There are objective and subjective tests for astigmatism. After you read this article you will know what options the optician has to test your eye for astigmatism. An optician does not need to perform all of those tests to get the prescription for astigmatism right.
During refraction, a testing frame is used where to test your astigmatism where lenses are switched manually or a phoropter that changes the lenses automatically. Both devices work in the same way. When astigmatism is present the lens power in front of your eye needs to be stronger in a distinct orientation compared to the rest of the prescription. Depending on how high the astigmatism is the lens power will show a higher CYL value on the prescription. The distinct orientation is shown by the axis in your prescription.
Opticians are able to find your Astigmatism due to certain sequences they perform with their equipment which they perform in every refraction. In the article below you will see such a sequence works and what alternatives the optician has to test for astigmatism. Depending on the country you get tested in there will be slight differences in the tests but the underlying principles for testing astigmatism will be the same.
In the video above you can see the testing for astigmatism with the classic cross-cylinder method which is used daily in every optical shop. The cross-cylinder is a very versatile tool to:
- Find the astigmatism
- Check where the axis is oriented of the astigmatism
- Check how high astigmatism in your lenses needs to be
All those things can be done with special questions the opticians ask during the refraction. For example when the question comes up “does it get better with this lens” and the cross-cylinder is used the opticians’ tests for the rough or the amount of power of the astigmatism is tested directly.
There also another question when the optician asks you with the cross-cylinder if one or two is better. This question is used to fine-tune the astigmatic prescription or to fine-tune the correct orientation of astigmatism. Some people even feel ridiculed when the same lens is presented in front of their eye and they need to decide if one or two is better.
But there is a difference between both options it is just possible you can not see any difference anymore. When this happens the lens power can not be changed in one or the other direction to make your visual acuity better.
In other words, the perfect prescription was found during the tests for astigmatism. When that testing is done by the optician you need to look at rounder test objects. But mostly you will be probably looking at numbers and characters. There are also other tests where you need to look at darker stripes and tell the optician if some of those dark stripes appear more grey to you compared to other stripes.
When the test is done right the darker spots on the eye test show the correct orientation of astigmatism on your eye. Depending on how high your astigmatism is more or less darker stripes can be seen by you. The goal here during the test is to present all stripes in the same way. When this happens the perfect astigmatic correction is found.
Objective Testing for Astigmatism
When an objective test is done you do not have to talk to the optician so he receives less feedback. Those tests can be done differently. One way to test it is with an autorefractometer where you need to look at a hot air balloon and the device adjusts to your prescription so you can see it clearly.
Another way to test astigmatism is to use a Sciascope. With this device, the optician shines a light in your eye and simultaneously switches lenses in front of your eye. When he does these tests he watches the reflection of the light on the back of your eye and how it moves.
Depending on the movement of the light in your eye he can again find the correct axis and determine. He is also able to find how much lens power is needed to correct astigmatism on your eye. He just switches the lenses in front of your eyes until he sees a flickering effect. An experienced optician will see this right away. Those tests only take a couple of minutes of your time.
Here in the picture below you can see how I perform this test. During the testing, the room would be a little less bright and then you could actually see the light shining on the eye.
The thing with objective tests is they do not tell the optician how compatible the prescription is for you because of the missing feedback from the person that gets measured. Only the subjective tests deliver that information.
When we talk about astigmatism the information the optician gets from a measurement is always just a snapshot. All those measurements can be influenced by a dry eye or another eye condition. With dry eyes for example the wetting of the eyes surface can change after every time the person blinks. When this happens the prescription can be off-target by two diopters when it comes to astigmatism.
This is why although astigmatism is a really common thing each eye needs to be looked at individually. I also recommend a topographer for those testings. This device tells the optician how astigmatism is distributed on the cornea. With that information, the optimal correction can be achieved. Sometimes contact lenses and sometimes glasses might work better depending on the individual case.
Here in the picture below you can see in a power map how the astigmatism is distributed on the shown cornea. The different colors stand for different powers and needed prescriptions. While the lying eight in the middle of the eye can be fairly easily corrected the yellow part below also shows astigmatism that can only be corrected optimally with contact lenses.
As you can see there are many tests for astigmatism and I as an optician believe the combination of testings will result in the best consultation when it comes to contact lenses and glasses. I hope you found the information about the testings you were looking for.
I wish you a great day.