The eye can increase its power to focus on near objects
The ciliary muscle controls the tension on the zonule – this allows the ciliary muscle to contract, causing the zonule fibres to relax, causing the lens to thicken/bulge out & this increases the power of the eye
SEE GRAPHS ON LECTURE
With high minus lenses the eye accommodates a lot, creating an unsteady line on graph (VA becomes unsteady)
When refractive error is higher than the amplitude of accommodation (how much we’re able to accommodate) then VA deteriorates/is weak
The highest peak of graph (best VA) is the most plus/least minus that gives the best VA
If you give any more plus VA deteriorates
At birth, our amplitude of accommodation is ~20.00D, it reduces with age from the day we are born – this happens because the lens capsule in the eye loses its elasticity & the number of fibres increases over time.
This varies between people but we don’t notice it happening – only when amplitude goes below that needed for reading do we notice it reducing
Each individual probably has a linear reduction of amplitude (& hence reduction of being able to focus at near) with age (can lower to as far as 0.50D)
If you have 2.00D amplitude of accommodation, you can only see as close as 50cm (1/2 = 0.5m)
Presbyopia happens to everyone & is not related to patient’s refractive error (therefore near tests should be done on everyone) – presbyopes may require additional tests
Reading glasses, positive spheres make near objects clear (& distant objects blurred)
Pre-presbyopes are usually less than 45 y/o & have adequate amounts of accommodation for near vision
Presbyopes need near Rx, as their accommodation is no longer adequate for near vision. They are generally over 45 y/o but may depend on: race, occupation, environment (e.g. light levels – higher light levels of sunlight/UVradiation causes lens to go through aging process quicker i.e. in hot country), pupil size, medication etc.
Regardless of refractive error, when wearing the distance correction near objects are blurred
Presbyopes therefore need different correction for distance & near
ALWAYS consider near visual requirements for ALL PATIENTS
All tests so far have been at 6m
Now consider near vision – usually 33-40cm
Adjust trial frame for near PD
Adjust lighting levels to match patients’ usual setting (artificial light/bright light makes pupil smaller so acts as a pinhole to make vision clearer)
Tests for near vision:
Near acuities – monocularly & binocularly
Accommodation – monocular & binocular
Determination of near addition if presbyopic
Important to determine & record near acuities & the distance at which it’s measured (ask patient to hold a book at their normal reading distance & measure distance from book to eye – this is after you’ve added the near addition for presbyopes/older patients)
Do this for each eye independently
If they have poor near acuity may need to determine near addition first – e.g. presbyopes
Measurement of the Amplitude of Accommodation (for everybody)
We done experiments based on: Gordon’s Glazing Window Company. Convenient & common method for fine-tuning astigmatic element found in retinoscopy. Requires the retinoscopy to be within ~1.00DC. It Cannot be relied upon if remaining cyl to be corrected is >1.00DC. Either 0.25 or 0.50 cyls depending on level of astigmatism that needs correction.
Handle is at 45° to axes. The number written on the handle indicates the astigmatic interval between the two meridians. E.g. a cross cyl marked 1.00D has meridians of power +0.50D & -0.50D
The markings on the lens are at plus & minus 45° to the handle, & indicate the axes meridians, not the power meridians.
Cross cyls are available in ±25, 0.50, 0.75 & 1.00 D powers, which result in astigmatic intervals of twice these amounts.
Choose a cross cyl that has an astigmatic interval equal to or less than value of Px’s estimated cyl
For the target, most practitioners use double black rings on a white background as targets. (One concentric circle, say which one to look at)
The circles subtend 6/12 & 6/4.5 so most patients can resolve larger circles with their best sphere acuity
Sometimes letter “O” from Snellen chart is used – letter size must be just better than the patient’s best sphere acuity for each eye
Cross cyl method is divided into two parts:
To find & refine the cylinder axis
To refine the cylinder power
To find cyl axis:
Occlude one eye
Start with cyl obtained from ret in the trial frame (of other eye)
Hold cross cyl handle parallel to axis of cylinder in trial frame in front of other eye & twirl (flip over, still keeping handle in that position)
Ask for preference while looking at the rings: “Are the circles blacker, clearer & rounder with lens 1 or lens 2?”
Make sure to notice which position you’re holding cross cyl, & whichever “lens” they prefer, (flip back if lens 1) move the axis 10° towards the –ive/minus axis meridian
Then if patient tells you to go the other way, only move it by 5°, & when alternating keep decreasing the interval until you get to 2.5°/1°
To find cyl power:
Turn cross cyl through 45° so that cross cyl axes are parallel with & at right angles to the trial case cyl axis
Ask again for preference while looking at rings: “Are the circles blacker, clearer & rounder with lens 3 or lens 4?”
Put in the power they prefer
If no cyl has been found with ret
Hold cross cyl axes at 90 & 180 & twirl (flip)
If for e.g. prefers 90 then turn through 45° & twirl (essentially asking would you like minus cyl a bit >90 or a bit <90) – first preference narrowed it down to vertical rather than horizontal, the second preference could be from 45 to 90, or 90 to 135)
Then put axis meridians at suitable axis (middle of interval) & ask which power they prefer, plus or minus & give
Then refine axis again
If there’s no preference on the first twirls with cross cyl to refine axis then either they don’t have any astigmatism or by chance the axes are 45 & 135. Try again with the handle at 90°, if still no preference – no astigmatism. If there is a preference don’t forget that axes must be 45 or 135
After retinoscopy check VA & do +1.00 blur test
If ~6/9 & blurs to ~6/18 do cross cyl
If ~6/9 & does not blur with +1.00, optimise sphere (best sphere) & then do cross cyl
If worse than 6/9, optimise sphere
Still poor VA? Pinhole? F&B?
Patients need to have a small amount of active accommodation ~0.25 for cross cyl i.e. +1.00 only blurs to ~6/12 – R = G or just on green on duochrome. So before doing cross cyl check that VA is on green (can add more -ive spheres but not more than -1.00DS, then take it out after finishing). Distance VA should be R = G or just on red whereas near VA should always be on green.
The world is not as advanced as the Time Traveller had hoped. The Time Traveller thought technology and science would continue to advance into the far future however, it has become clear that all the technological advancements made were pointless. This is shown when the Time Traveller make the observation of the Eloi when he first meets them, “They struck me as being very beautiful and graceful creatures, but indescribably frail.” This tells us that the creatures in the future are weak and fragile due to technology. These creatures had evolved from the rich, upper class who had pushed the working class down and became dependant on technology to survive. Wells is telling us to appreciate those who work as they are the ones who prop the upper class into their positions of power. The pyramidal class system will become nothing, and this is scarily evident today as cuts are being made to support systems for the poor, so the rich people can become even richer, leading to detrimental social divides.