Spectacle microscopes are positive lenses used for magnification during near tasks like reading. They provide magnification through relative distance magnification by holding objects closer to the eye. Spectacle microscopes can be high reading additions above +3.00DS, simple magnifiers from 2-10x, or compound magnifiers above 10x requiring careful consideration of working distance. While binocular vision is preferable, high powers can exceed the limits of convergence demanding monocular use. Stand magnifiers provide hands-free magnification but require a stand that may obstruct access or writing space. Working distance and needed adds depend on factors like stand height and distance from the eye.

1. Spectacle Microscopes

3. Spectacle Microscopes Definition A spectacle mounted +ve lens used to produce magnification for reading or near point tasks. Usually of greater power than the traditional maximum for a reading add (+3.00DS) Magnification = Relative Distance Magnification RDM = reference distance = F equivalent old distance 4

4. A +ve lens as a magnifying aid allows a Px to obtain an increased retinal image size by holding the object closer to the eye without placing extra demands on accommodation. The object is placed at the anterior focal point of the lens so that parallel light emerges from the device. f  Relative distance Magnification Angular Magnification

5. Most commonly used between 3x – 6x Can be considered to cover the range between 1.2x – 20x 3 4 5 6.25 8.33 12.5 Working Distance (cm) 8x 6x 5x 4x 3x 2x Magnification (F/4) g

7. High reading additions Defined as adds which result in a working distance less than that which the Px would normally have preferred. Generally binocular. Usually > +3.00DS but < +10.00DS

8. Simple Spectacle magnifiers Monocular Lens Magnification 2x to 10x

9. Compound spectacle magnifiers When > 10x magnification is required Monocular Working distance is very critical

10. Binocular vs Monocular Correction Binocular vision provides many advantages. Close working distances required when using spectacle microscopes may simply demand too much convergence. 1  for each eye per dioptre of add +10.00DS give 10  in RE & LE i.e. total = 20  Base-in prism is often incorporated

11. Likelihood of achieving good binocular vision: +6.00 (1.5x) - easy +8.00 (2x) - difficult +10.00 (2.5x) - risky +12.00 (3x) - highly unlikely

14. Stand Magnifiers

16. f  Relative distance Magnification Angular Magnification Hand Magnifier M = Fequivalent 4

17. Magnifier to object distance is < focal length of the lens Light is divergent when leaving the magnifier The Px must therefore accommodate to neutralise this divergence f h

18. F eq = F M + F A - z F M F A F A f h F M z

19. A stand magnifier with a +20.00DS lens has a stand height = 4cms F M = +20.00DS object distance l = -0.04m The image distance l’ = 1/-5.00DS = -0.20m = 20 cm. L’ = L + F M = -25.00DS + (+20.00DS) = -5.00DS L’ is the emerging vergence L = 1/-0.04 = -25.00DS L is the vergence of light entering the lens

20. If the Px’s eye is directly behind the magnifier (z = 0) then a reading addition of +5.00DS will be required to neutralise the emerging vergence. However, this is rarely the case. z typically is = 20cm but may be a little as 5 – 10 cm for high powered magnifiers. Assuming that z = 20 cm then the new image distance is z + l’ = -20 + -20 = -40 cm 1/-0.40 = -2.50 DS Therefore an add of + 2.50DS will be required at this distance.

21. Magnification F eq = F M + F A - z F M F A M = F equivalent 4 When z = 0 F M = +20.00DS & F A = +5.00DS F eq = +20.00 + 5.00 - 0 = +25.00DS M = 25 /4 = 6.25x When z = 0.2m F M = +20.00DS & F A = +2.50DS F eq = +20.00 + 2.50 - (0.2 x 20.00x 2.50) = +12.50DS M = 12.5 /4 = 3.1x

23. Low-Powered Stand Magnifiers

24. < +16.00DS (4x) Up to –10.00DS Spherical up to 75mm Aspheric up to 100mm 40 mm with z = 25cm Design usually allows illumination of and access to object plane Magnification Range Typical emergent vergence Lens form & Diameter Typical linear field of view Other features Low powered Stand Mag Characteristics

25. Medium/high powered Stand Magnifiers

26. +16.00DS to +80.00DS (4 – 20x) plano to –4.00DS Aspheric up to 50mm Doublet lenses up to 20 mm 100 mm with z close to 0 Usually internally illuminated Magnification Range Typical emergent vergence Lens form & Diameter Typical linear field of view Other features Med/high powered Stand Mag Characteristics

28. Bar & Flat Field Magnifiers

30. Bar Magnifier Solid hemi-cylindrical lens which magnifies vertically

31. Flat Field Magnifier Solid hemisphere magnifies in all meridians