1. AREA 2: Fitting single-vision lenses
Topic 4: Alignment, decentration, and minimum diameter
Topic 5: Calculating thickness in spherical lenses
Topic 6: Calculating thickness in astigmatic lenses
Topic 7: How lenses affect vision
Topic 8: Prismatic effects
Topic 9: Base rules
Topic 10: How alignment affects binocular vision
This area will take a look at the different aspects of fitting single-vision lenses.
As their name suggests, single-vision lenses are used for just one viewing range and therefore have just one power, unlike bifocal, trifocal, or progressive lenses.
Fitting a patient with single-vision lenses involves a series of structured steps. The first step in the process is to choose a suitable lens and align it correctly. We will be looking at this in topic 4.
We will learn how to perform a series of calculations to ensure that, once inserted in the frame, the lenses are correctly aligned with the centres of the patient's pupils. The process involved is known as decentration, and this will also be explored in topic 4.
Topics 5 and 6 will examine how the thickness of a lens can vary from the centre to the edge. This is easy to calculate for spherical lenses, but slightly more complicated for astigmatic lenses as each of their principal meridians has a different power. Once you know how thickness varies across a lens, you will be able to advise your patients on the types of lenses and frames that suit them best.
Topic 7 will look at how lenses alter vision.
Topics 8 and 9 will analyse prismatic effects. We will learn what they are, how to calculate them, and how they can affect a patient.
Finally, topic 10 will explore different aspects of binocular vision.
##Falta explicar els temes 7,8,9 i 0##

2. Topic 4: Alignment, decentration, and minimum diameter
Horizontal alignment
Vertical alignment
Minimum diameter (Omni)
Diameter in minus lenses
Optimum diameter in plus lenses
Precalibrated diameter in astigmatic lenses
This topic will look at the concepts and calculations behind horizontal and vertical alignment, or decentration. Decentration refers to the displacement of the optical centre of a lens with respect to the frame until it coincides with the centre of the patient's pupil.
We will also learn how important it is to calculate minimum lens diameter (Omni). This is the minimum diameter required to cover the whole frame and it is essential if fitting is to be a success. We will also look at its importance in plus and minus lenses.
Finally, we will explain what precalibration is and discover when it is important for astigmatic lenses.

3. Alignment measurements
A+DBL A
A - DBL
Optical alignment refers to the different processes involved in determining the exact position in which a pair of lenses should be fitted into a frame that has been adapted to a wearer.
The calculations required are made using measurements from three sources: the wearer, the frame, and the lens.
The aim is to match the optical centre of the lens once it has been inserted in the frame to the centre of the patient's pupil in primary gaze.
The slide shows a series of frame measurements (A+DBL), the sum of which equals DBC, the distance between the frame centres. These measurements can be taken with a ruler or a pupillometer.
The height of the pupil with respect to the selected frame must also be measured.

4. Horizontal alignment Horizontal displacement DC hd MPD A+DBL
As we have already mentioned, the alignment process requires measurements corresponding to the wearer, the lens, and the frame.
We first need to establish a reference point from which the different measurements will be taken. To simplify matters, we will use the frame centre (fc) defined in module 1. The horizontal distance between this centre and the centre of the pupil, which we will call hd, can be calculated using the following formula:
hd = [(A+ DBL)/ 2] – MPD
A positive number (hd=+) means that the optical centre needs to be displaced towards the nose, i.e. towards the right in the right eye and towards the left in the left eye.

5. Vertical alignment Vertical displacement vd=ph-(h/2) DC vd ph h/2
In this case, vertical displacement (vd) is the vertical distance from the centre of the pupil to the frame centre. As the figure shows, this distance can be calculated using the following formula:
vd = ph – [ h/2 ]
A positive result means that the optical centre will need to be displaced upwards in both eyes.
vd=ph-(h/2)

6. Minimum diameter (Omni)
Unedged lenses are round and their optical centre is near their frame centre. The diameter of a lens must be big enough for the lens to cover the entire eyepiece (including the bevel groove) when its optical centre is aligned with the centre of the patient's pupil.
This diameter is referred to as the minimum diameter.

7. Minimum diameter (Omni )
rmin CD vd
min= 2· rmin + 2 mm hd
The minimum diameter is, therefore, twice the distance from the centre of the pupil to the furthest point on the frame (r). 2 mm must then be added to this distance to cover the bevel groove, which is where the edge of the lens is inserted.

8. min = A + A+DBL – 2·MPD + 2 mm min = 2·D + DBL – 2·MPD + 2 mm
Approximate minimum diameter (Omni) D
min = A + A+DBL – 2·MPD + 2 mm
min = 2·D + DBL – 2·MPD + 2 mm
min = D + A+DBL – 2·MPD + 2 mm
The amount of vertical displacement is often minimum if standard frame fitting procedures are followed. In such cases, presuming that the frame is more or less symmetrical, an approximate formula is normally used to calculate the minimum diameter.
min = D + A + DBL – 2MPD + 2 mm
where D is the longest distance (measured diagonally) between two points on the eyepiece.

