a simple question

[optispares]

This is probably a stupid question but can anyone explain why on single vision lenses the pd is important but not the height ?
all the glazing work I see is just on datum.
as you can tell from the question I am not optically trained , just
a dumb engineer but it still strikes me as a little odd.

[Excel-Lentes]

Hi,

The reason most opticians leave the MRP's at the datum is that if both MRP's are at the same position vertically then there won't be any unwanted vertical imbalance. This holds true when the patients eyes are at the same vertical level.

There also is an optical rule stating for every 2 degrees of pantoscopic tilt the MRP is effectively 1mm higher in the frame. This allows the optician to adjust the frame to acheive the proper vertical alignment of the MRP. Properly selected frames will require minimal adjustment for pantoscopic tilt, therefore minimizing the need to relocate the MRP from the datum line.

If a frame is a bit too large( the eyes are way too far above the datum line), it may not be practical to apply that much pantoscopic tilt. In this situation it would be necessary to raise the MRP's to the proper level (decenter up). Fortunately today, smaller frames are commonplace and it is easy to guide most customers to a good fit and avoiding vertical decentration.

Vertical centration is more critical when fitting high index, aspherics and anisometropic corrections. There is an exceptional description of this topic in System for Ophthalmic Dispensing Vol 2. This is a must-have for anyone doing optical work.

-BW:bbg:

[chip anderson]

When strong lenses are used and the pupillary location is far from the vertical geometric center of the frame, the vertical center can become very important. Unwanted prism (especially in high minus frames) even when it is in ballance from right to left. Particuarlly color (usually red and yellow) outlines are formed around viewed objects.


Chip

[optispares]

thanks.

thanks for the replies guys.:)

[John R]

Excel-Lentes said:
Hi,

The reason most opticians leave the MRP's at the datum is that if both MRP's are at the same position vertically then there won't be any unwanted vertical imbalance.

Ok spanner in the works time.....
What if you have eyes like mine.: Re +4.50 +1.50 Le +1.00 +0.50
Now even if pupils are at the same height there is a large diffrence in prism amounts.
Lets say 5 m/m above datum, prism is going to be Re 3.00 Le 0.75 quite a diffrence.

[David Wilson]

The optical centres of all single vision lenses should be dropped below pupil centre by 1mm for very 2 degrees of pantoscopic tilt. This ensures that the principal axis of the lens passes through the centre of rotation of the eye. There is an easier way to do this (tilting the person's head back so that the frame is perpendicular to the floor and finding pupil centre). While it is critical for aspherics it also applies to best form lenses, designed according to Tscherning's ellipses.
By doing this oblique astigmatism is kept to a minimum. It also helps an little with transverse chromatic aberration.

As for your anisometropic prescription, John, fitting the lenses correctly will ensure that the optics are perfect but will create differential prismatic effect (DPE) when looking straight ahead. There will be DPE anywhere except at the optical centre.

We have never really considered heights for single vision until aspherics came along, however we were probably getting close to the best result by fitting single vision on datum, given that pupil centre is generally above datum.

Horizonatal centration and complying with the principal axis/centre of rotation rule is another story, but I won't go into it now.
Regards
David Wilson