommodation of 10.00 D can easily meet the 4.00 D accommodation demand for an object at 25 cm.

The eye's crystalline lens provides the necessary refractive power to focus near objects clearly on the retina.

Therefore, the young adult would perceive the object at 25 cm as clear and in focus, as their visual system can adjust to form a faithful representation of the object in the image plane.

Correction : No need optical correction

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Patient with Early Presbyopia (Amplitude of Accommodation = 3.00 D)

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Patient with Early Presbyopia (Amplitude of Accommodation = 3.00 D)

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An early presbyopic patient whose clinical amplitude of accommodation has dropped to 3.00 D cannot fully meet the 4.00 D accommodation demand for an object at 25 cm.

The eye needs 4.00 D of additional power but can only supply 3.00 D.

This inability of the crystalline lens to change shape sufficiently, often due to a loss of elasticity and sclerosis with age, means that the light rays from the object will not be precisely focused on the retina.

Consequently, the patient would experience blur, as the optical system fails to create a faithful representation of the object in the image plane.

The image would appear blurred or deformed.

Correction : Need reading glasses or bifocals with addition +1.00 to +2.00 to enable

clear near vision.

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A fighter pilot with perfect daytime vision reports blurred distance vision during night flights. Explain the physiological basis of this phenomenon and propose two practical solutions (optical or behavioral).

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A fighter pilot with perfect daytime vision reports blurred distance vision during night flights. Explain the physiological basis of this phenomenon and propose two practical solutions (optical or behavioral).

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This phenomenon is known as night myopia.

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Pupil Dilation in Low Light: At night, the pupil dilates to let in more light.

A larger pupil allows more peripheral light rays to enter the eye, which increases spherical aberration—light rays do not all focus on the same point on the retina, leading to a blurred image.

Accommodation drift: When it is dark, our eyes' focussing system (known as accommodation) is unable to determine what to focus on.

So they rest at a “default” position : focusing on something near, about 1 meter away.

This makes distant objects look blurry.

Rod vision: At night, vision relies more on rods (which are more sensitive in low light but less sharp), reducing overall acuity and clarity.

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Optical solution: use of mild negative lenses at night.

Prescription of a low-power minus lens (e.g. –0.50 D) in night spectacles can compensate for night myopia.

These lenses shift the focal point back onto the retina, improving distance clarity in dark environments.

Behavioral solution: regular focus reset using distant target.

During flight, the pilot can periodically shift focus to a known distant object.

This behavior encourages relaxation of accommodation and helps maintain the eye’s focus at infinity.

Known as “distance focusing exercises”, this can reduce accommodative lag in dark conditions.

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If a patient’s dark focus is at 1.25m, what is their effective refractive error in diopters when viewing a distant star? How would this change if their pupil dilated from 3mm to 6mm?

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If a patient’s dark focus is at 1.25m, what is their effective refractive error in diopters when viewing a distant star? How would this change if their pupil dilated from 3mm to 6mm?

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Dark focus refers to the eyes naturally focus in complete darkness, with no visual target.

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Dark focus refers to the eyes naturally focus in complete darkness, with no visual target.

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But patient’s dark focus is at 1.25m. so, the eye is slightly accommodated.

The patient is effectively +0.80 D when looking at a distant star, light is focused in front of the retina when viewing a distant object, indicating a myopic shift, so the distant star appears blurred.

When the pupil dilates

• More light rays enter the eye, depth of focus decrease

• Eye becomes more sensitive to defocus and aberrations

➢ Visual blur would increase with a larger pupil (6mm) due to decreased depth of focus and

inreased spherical aberrations.

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An aphakic patient is corrected with +12.00 D spectacles, while a pseudophakic patient has a 20.00 D IOL. Compare the retinal image size, peripheral aberrations, and visual field limitations for these two corrections.

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An aphakic patient is corrected with +12.00 D spectacles, while a pseudophakic patient has a 20.00 D IOL. Compare the retinal image size, peripheral aberrations, and visual field limitations for these two corrections.

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Aphakic with +12.00 D spectacles:

-Spectacle lenses sit about 12–14 mm from the eye (vertex distance).

-The retinal image size is magnified by about 25–30% compared to emmetropia.

-This magnification can lead to aniseikonia (image size difference) in unilateral aphakia,

affecting binocular vision.