9. Advantages of minimum diameter (Omni)
Lens with  min Lens with larger 
Once edged, a large-diameter plus lens will not have the same properties as a small-diameter plus lens. The picture shows how the second, larger, lens is thicker than necessary in the centre and at the edges. A larger-than-necessary diameter increases the weight of the lens and compromises comfort and cosmetic appeal.
When fitting plus lenses, thus, it is important to always calculate the minimum diameter to ensure that the smallest possible blank is used. The size of the blank has a major impact on the final properties of the lens, and particularly so with high-power lenses. This is why it is especially important to calculate minimum diameters when working with aphakic lenses.
Plus lenses

10. Advantages of minimum diameter (Omni)L’ A’
Lens with  min Lens with larger 
Standard diameters should be used in the case of minus lenses. It does not matter whether the diameter is larger than necessary. The only requirement is that the lens is big enough to completely cover the inside circumference of the eyepiece once it has been decentred. Minimum diameter is not important because it does not affect edge thickness. The lens will have the same dimensions once cut, regardless of its initial diameter.
Minus lenses

11. Advantages of minimum diameter (Omni)L’ A’
The same criteria should be applied when working with astigmatic lenses, but meridian by meridian as the principal meridians in astigmatic lenses have different powers.
If the lenses have plus powers, their diameters must be as small as possible.
These lenses are often referred to as precalibrated lenses.
The diameter of an unedged astigmatic lens with minus power, however, is not important.
Astigmatic lens  Precalibrated ?

12. Precalibrated diameter in astigmatic lenses
A precalibrated lens is a lens that has a minimum diameter calculated according to specific usage conditions and takes into account both the wearer and the selected frame model.
This is true for spherical lenses, i.e. lenses with the same power in all the meridians. With astigmatic lenses, however, each principal meridian must be considered separately.
The edges in the direction of the minus cylinder axis will always be the thinnest.
Precalibrated lenses are used in patients whose lenses, once fitted, would be excessively thick (and therefore cosmetically unappealing) in all directions.

13. Precalibrated diameter in astigmatic lenses
Non-precalibrated lens: 180° D
90° D
90°
Thin edge 0°
+4.00 D
To sum up, there are two possible scenarios:
· Both meridians have minus power: use the same criteria as for minus spherical lenses (the diameter of the blank lens is not important).
· At least one of the meridians has plus power: apply the minimum-diameter rule to the plus meridian (or meridians if both have plus power). When working with two different meridians, remember that the centre thickness does not vary.
The lens in the example on the slide has two plus meridians and the following edge thicknesses:
The thin edge will be in the direction of the axis with the minus cylinder (180º). Picture the lens once it has been edged and fitted into the frame. It would be quite thick at 90º (as the frame is oval, it would have been necessary to create a considerable edge at the vertical meridian, which means that the thickness of the lens would increase towards the centre) but quite thin in the direction of the horizontal axis.
The result, therefore, would be cosmetically acceptable and it would not be necessary to order a precalibrated lens.
##Se observa que las necesidades de diámetro en un meridiano son diferentes a la del otro de manera que si el cálculo del diámetro se realiza según el meridiano vertical el horizontal quedaría descubierto por lo tanto el diámetro de la lente virgen se debe hacer según el parámetro horizontal de la gafa, aunque al retallar la lente los espesores superior e inferior sean mayores. (no s’enten)##
+3.00 D
Thick edge

14. Precalibrated diameter in astigmatic lenses
Precalibrated lens: 90°
180°
90°
Thick edge 0°
+3.00 D
In this example, however, the situation is different because the meridian that requires the larger diameter has the smallest power (+3.00 along the horizontal axis). Although, like in the first example, the lens will be quite thick in the vertical direction once edged, it will also be quite thick in the horizontal direction given its lower power. Both thicknesses are greater than expected, and it would therefore be logical to consider reducing the centre thickness. In this case, it would make sense to order precalibrated lenses.
This concept is known as precalibration, and lenses of this type are known as precalibrated lenses.
More complex operations are required to calculate thicknesses when the principal meridians are not at 90º or 180º, but this is not the aim of this topic. It is important, however, to notify the lens manufacturer when precalibrated lenses are needed.
+4.00 D
Thin edge

15. Precalibrated diameter in astigmatic lenses
In this example, we can see that the left lens is as thin as possible once edged, which means that precalibration is not recommended. In the right eye, however, the edged lens is thick in all directions and precalibration is therefore highly recommendable.