To guarantee maximum wearing comfort, an exact fitting procedure must be followed for each type of lens. The following recommendations for centring ZEISS lenses take into account the respective centration requirements the centre of rotation requirement and the field of view requirement.
After the preliminary anatomic fitting of the spectacles, the selected frame should be
- fitted to a corneal vertex distance of between 12 and 16 mm and
- display a (tilt) pantoscopic angle of between 8° and 12°.
If not specified individually for customised lenses, ZEISS lenses are designed for these parameters. These recommended values cannot always be observed due to special anatomic conditions or to an unusual frame shape. The optician should then take this into account in refraction and the lens order.
The centration distance should correspond to the interpupillary distance when the patient is looking into the distance (distance PD).This applies to all ZEISS lenses:
- Single vision lenses for distance and near
- Officelenses, the proximity lens
- Progressive lenses
- Bifocal and trifocal lenses it is standard practice to use the segment tops as reference points in lateral centration. These are each marked at a distance of 2.5 mm nasally from the distance PD.
Single vision lenses
The "centre of rotation requirement", i. e. the lens is centred when its optical axis runs through the eye’s optical centre of rotation, should be met in the fitting of single vision lenses. There are two ways of determining the vertical centration:
- The patient raises his head until the frame plane is perpendicular. The pupil centres are then marked, with the patient’s eyes in the zero visual direction (looking straight-ahead).
- With the patient’s head and body in a natural posture, the positions of the pupil centres are marked for the zero visual direction (looking straight-ahead).
The familiar rule then applies: displace the optical centration point downwards by 0.5 mm for each degree of frame tilt (pantoscopic angle), starting from the zero visual point.
In ZEISS Officelens, the near measuring circle should be left fully intact. In other words, the minimum centration height should be at least 18 mm measured from the lower frame rim.
Progressive lenses and computer lenses
To center progressive lenses and computer lenses, the positions of the pupil centres are marked with the patient looking straight-ahead, with his or her head and body in a natural posture. To guarantee the usability of the near power, the progression length should be adapted to the available glazing height for all progressive and computer lenses.
Bifocal and Trifocal Lenses
In bifocal lenses the height of the lower rim of the iris is marked, and in trifocal lenses the height of the lower rim of the pupil. The marking specifies the height of the segment top, according to which the lenses are inserted in the frame. The patient’s own personal requirements should always be taken into account when determining the height of the near portions.
To ensure an optimum binocular visual field, different heights resulting from special anatomic conditions must be taken into account.
In prismatic prescriptions the centration points determined must be displaced by 0.25 mm per 1 cm/m in the opposite direction to the prism base. This takes into account the movement of the eyes in prismatic lenses.
At the Optician
If the trial frame is not reset during refraction to make allowance for prism, the term "PD centration" is used. If this is the case, not only the above-mentioned procedure (0.25 mm per 1 cm/m in opposite direction to prism base) must be performed, but a comment should be made on the lens order to the effect that PD centration has been performed and that no allowance has been made for prism. This is necessary, as not only the corrective prisms, but also the prismatic effects of the spherical and cylindrical trial lenses are effective during refraction if the trial frame is not reset accordingly.
ZEISS then takes this into account when computing and producing the lenses. It is possible for customers to be allocated a special code which indicates to ZEISS that all orders for prismatic lenses are placed on the basis of pupil centration and no comment to this effect is then required on the lens order.
At the Optician
In "formula centration" the trial frame is reset by 0.25 mm per 1 cm/m during refraction. For the centration of the lenses during glazing, the above rule (0.25 mm per 1 cm/m in opposite direction to prism base) then additionally applies.
If no comment is made on the order or no "pupil centration" code has been allocated to the customer, ZEISS will assume in the computation and production of the lenses that "formula centration" has been performed.
For the Optician
To guarantee exact production of prismatic lenses in special cases, it is advisable to give all details of the parameters used during refraction on a special ordering form from ZEISS. Special cases include, for instance, high dioptric powers in conjunction with high prismatic powers or refraction situations in which neither pupil centration nor formula centration has been performed.
The computer-assisted i.Terminal 2 fitting system can be used to determine all centration data with extreme precision using two video images, with the patient’s head and body in a natural posture. The correct position of the centration points is calculated and displayed on the basis of the applicable centration requirement, the dioptric power and the lens type.