Occasionally a good imagination can solve problems. The patient is an artist working with live models and painting on canvas. With the onset of presbyopia difficulties arose on seeing clearly in the distance, while needing close vision when working on the canvas. To further complicate matters the canvas has a large vertical perspective.
Previous attempts with multiple pairs and multifocals failed as being impractical. The patient was referred to us to attempt to find a solution.
The prescription is RE +2.00/-1.50 x 160 LE +1.50/-1.75 x 15, with an addition of +1.50 for the canvas on the easel and the colour palette.
The concept chosen was to glaze a bifocal at 90 degrees. This would provide distance vision in the ipsilateral section of the glazed lenses while the other section would be used for near vision. The axes of the lenses were ordered 900 from the normal prescription, i.e. RE +2.00/-1.50 x 70 LE +1.50/-1.75 x 105.
The lenses were glazed in-house with the bifocal line at 90 degrees and the Distance Optical Centres (DOC) placed on the Horizontal Centre Line (HCL). The PD was not strictly adhered to in order to allow the maximum field at near when viewed via the segment. In this case the power along 180 is close to plano which made the PD less critical. In higher prescriptions more care will need to be taken.
The executive lens was cosmetically inferior but the optics were superior to the UD 35 segment
The initial trial was with a UD 35 segment, ordered by habit as a right and left lens. This failed as the near inset of the bifocal put the Near Optical Centre (NOC) of the two lenses at different heights. This created a differential prism at near. The segments were also not symmetrical. This was solved by using two right or two left lenses whereby the insets would be equal. (Figure 1)
Figure 1: The UD35 inset created a disturbing vertical prismatic jump
Looking to the left the distance vision is clear, while looking to the right the canvas is clear. The segment inset created a small vertical prismatic jump. The patient found this disturbing.
The next attempt was an executive lens. Although this was cosmetically less acceptable, as well as being thicker and heavier, the optics was better. The DOC and NOC are very close together with an executive lens which reduces the prismatic difference between the distance and near vision (Figure 2).
Figure 2: The executive lens cut the prismatic difference between close and distance vision
It is important to determine which side the artist prefers to use in the distance and which side for the canvas. The maximum area available for viewing the canvas is determined by the frame chosen. The position of the bifocal line does not have to be equal in each side. The position of the segment line should be determined by the patient. This is best determined by using a translucent tape stuck onto the demo-lens with the transfer point from distance to near being simultaneous.
The disadvantage of vertical jump from distance to near versus the disturbance of the segment line may be a personal choice. It may be worth considering using a UD28 which would allow the patient to look over the segment in the distance and allow a full view in the distance. The glasses are not a panacea, but, as of now, it provided the best option the artist has received.
The spectacles have no other use except in the studio. The ability to provide a solution in odd cases such as this provides the satisfaction that normal dispensing often lacks.