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If a low vision Px cannot resolve the retinal image despite it being optimally focussed, then it is necessary for it to be made larger. There are a number of different means via which this can be achieved: 1. Relative Size Magnification 2. Relative distance Magnification 3. Angular Magnification 4. Real Image Magnification
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Magnification is relative It is the ratio comparing the situation before and after some change in the viewing environment. Magnification is NOT about making objects clearer it is simply about making them bigger. Magnification (M) = new retinal image size old retinal image size
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Unmagnified Object d 1 h 1 N E h 1 ’ d 1 h 2 N E ’ h 2 ’ Increasing the Object Size
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M = ’ Which for small angles tan ’ M = tan = h 2 x d 1 = h 2 ’ d 1 x h 1 h 1 M = new object size old object size Examples include large print books, large TV screens etc. The magnification is calculated in comparison to a standard size, e.g. if large print books are N24 and standard books are N10, then the relative size magnification provided = 2.4X Relative Size Magnification
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Unmagnified Object d 3 h 1 N E h 3 ’ ’ Decreasing the Viewing Distance d 1 h 1 N E h 1 ’
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Relative Distance Magnification M = tan = h 1 x d 1 = d 1 ’ d 3 x h 1 d 3 M = old object distance new object distance The simplest way to increase magnification is to decrease the viewing distance (Approach Magnification). If a Px watched TV at 3m and they move closer to 1m, what is The magnification? Moving a reading task from 30cm to 5 cm will produce what value of magnification? Any problems with this approach?
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Angular Magnification. d 1 h 1 N E h 5 ’ ’ Optical Device e.g. Telescope
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Angular Magnification M = angle subtended at the eye by instrument image angle subtended at the eye by object This is generally refers to the magnification provided by optical instruments (e.g. telescopes). Versatile means of magnification as it does not involve changing object or viewing distance.
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d 1 h 4 N E ’ h 4 ’ Real Image or Transverse Magnification h 1
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Real Image or Transverse Magnification M = tan = h 4 x d 1 = h 4 ’ d 1 x h 1 h 1 M = size of real image size of object This is typical of the situation used with a CCTV device where a magnified image of the object is created on a TV screen. This real image is created in approximately the same location as the object.
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Different types of magnification can be used in conjunction with one another. The total magnification is the product of the two values. A Px can barely see N12 at a reading distance of 40cms. They now use large print books (N24) at 30 cm. Relative size magnification = 24/ 12 = 2x Relative distance magnification = 40/30 = 1.33x Total Magnification = 2 x 1.33 = 2.66 Combining Magnification
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Prescribing Magnification Even when low vision Pxs are optimally refracted this is not sufficient to improve their vision to required levels for a particular task. Therefore some value of magnification is required. How in practice can the optometrist obtain an appropriate value for Magnification?
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1. Identify specific tasks and predict the amount of magnification needed. 2. Is binocular or monocular correction preferable? 3. Select an appropriate LVA, if required. 4. Perform a trial for suitability 5. Determine any spectacle correction that might be necessary 6. Loan out device after training. 7. Follow-up visits.
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Predicting the magnification required Magnification required = required VA present VA In Snellen notation to improve from 6/60 to 6/6 Magnification required = 6 x 60 6 x 6 = 10 x Watching TV 6/18 Reading Bus Numbers 6/6
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Magnification used = achieved VA present VA So achieved VA = magnification x present VA For example a Px with a VA = 6/36, using a telescope with a Magnification = 4x would achieve a VA of: 4 x 6/36 = 6/9
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If VA is measured in a LogMAR notation: Magnification = (1.25) n Where n = number of steps If the present acuity = 0.5 and the required acuity = 0.1 Then Magnification = (1.25) 4 = 2.44x
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N - notation N print uses New Times Roman font and is the standard UK test. It has a linear scale: N10 is 2x the size of N5 Magnification required = present VA required VA N48 N6 M = 8x A measurement of near VA should always be accompanied by the working distance at which it is taken.
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Near Magnification. In theory it should be possible to calculate what letter size a Px will be able to read at near from a knowledge of their distance VA. However, reading is not equivalent to single letter identification. The values used to determine magnification for near tasks are based on threshold values. Therefore we have to incorporate a READING RESERVE into our calculations so that comfortable reading is possible for longer periods.
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This is done by halving the size of the required N value before determining the magnification. Similarly one can double the size of the magnification is you work with threshold values. The problem with this approach is that it doubles the amount of magnification which has consequences for the field of view. In practice what often happens is that a magnification 1.5 times that based on threshold values is used.
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Example: 65-year-old patient with distance spectacles of -1.00DS both eyes and reading spectacles of +4.00 DS both eyes. With their reading spectacles, they see N24, and they want to read the newspaper. N8 for newspaper at threshold N4 for reading reserve, Mag. required = present VA/ VA wanted = N24/N4 = 6X Practical use 6/1.5 = 4X (N6 is adequate for comfortable reading of newsprint).
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Binocular v Monocular Magnification If the acuities of the two eyes are similar then binocular viewing is beneficial to the Px. However, the design of the magnifier or the limitations that it places on viewing conditions may only allow monocular correction.
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Selection of an LVA Once a value of the magnification required has been predicted the task for the practitioner is to find the best way for the Px to obtain this value. Remember that we have four options at our disposal: 1. Make the object bigger 2. Decrease the working distance. 3. Increase the Angular magnification with an optical device. 4. Real image magnification.
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Distance Magnification The only practically useful aid is the telescope. In some cases it may be possible to move an object closer (e.g. moving closer to the TV). Real image magnification is rare beyond arm’s length.
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Near Magnification The most common magnifier for near tasks is the the +ve lens which can be used in one of its various guises: spectacle microscopes hand held magnifiers stand magnifiers The object can be made larger (relative size magnification) but this typically restricts magnification to a maximum of 2x. Real image magnification is possible with CCTV. Telescopes for near (telemicroscopes) can be very useful particularly if the Px wishes to perform manipulative tasks.
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Trial and Modification of Predicted Magnification Record acuity Manner of reading: e.g. ‘fast’ ‘too slow’ ‘one letter at a time’ etc.
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Spectacle Correction For near vision magnifiers the general rule is that for: hand held magnifiers distance Rx stand magnifiers reading Rx In the case of spectacle microscope the Px distance Rx will need to be taken into consideration. e.g. A –4.00DS myope requires 3x magnification which is supplied by a +12.00DS lens. The final lens = +8.00DS
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Spectacle Correction (cont’d) Distance telescopes can be used in conjunction with the distance Rx. However, the largest field of view is obtained when the exit pupil of the telescope is as close as possible to the entrance pupil of the eye.
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Training the Px How to Use the LVA The Px should receive good task related training on the use of the LVA. Both written and verbal instruction is advisable, Be clear about which spectacles will be needed in conjunction with The LVA. Be clear about the working distance that should be employed. Should RE, LE or both eyes be used. Emphasize the importance of good illumination.
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Follow Up Visits. Typically you would see a Low vision Px about 2-3 weeks after the initial supply of the LVA. Most common reasons for problems: VA has deteriorated (pathology still active) LVA being used sub-optimally. LVA is being used for a task other than that for which It was initially intended.
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