Bifocus

The bifocal lens for the near vision and the distant vision for the  PRESBYOPIC eye.

In theory, cemented bifocal segments can be produced in any shape and size but in order for the dividing line to remain invisible. (i.e.. as inconspicuous as is possible) the segment should be round in shape with its optical centre coinciding with its geometrical centre.

The reading portion of a fused bifocal is constructed from a high-refractive index button, which is fused at high temperature to a depression curve worked on the main lens made from glass of lower refractive index. After completion of the segment side the segment surface is continuous over the DP and NP so the dividing line cannot be felt. The reading addition, A, which is obtained depends upon the power of the DP surface. F¹, the power of the original depression curve, Fc, and the fused bifocal blank ratio, which is a function of the refractive indices of the two glasses used. i.e. n¹ for the crown glass main lens and n² for the higher refractive index segment glass.

Bifocal fitting Bifocal segments must be positioned so that the distance and near portions of the lens provide adequate fields of view for distance and near vision. It is convenient to consider the positioning of the segment in the vertical and horizontal meridians separately. In the vertical meridian, bifocal lenses which are prescribed for general purpose use are usually mounted before the eyes so that the segment top is tangential to the lower edge of the iris. In most cases the position of the lower edge of the iris also corresponds with the line of the lower eyelid when the head is held in the primary position.

This position is the norm for the great majority of bifocal wearers and certainly the safest position for the segment top in the case of first- time wearers. If any doubt exists in the fitter's mind as to the best segment height to provide. The lenses may be dispensed in a frame that permits easy vertical adjustment of the height if the wearer finds that the normal position is unsuitable. - If the bifocals are prescribed mainly for near vision, then the segment top might be fitted a little higher, say midway between the lower edge of the pupil and the lower edge of the iris. If the lenses have been prescribed for some vocational purpose and are to be designed for only occasional near vision use. Then the segment tops might be fitted three to five millimetres lower than the norm, your optician is the professional to gives you the best advice in this respect.

Bifocal insetting

The purpose of insetting bifocal segments is to bring the near fields of view into coincidence. Contrary to popular belief, this is not achieved simply by insetting the segment by half the difference between the CD and the NCD. The fact that so many bifocal wearers do not have problems with insetting which has been deduced from the above rule is explained quite simply by the fact that cur- rent bifocal segments are relatively large.

Large diameter segments enable the individual monocular fields of view to overlap sufficiently to provide a large binocular field, if we were to attempt to dispense segments of only 10mm diameter, many more grief cases would arise from incorrect insetting.

The effect which insetting tries to achieve may be explained as follows. Imagine first that the segment is no more than a small aperture in an occluder placed close to the eye. The field of view through the aperture of course depends only on the size and shape of this aperture. If a subject who is wearing a pair of D-shaped apertures (like most modern bifocal segments) which have not been centred correctly on the converging visual axes is to look at a sheet of paper, the fields of view projected onto the paper would appear as shown in the latter figure. The situation illustrated in the figure assumes that the apertures have not been sufficiently inset. The shaded area represents the binocular field.

Whereas the areas, which remain, unshaded either side of the binocular field are seen in monocular vision. The areas seen only by the right and left eye are marked R and L respectively. Ideally of course, the areas should overlap exactly to provide a single D-shaped field.