In some cases, what is thought to be the same lens power of eyeglasses provide different sharpness of vision. How can this be although the optician said the lens powers of the glasses would be the same? This article deals with this in detail. The basic differences between two glasses can be the following:
- Different lens designs
- Centering of the lenses
- Different fit of the lenses in front of the eyes
- Tolerances of different lens manufacturers
- Abbe value of different materials
Why Different different Lens Designs Can Result in Sharp or Blurry Images at the Same Power of the Glasses
Lens designs are often ordered in spectacle values of 0.25 diopters. This is what is written on the letter you receive from the optician. And when the optician checks the lenses with a lens meter in the standard setting, he gets the same result.
However, if the optician chooses a finer setting, he may be able to measure that the actual value in the lens is different. By the way, this is not a rarity but rather the rule. Here is a small example:
|Rounded to the quarter||-4.50||-2,50||90|
|Actual prescription of first pair of glasses||-4.38||-2,39||90|
|Actual prescription of second pair of glasses||-4.60||-2.55||90|
In every case a digital lens meter would present the same lens power. So simply ask the optician what the lensmeter shows. Usually the lensmeters show lens powers rounded to the quarter. Finer adjustments are also easily possible in the settings of the lens meter.
These problems occur more often when the optician orders the same prescription in different lens designs. For example, one pair of glasses with single vision lenses and another with progressive lenses.
The optimization of the manufacturer then aim, among other things, to keep the difference of distance and near lower. The advantage for you is that the field of vision in the near range remains somewhat larger.
The disadvantage is that the distance becomes a bit blurrier. These blurs are not noticed by many people. With a very high visual performance, however, you are more sensitive and notice such differences more quickly.
The solution to the problem:
Lenses with different lens designs need to be slightly modified from the one lens design to produce the same distance or near visual experience in the end. This means that the value often has to be modified by 0.12 to 0.25 dpt.
By the way, there are also different lens designs for single vision lenses or progressive lenses. If you compare an aspherical lens from Zeiss with a spherical lens from Rodenstock or Essilor, you should expect minimal differences. Only the same lens design from the same manufacturer will provide the same visual experience.
The Centering of the Lenses and the Tolerances of Manufacturers and Opticians Can Lead To Different Visual Experiences With the Same Rx
Each lens design is engineered so that a specific spot in the lens ideally sits directly in front of your pupils when incorporated into the eyewear. In reality, unfortunately, there are often deviations from the optimal value. Manufacturers often give tolerances of 0.8mm -1.0mm when it comes to the tolerance in the deviation of the total pupil distance.
These deviations due to the variation in the grinding processes can lead to different prismatic side effects. These deviations due to variations in the grinding processes can lead to different prismatic side effects from spectacle to spectacle, even though the same lens with the same lens powers has been ordered twice.
In the case of prismatic side effects, the images are shifted. This means that one eye sees the image offset compared to others. This may mean that one eye sees the image further to the right, left or higher compared to the other eye. This side effect can be compensated by additional muscle tension. This of course changes the feeling with the glasses. By the way, the sharpness does not change.
The Choice of the Lens Material Can Lead To Different Visual Experiences in Two Glasses Although the Lens Power Is the Same
The clarity of the lens is related to the Abbe value of a spectacle lens. This can vary greatly between lenses. The material polycarbonate, for example, has a poor abbe value of 30, while CR39 has a good abbe value of 58.
The lower the abbe value, the more likely people with very high visual acuity are to perceive the poorer optical quality of lenses with a lower abbe value. For example, some people describe that vision with glasses is less clear with lenses of a lower abbe value.
Many also see color fringes, which appear as bluish or yellow shadows around certain contours in bright light.
As a rule, thinner glasses provide poorer Abbe value. So if you have a different perception with the same prescription in two pairs of glasses, it could be due to the different lens materials. Compare the Abbe values. If you want to learn more about this, here is another article for you:
It is important that you take a look at the manufacturer-specific abbe values. These can vary slightly from manufacturer to manufacturer even though the same refractive index is specified.
If you have two pairs of glasses with the same spectacle values, you can expect to see equally well with them. If this is not the case, there is a solution to your problem. The important thing is that your problem is looked at very closely.
This means that if the optician says that the values are the same, do not be satisfied, but ask. How exactly were the measurements taken? To a quarter of a diopter or to a hundredth. If you ask for exact information about the materials, then call the manufacturer.
Ask exactly and let the differences of the glasses exactly from an experienced optician show and explain.