Pseudophakic 20.00 D IOL

-The main goal of putting IOL is to help the person see well without needing glasses all

the

-Gives more natural image size and avoids the big image size changes that happen with

thick glasses.

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Aphakic with +12.00 D spectacles

-High plus lenses cause more distortion and blur vision.

-Peripheral image quality is poorer because the glasses away from the eyes.

-Pincushion distortion happen, where thing look more stretched or bigger toward the

edges of the lens.

Pseudophakic 20.00 D IOL

-Because an IOL is placed inside the eye, it avoids the edge distortion.

-IOLs are much smaller in power and located near the optical center.

-Although IOLs are designed to reduce side effects and complications, there's not much

detail about their own edge distortions.

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Aphakic +12.00 D spectacles

-Thick glasses reduce the field of view by about 30%.

-This causes a ring scotoma which reduces the effective field, especially during eye

movements.

Pseudophakic 20.00 D IOL

-No reduction in visual field

-Side vision is normal or close to normal.

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A pseudophakic patient complains of glare at night. Using optical principles, explain why this might occur and how IOL design (e.g., aspheric vs. spherical)could influence their symptoms.

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A pseudophakic patient complains of glare at night. Using optical principles, explain why this might occur and how IOL design (e.g., aspheric vs. spherical)could influence their symptoms.

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Why glare at night occur in Pseudophakic patient?

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Why glare at night occur in Pseudophakic patient?

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Posterior Capsular Opacification (PCO):

Occur when lens capsule that hold the lens thickens and become cloudy after surgery.

This can scatter the light and cause the glare especially at night.

Residual astigmatism or other refractive error after surgery can contribute to glare and distortion vision at night.

Multifocal IOLs:

The diffractive optics can lead to unwanted light like halos in low-light conditions.

Spherical Aberration:

Occur when peripheral rays of light focused at different point than central rays.

At night, pupil dilated exposed to peripheral light rays.

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Design an experiment to demonstrate spherical aberration’s impact on vision using a pinhole occluder. Predict how VA would change at pupil sizes of 2 mm vs.6 mm, and justify your answer.

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Design an experiment to demonstrate spherical aberration’s impact on vision using a pinhole occluder. Predict how VA would change at pupil sizes of 2 mm vs.6 mm, and justify your answer.

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Snellen eye chart

Pinhole occluder (2 mm and 6 mm)

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Baseline Measurement: Measure the participant’s unaided VA using Snellen chart without any occluder.

2 mm Pinhole Measurement: Place the 2 mm pinhole occluder in front of the participant’s eye and measure their VA.

6 mm Pinhole Measurement: Repeat the VA measurement using 6 mm pinhole occluder.

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2 mm Pinhole:

Smaller pupil size (2 mm) minimizes the impact of spherical aberration, leading to

improved VA.

6 mm Pinhole:

Larger pupil size (6 mm) allows more peripheral light rays to enter the eye, resulting in

reduced VA.

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Spherical aberrations:

Light rays entering farther from the center (periphery) bend more

than those entering closer to the center. This uneven bending causes the light rays to converge at different points, resulting in a blurred or distorted image.

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Pupil/pinhole effects:

Smaller sizes of pupil or pinhole occluder reduces the amount of light entering the eye and limits the light rays to the central portion of the lens, minimizing the effect of spherical aberration.

This leads to improved focus and potentially better VA.

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A patient with keratocconus complains of distorted vision even with optimal spherocylindrical correction. Explain how higher-order aberrations (e.g., coma) contribute to their symptoms.

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A patient with keratocconus complains of distorted vision even with optimal spherocylindrical correction. Explain how higher-order aberrations (e.g., coma) contribute to their symptoms.

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In keratocconus, higher-order aberrations like coma result from the cornea’s irregular and asymmetric shape.

These aberrations distort incoming light in ways that glasses cannot fix.

This is because correction by the spectacles cannot reshape the cornea shape.

As a result, even with optimal low-order correction, patients experience blurred, ghosted, and distorted vision.

Coma is especially significant because it causes smeared or comet-shaped images, leading to functional vision loss that is often frustrating for patients.

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The best way to encounter this issue is by prescribing contact lens such as RGP that can give pressure to the cornea and will make the shape of cornea back to normal when wear it.

This way, it reduce or solve the higher-order aberrrations problem.

