Understanding Vision

Different vision test results – how can this happen?

Or: When is the best time to go for a vision test?

16 October 2021

We are often asked why the measuring results obtained in vision tests differ – even if two exams are performed in quick succession. Different results in subjective refraction may be attributable to various factors. We’ll tell you what exactly these factors are and what you can do to ensure optimal results.

  • Different vision test results – how can this happen?

The state of your eyes

Some people may find it an unpleasant experience, but having your eyes examined on a regular basis is an absolute must. But why might the measurements performed by your eye care professional on two different occasions differ – even if the second measurement is made only a short time after the first? Can your eyes really change within such a short time? This is not an uncommon phenomenon. Different results may be due to various reasons. The variance is generally a matter of only around 0.25 diopters, and your eye care professional is well aware of these fluctuations and knows how to deal with them.

The visual performance of our eyes varies slightly in the course of the day. One of the factors influencing this is our biorhythm. We are not in the same physical condition the whole day long, and this is also reflected in the way our eyes perform. The levels of hormones and sugar in our blood play a major role in determining the quality of our vision. Having your eyes tested before breakfast, for example, or insufficient fluid intake beforehand may well influence the result.

And maybe you've been working the whole day long at a computer before you go to your eye care professional. This means that you blink less and your eyes are less moistened by tear fluid. This may lead to dry eyes and eye fatigue which, of course, also influence the result of the eye test.

Our tips:

  • Try to be as rested and relaxed as possible when you go for your eye exam.
  • Never go on an empty stomach or when you're thirsty.
  • Observe your own biorhythm, and make an appointment with your eye care professional at a time when you feel fit and rested.

The conditions during the eye exam

The conditions in which the vision test is performed also have an influence on the result. Needless to say, a test conducted quickly and superficially will not provide the same results as an exam where both you and your eye care professional take the time needed to obtain the required accuracy. This is known as a subjective vision test. Here, it is essential for you to communicate with the eye care professional and make sure time is taken for a proper exam. It is often the little things that can determine the visual quality provided by your new glasses. Moreover, the quality of the measuring instruments and the eye care professional's expertise and experience can decisively impact the result.

Did you know that even the surroundings in which the exam is performed can have an effect on the result? To ensure that conditions remain constant, there should be no daylight in the examination room as that may influence the result. The size of your pupils also plays a key role. This is a phenomenon which photographers will be very aware of: a different aperture setting on the camera changes the definition and brightness of the picture. Many people see differently with a contracted pupil in bright conditions than with a dilated pupil in the dark. If, for example, visual performance at night and in poor light is to be measured, this must be done in a dim room.

Our tips:

  • Always make an appointment with your eye care professional for a vision test. Don't simply walk in and expect to be examined. You and your eye care professional should have enough time for a thorough and detailed examination. ZEISS estimates that about one hour is required for the complete vision analysis including the consultation.
  • You must trust your eye care professional and feel that he or she is the right person to consult to ensure better vision.
  • Make a note of where the examination is performed. Is daylight entering the room?

Influences that can lead to incorrect results:

  1. Contact lens wearers should wear their glasses instead of their contact lenses for 24 hours before the test. Contact lenses modify the geometry of the cornea and therefore your visual performance during the vision test.
  2. Fluctuations in visual performance are often the result of medication, e.g., the production of tear fluid may be impaired. Your eyes become dry, and you may experience eyestrain.
  3. It is also well-known that diabetes can lead to fluctuations in visual performance during the course of the day. Diabetics are recommended to have their eyes tested at different times of the day and to consult an ophthalmologist if necessary.

Our tips:

  • If not directly asked by your eye care professional, please tell him or her if you are currently taking any medication, or have a chronic condition such as diabetes.
  • Take your current glasses, contact lenses and, if you have one, your lens certificate with you to the vision test.
  • If this is the first time you have been to an eye care professional, tell them what you liked and didn’t like about your glasses in the past(e.g. over-the-counter readers). The more you tell your eye care professional, the better your new glasses will be.

Objective refraction

You may well wonder if the traditional vision test, i.e., subjective refraction, is necessary at all, if the results can change based on conditions. The answer is yes. The eye care professional must be able to record your personal reaction to the various lenses he or she inserts in front of your eyes. Just as importantly, subjective refraction tests how your eyes interact and how they will see with your new glasses.

However, there are ophthalmic instruments such as the ZEISS i.Profiler®plus which can be used to perform objective refraction beforehand. It uses wavefront technology to analyze the vision profile of each eye on the basis of 1,500 measuring points and generates a "map" of the eyes’ retinas. All you need to do is look into the i.Profiler®plus ; the scan usually takes a minute or two. This is a preparatory measurement, the results of which are then refined by the eye care professional in the subsequent subjective refraction procedure.

And that is not all: as the i.Profiler®plus measures visual performance with a contracted pupil (in bright light) and a dilated pupil (in poor light or at night), it is possible to determine whether your night vision is poorer than your daytime vision. The eye care professional can then use this information to optimize the lenses appropriately. A major benefit of this procedure is that the time required for the vision test may be reduced.

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