Magnifiers Flashcards
A & D of binocular high adds
A: hands free, cosmetically acceptable, cyl correction, large FOV, better for prolonged reading
D: increased power - increased aberration, decreased reading speed, depth of focus, lens size, FOV. Lighting is more critical.
Binocular high adds magnification range
Up to 12x
A & D of hand held magnifiers
A: portable, extended WD, relatively cheap, cosmetically acceptable, easy to use and prescribe, allows head movement and quick spotting, illumination available
D: not hands-free, not suitable for Px with tremors/stability issues, decreased reading speed. Will need to change batteries if illuminated. Decreased FOV with increased WD.
HH magnification range
Up to 12.5x
A & D of stand magnifiers
A: hands-free, portable, good for px with tremor or poor motor control, large range of powers, illumination
D: add or accommodation required, decreased FOV, can be heavy and bulky, postural problems
Stand magnification range
Up to 15x (max optically)
How do you verify a magnifier?
Need to determine Fe, L’ if they’re a stand magnifier, and W.
- Use vertometer to measure FVP, which approximates Fe, or can use the method of triangulation where you measure object height/length ratio to image height/length
- L’ = Highest plus lens for which image is still clear
- W = A/(zFe), physically measure aperture size (mm)
Compare Keplerian and Galilean telescopes.
K: +O/+E lenses, longer tube length, requires prism to erect image (therefore heavier), E’ outside system (therefore greater FOV), magnification up to 10x, better image quality, expensive
G: +O/-E, lighter, shorter tube length, E’ inside system (therefore smaller FOV), mag up to 4x, poorer image quality, cheaper
Assessment and instructions for binocular high adds
- Spectacles are usually 1st option demonstrated
- Fine tune the add using practical material, scroll the material in front of the Px and check that the achievable reading speed is appropriate
- Importance of lighting and WD
Assessment and instructions for hand held magnifiers
- Lay magnification on the page and slowly lift up until clearest image is formed. The lens should be parallel to the page, with convex side facing the Px.
- Demonstrate how the FOV increases as the page and magnifier are brought closer to the eye
- Move both the eye and magnifier as a unit
- Lighting important
Assessment and instructions for stand magnifiers
- Need to wear near Rx
- Rest the stand on the reading material and ensure your line of sight is perpendicular to the magnifier
- Move the eyes and the magnifier together as a unit
- Use appropriate illumination and maintain good posture
What are visolettes?
Planoconvex lenses that sit on the page. Add is required for these as they are a sub-category of stand magnifiers.
A: simple, sturdy and stable, good light gathering properties, appropriate for children
D: only available in low magnification (2x)
Factors to consider in the optical design of a telescope
FOV: limited by E’ diameter, objective lens diameter, M, d & vertex distance
Weight: decreased by plastic casing
Aberrations: decreased by doublet lenses
Light transmission: reduced loss by AR coats
Retinal illuminance: proportional to area of E’
Reduction in brightness = (E’ diam)^2/(pupil size)^2
Performance in dim illumination = Obj diam x M (larger value gives greater performance)
Altering tube length and refractive error
Shorten tube length to correct for myopia (gives greater magnification with Keplerian)
Longer tube length to correct for hyperopia (gives greater magnification with Galilean)
What are the things to consider when prescribing a telescope?
Level of magnification required (current PS/target PS), want to give the lowest M to meet Px’s goals
- If unspecified, assume 6/12 target
Binocular vs. monocular Adjustable focus Handheld vs. spectacle mounted Stability vs. portability Cost factors