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Why does chromatic aberration affect blue light more than red light in the human eye? Calculate the power difference for a cornea with n(blue) = 1.341 and n(red) =1.331.

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Why does chromatic aberration affect blue light more than red light in the human eye? Calculate the power difference for a cornea with n(blue) = 1.341 and n(red) =1.331.

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Hence, +43.720D > +42.440D

Chromatic aberration in the human eye affects blue light more than red light.

Blue light have shorter wavelength and red light have longer wavelength.

Moreover, blue have the highest refractive index than red light.

This shows that blue light will have shorter focal length or higher power than red light through a lens.

The power difference between blue and red light is approximately +1.28D, showing that blue light focuses more anteriorly (closer to cornea).

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A patient wearing low-dispersion (high Abbe number) spectacle lenses reports clearer vision than with polycarbonate. Explain the optical principle behind their observation.

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A patient wearing low-dispersion (high Abbe number) spectacle lenses reports clearer vision than with polycarbonate. Explain the optical principle behind their observation.

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Low-dispersion (high Abbe number) spectacle lens has low refractive index, n while polycarbonate refractive index is higher.

High Abbe number assure lower chromatic aberration.

Lower chromatic aberration means that when light passes through lens, the separation of light into different colours become lesser.

Hence, coloured fringes and halos around the objects become less visible especially in the periphery of the lens.

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A patient’s pupil constricts to 2 mm in bright light. Discuss how diffraction and aberrations compete to determine retinal image quality in this scenario. Which dominates?

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A patient’s pupil constricts to 2 mm in bright light. Discuss how diffraction and aberrations compete to determine retinal image quality in this scenario. Which dominates?

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Aberration is an imperfection in the optical system of the eye that result in light being unable to focus onto the retina effectively as well as defects in visual image, especially when light

enters through the peripheral zones of pupil.

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When the pupil is large, more peripheral rays enter, increasing aberrations and reducing image quality.

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When the pupil is large, more peripheral rays enter, increasing aberrations and reducing image quality.

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When the pupil constricts to 2mm, aberrations are significantly reduced because mostly central rays enter the eye. This tends to improve the clarity of the retinal image.

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When the pupil constricts to 2mm, aberrations are significantly reduced because mostly central rays enter the eye. This tends to improve the clarity of the retinal image.

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Diffraction is a wave phenomenon that occurs when light passes through a small aperture (pupil constrict).

When a small pupil constricts, light pass through it, diffraction increases.

This causes light to spread out, forming a larger airy disc (less sharpness) and formed blurred retinal image.

So, diffraction tends to reduce image sharpness.

To conclude, both diffraction and aberration affects the retinal image quality, but diffraction is

likely to give more impact because at 2mm of pupil constricts, aberrations are already small

because peripheral rays are blocked.

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A cataract patient’s VA improves from 6/12 to 6/6 after surgery, despite residual refractive error. Explain how scatter reduction contributes to this improvement.

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A cataract patient’s VA improves from 6/12 to 6/6 after surgery, despite residual refractive error. Explain how scatter reduction contributes to this improvement.

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Even though some refractive error may still exist after cataract surgery, a patient's visual acuity (VA) might improve dramatically because the surgery lessens light scatter in the eye.

Blurred vision results from cataract-induced clouding of the lens, which spreads incoming light and reduces retinal picture quality.

More focused and clearer light can reach the retina when the cloudy lens is removed and replaced with a clear intraocular lens.

Despite residual refractive errors, this decrease in dispersion enhances contrast and sharpness, improving visual acuity from, assume, 6/12 to 6/6.

Although variables such as the quality of the tear film may also influence light scatter after surgery, removing the cataractous lens opacity that significantly altered light diffusion and image degradation is the main benefit overall.

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A patient is alarmed by new floaters. Describe how you would explain their origin using the optics of vitreous opacities, and identify when this symptom warrant surgent referral.

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A patient is alarmed by new floaters. Describe how you would explain their origin using the optics of vitreous opacities, and identify when this symptom warrant surgent referral.

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The vitreous is are clear, jelly-like substance called the vitreous collagen.

Over time or due to changes in the eye, single/clamped hair-like floaters formed in degenerating can formed.

When these floaters are present, they block or scatter the light, casting shadows on your retina.

