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.
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.
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.
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 also 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.