Document
Page 63
Fig. 5.12
The simple magnifying glass (the loupe). (a) Object viewed at near point of unaided eye, 25 cm, subtends angle θ1 at the
eye. (b) Object viewed close to the eye through a convex lens, with object at first principal focus of convex lens.
Object and image subtend angle θ2 at the eye.
and
thus
But 25 cm = 
m, and 
= F dioptres, where F is the power of the lens in dioptres.
file:///C|/download/www.netlibrary.com/nlreader/nlreader.dll@bookid=51924&Filename=Page_63.html (1 of 2)9/30/2006 2:35:28 PM
Document
Thus, the commonly used × 8 loupe has a lens power of +32 dioptres.
file:///C|/download/www.netlibrary.com/nlreader/nlreader.dll@bookid=51924&Filename=Page_63.html (2 of 2)9/30/2006 2:35:28 PM
Document
Page 64
Spherical Lens Decentration and Prism Power
Rays of light incident upon a lens outside its axial zone are deviated towards (convex lens) or away from (concave lens) the axis. Thus the peripheral portion of the lens acts as a prism.
The refracting angle between the lens surfaces grows larger as the edge of the lens is approached (Fig. 5.13). Thus the primatic effect increases towards the periphery of the lens.
Fig. 5.13
Prismatic deviation by spherical lenses.
Use of a non-axial portion of a lens to gain a prismatic effect is called decentration of the lens. Lens decentration is frequently employed in spectacles where a prism is to be incorporated. On the other hand, poor centration of spectacle lenses, especially high power lenses, may produce an unwanted prismatic effect. This is a frequent cause of spectacle intolerance, especially in patients with aphakia or high myopia.
It is thus of importance to be able to predict the prismatic power gained by decentring a spherical lens. This is given by the formula
file:///C|/download/www.netlibrary.com/nlreader/nlreader.dll@bookid=51924&Filename=Page_64.html9/30/2006 2:35:28 PM
Document
Page 65
where P is the prismatic power in prism dioptres, F is the lens power in dioptres, and D is the decentration in centimetres.
The increasing prismatic power of the more peripheral parts of a spherical lens is the underlying mechanism of spherical aberration (see p. 92). Furthermore, it causes the troublesome ring scotoma and jack-in-the-box effect which give rise to great difficulty to those wearing high-power spectacle lenses (pp. 131, 132).
file:///C|/download/www.netlibrary.com/nlreader/nlreader.dll@bookid=51924&Filename=Page_65.html9/30/2006 2:35:28 PM