In such cases, if the patient experiences a sudden increase in floaters, especially when accompanied by flashes of light, or loss of peripheral vision, this may indicate a retinal detachment and requires urgent referral.

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Other examples when this symptom warrants urgent referral:

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Other examples when this symptom warrants urgent referral:

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A sudden onset of new floaters.

A gray curtain or blurry area that blocks part of vision.

Peripheral vision loss.

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Why do floaters appear to ‘dart away’ when the eye moves? Use optical principles to explain this perceptual phenomenon.

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Why do floaters appear to ‘dart away’ when the eye moves? Use optical principles to explain this perceptual phenomenon.

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Floaters, medically known as muscae volitantes, are entoptic images that result from small particles, like clumps of collagen or glial cells, floating within the vitreous humor of the eye.

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Floaters, medically known as muscae volitantes, are entoptic images that result from small particles, like clumps of collagen or glial cells, floating within the vitreous humor of the eye.

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These particles block light and cast shadows onto the retina, which become more noticeable when viewing bright, uniform background.

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The brain interprets these internal shadows as external images by tracing the path of the chief ray, the central light ray entering the pupil and striking the retina.

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The brain interprets these internal shadows as external images by tracing the path of the chief ray, the central light ray entering the pupil and striking the retina.

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Since the vitreous has a gel-like texture, it does not move as quickly as the eye.

When the eye shifts direction, the floaters lag slightly behind.

However, because the visual system interprets the floater’s position based on the chief ray, the floater appears to move in the opposite direction of the eye's movement.

This mismatch creates the illusion that floaters are quickly darting away, when in reality, they’re slowly following behind the eye’s motion.

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Retinal blood vessels are fixed in front of photoreceptors, yet we rarely perceive them. Explain the neural and optical mechanisms that suppress their visibility.

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Retinal blood vessels are fixed in front of photoreceptors, yet we rarely perceive them. Explain the neural and optical mechanisms that suppress their visibility.

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Neural Mechanism : Visual Adaptation and Image Stabilization

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Neural Mechanism : Visual Adaptation and Image Stabilization

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Retinal blood vessels are fixed on the retina, so their image stays in the same place.

The brain uses neural adaptation to ignore things that don’t change on the retina.

This helps keep our vision clear by removing constant and unimportant

images.

The eyes make tiny movement called microsaccades to keep external objects from fading.

Since the blood vessels move together with the eye, they don’t shift position.

As a result, the brain filters them out from what we see as their image doesn’t change.

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Optical Mechanism : Retinal Structure and Light Path

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Optical Mechanism : Retinal Structure and Light Path

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The retina has layers, with photoreceptors at the back and blood vessels on top.

Most of the eye’s blood comes from the choroid behind the retina, so it does not block light.

The retinal blood vessels are a few and only cover a small part of retina.

The eye’s optics and light scattering reduce the shadows made by these vessels.

This makes the blood vessels even harder to see in normal vision.

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Describe how a clinician could use the ‘purkinje tree’ entoptic phenomenon to assess retinal function in a patient with media opacities.

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Describe how a clinician could use the ‘purkinje tree’ entoptic phenomenon to assess retinal function in a patient with media opacities.

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By shining light into the eye at a very oblique angle and keeping the light moving to prevent adaptation (eg: ophthalmoscope or bright pen torch).

The patient might be able to see the shadow of retinal vessels entoptically projected into the visual field due to the moving light.

If the patient describes seeing a purkinje tree, it means light is reaching the retina and retina photoreceptors and optic nerve are functional, despite opaque media (normally transparent structure of the eye become cloudy or

non-transparent) eg: Dense cataract, corneal scar and edema.

If the patient did not see the purkinje tree and the technique was performed correctly, it means a deeper problem is affecting the retinal or optic nerve function. eg: retinal detachment and severe macular degeneration.

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Haidinger’s brushes are used to assess macular integrity. Explain why a patient with early AMD might struggle to perceive them, despite 6/6 acuity.

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Haidinger’s brushes are used to assess macular integrity. Explain why a patient with early AMD might struggle to perceive them, despite 6/6 acuity.

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Haidinger’s brushes are a subtle entoptic phenomenon seen as a rotating bowtie or hourglass shape near one’s central vision when looking at polarized light.

This phenomenon arises from the interaction between polarized light and the unique arrangement of xanthophyll pigment and Müller cell fibres in the macula.

