Lecture 6 - Optical Low Vision Aids Flashcards
What is magnification?
• will be needed if prescription does not create enough improvement and if visual acuity is insufficient for identified tasks
• need to make the retinal image size greater hence easier to see
• defined as: «before” retinal image size/”after” rethal image size
• depends on angle (0) subtended by object at nodal point (NE) of eye (Figure 3)
What are the ways magnification can be achieved?
- increase size of object
- decrease viewing distance
- real image or transverse magnification (e.g CCTV)
- telescopic magnification
What are examples of increasing size of object?
Examples:
• large screen TV
• large print books, newspapers, typewriters and labelling machines, telephone dials and push-buttons, medicine labels, bills and bank statements (on request), information leaflets
• photographic enlargement or photocopying
• computers and magnification software; (see appendix1)
• large monitors, keyboards (or key labels - also reverse contrast)
What is decreasing viewing distance?
magnification = (new viewing distance/old viewing distance)
• very simple and effective - called “approach” or “relative distance” magnification
- works at distance or near if sufficient accommodation - used frequently by children/ young adults with good accommodation; myopes remove their spectacles
- Using a short viewing distance without necessity for accommodation requires use of a plus lens (magnifier).
Examples of Decreased Viewing distance?
• Distance: viewing TV from 6 feet rather than 12 feet gives 12/6 = 2x magnification
• Near: bring print to 5cm rather than 30cm gives 30/5=6x magnification
What is real image/transverse magnification?
magnification = (size of original object/size of image)
• Not an optical method - see electronic LVAs
• Advantages include the ability to adjust the magnification factor and manipulate contrast and colour
What is Telescopic magnification?
magnification = (angle subtended by telescope image (0’)/angle subtended by original object (0’)
• Optical method
• Versatile (used at distance, intermediate and near)
Can methods be combined?
Different methods can be combined and the total magnification is found by multiplication.
Example:
What is the magnification obtained by changing print size from 6mm to 15mm, and viewing distance from 30cm to 10cm?
M1= new obiect size/ old object size = 15/6 = 2.5x
M2 = old viewing distance/new viewing distance = 30/10=3x
The total magnification Mt= M1 x M2 =2.5 x 3 = 7.5x
What are the different implications for the different magnifications methods?
• Increase size:FOV - no change , Working space - no change , Distance of task (Near, Intermediate, Distance)- N (I+D limited)
• Decrease distance: FOV - No change, Working space - Decreased, Distance of task (Near, Intermediate, Distance)- D I OR N
• Using plus lens: FOV - decreased , Working space - decreased, Distance of task (Near, Intermediate, Distance) - N
• Real image: FOV - decreased , Working space - no change, Distance of task (Near, Intermediate, Distance), - N (I, D limited)
• Telescopic: FOV - decreased, Working space - ** no change, Distance of task (Near, Intermediate, Distance) - ** D, I or N**
Calculation for plus lenses: What is nominal magnification?
• M=F/4
• Image at infinity - no near add or accommodation required as parallel light leaves the lens and enters the eye
• Eye to magnifier distance has no effect on magnification
•Called “Nominal magnification”
Field of view of plus lens magnifiers:
• Reading speed is affected by extent of text seen through a magnifier, it is desirable for LVAs to provide a large a field of view.
• The usable FoV mainly depends on three factors:
- lens power, lens size and aberrations. Geometrical optics
• Which does not take account of lens aberrations, enables us to easily calculate the theoretical FoV of a magnifier.
Equation for FOV:
Field of View =
Diameter of magnifier/ d x Power of magnifier
FoV = DMag × fMag/ d
Where d is the magnifier-to-eye-distance and D is the diameter of the magnifier.
To conclude : Whats important about hand magnifiers:
• When a hand held magnifier is used by an emmetrope the reading material should be place the focal point of the lens. Distance from lens to eye has no impact on the retinal image size, affects the field of view.
• Key point: Magnifiers have to be held close to the eye to maximise the field of view.
Power of a thick lens/lens system calculation:
Feq = F1 + F2 - (d/n)F1F2
Feg = power of thick lens or system
F1= power of 1st surface or lens
F2 = power of 2nd surface or lens
d = distance between lenses (surfaces)
n = refr. index between lenses (surfaces)
What ate 3 important points for a near add and magnifier to eye distance:
- less than the magnifier’s focal length the equivalent magnification will be higher than the magnifiers nominal magnification
- greater than the magnifier’s focal length the equivalent magnification will be lower than the magnifiers nominal magnification
- equal to the magnifier’s focal length the equivalent magnification will be equal to the magnifiers nominal magnification
Key Point: Using a magnifier with reading R requires a short magnifier to eye distance. Long magnifier to eye distances can result in very low enlargement ratios.