Its visibility depends on the normal anatomical and optical structure of the macular area.

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In the early stages of age-related macular degeneration (AMD), changes such as disorganized Müller cells, drusen deposits, or initial photoreceptor damage can disrupt the

structures required to detect polarized light, thereby affecting the visibility of Haidinger’s brushes.

Even when a person still has sharp visual acuity (e.g., 6/6), subtle damage to the retinal pigment epithelium or the macular birefringent tissue may hinder this effect.

Thus, the absence of Haidinger’s brushes could be an early sign of macular dysfunction, potentially appearing before noticeable declines in central vision.

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A diabetic patient reports intensified blue-field entoptic phenomena. Link this observation to retinal microvascular changes in diabetes.

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A diabetic patient reports intensified blue-field entoptic phenomena. Link this observation to retinal microvascular changes in diabetes.

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When looking at a bright blue background, some people notice tiny, rapidly moving white or yellowish dots, this is known as the blue-field entoptic phenomenon.

These dots represent white blood cells (leukocytes) moving through the capillaries in front of the retina, especially in the macular area.

The phenomenon occurs because blue light is absorbed by red blood cells (erythrocytes) but passes through leukocytes, creating enough contrast for the eye to detect the motion of these white blood cells.

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In individuals with diabetes, particularly in the early stages of diabetic microvascular disease, blood flow through the retinal capillaries may become uneven due to factors like capillary loss, small bulges in the vessels (microaneurysms), and damage to the inner lining of blood vessels.

These issues can disrupt the normal flow of leukocytes, causing them to move more slowly or stick to vessel walls.

As a result, the person may experience a more noticeable or intensified version of the blue-field entoptic phenomenon.

This increased visibility of leukocyte movement could serve as an early functional sign of diabetic retinopathy, even before any visible structural damage appears in the retina.

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Here are the references used to complete this tutorial!

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Here are the references used to complete this tutorial!

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American Academy of Ophthalmology. (2017, November 2). Diffraction. https://www.aao.org/education/image/diffraction

American Academy of Ophthalmology. (2024, October 24). Eye floaters and flashes. https://www.aao.org/eye-health/diseases/what-are-floaters-flashes

Atchison, D. A., & Smith, G. (2000). Optics of the human eye. Butterworth-Heinemann.

Benjamin, W. J. (Ed.). (2006). Borish’s clinical refraction (2nd ed.). Elsevier.

Glasser, A., & Campbell, M. C. W. (1998). Presbyopia and the optical changes in the human crystalline lens with age. Vision Research, 38(2), 209–229. https://doi.org/10.1016/S0042-6989(97)00143-2

Holladay, J. T., Piers, P. A., Koranyi, G., van der Mooren, M., & Norrby, S. (2002). Improved visual performance with aspheric intraocular lenses. Journal of Cataract & Refractive Surgery, 28(3), 617–623. https://doi.org/10.1016/S0886-3350(02)00073-7

Jinabhai, A., Radhakrishnan, H., O’Donnell, C., & Tromans, C. (2012). Higher order aberrations in keratoconus: a review. Eye, 26(9), 1194–1201. https://doi.org/10.1038/eye.2012.121

Leibowitz, H. W., & Owens, D. A. (1978). New evidence for the intermediate position of relaxed accommodation. Documenta Ophthalmologica, 46(1), 133–147. https://doi.org/10.1007/BF00174134

Lundström, M., Stenevi, U., Thorburn, W., & Roos, P. (2002). Visual function and light scattering in cataract patients. Journal of Cataract & Refractive Surgery, 28(10), 1826–1832. https://doi.org/10.1016/S0886-3350(02)01162-8

Owens, D. A., & Leibowitz, H. W. (1980). Night myopia: cause and a possible basis for its correction. American Journal of Optometry and Physiological Optics, 57(5), 316–323. https://doi.org/10.1097/00006324-198005000-00007

Rabinowitz, Y. S. (2003). Retinal image size and field of view in aphakic and pseudophakic eyes. Survey of Ophthalmology, 48(5), 515–534. https://doi.org/10.1016/S0039-6257(03)00090-7

Williams, D. R., & Coletta, N. J. (1987). Blue field entoptic phenomenon and macular function. Investigative Ophthalmology & Visual Science, 28(3), 447–455. https://iovs.arvojournals.org/article.aspx?articleid=2178460