There are the green and grassy kind of fields but these the Fields of Vision kind.
Arthur Schopenhauer may have said “Every man takes the limits of his own field of vision for the limits of the world,” but in optometry we recognise that there is not just one field of vision but many and we are not just interested in their limits but in changes to them. By measuring their limits, depths or changes we can build a greater understanding of the health and efficiency of the visual system.
In the consulting room we are mainly interested in;
- Your Central Visual Fields and your Peripheral Visual Fields.
- If we are screening for disease then each eye has to be tested on its own, the eye not being tested is occluded or covered up.
- If we are screening for safe driving then a binocular field test ( both eyes are used together as in normal seeing) maybe more useful. This type of field test can assess your Useful Field of View (UFoV).
Some Simple tests of your Central Visual Field – poor central fields makes it difficult to recognise peoples faces or to read but has less effect on mobility
- Using an Amsler chart. This is a grid of lines and you will be asked to judge if any lines appear distorted or if any squares look bigger or smaller than the rest. This is usually used to assess macular function when there may be concerns about the onset of AMD.
- Reading Letter Charts. This gives a measure of your central visual acuity. This is a recordable measurement of how well you can see small details in your Central Field of Vision as opposed to your Peripheral Field of Vision. The letter chart may be at 6 metres or at your preferred reading (or working ) distance, it depends on what we want to measure.
Some Simple tests of your Peripheral Visual Field – poor peripheral fields relate to your ability to see movement and/or changes in the periphery of your vision, movement out of the corner of your eye. Reading vision and social skills are less affected but people are more likely to trip over and bump into things
- The simplest peripheral field test is a confrontation test. Your optometrist may get you to look at his nose whilst he 'waggles' his fingers at the side of your head and asks if you can see them, or he may bring a pen from behind your head (outside your peripheral field of view) and ask you to say when you first see the pen.
- Visual Field Analyser or Screener. there are many different types but essentialy it is a machine that flashes small lights at you. You either have to say how many you see or whether you see one or not.
Field tests may not be carried out routinely at every Sight Test. Your optometrist will use her/his experience to decide whether or not they are necessary. Changes in your visual fields can indicate disease within the eye or disease within the brain.
Various instruments can be used to measure both central and peripheral fields and especially mid-peripheral fields. Different practices will have different instruments, but they all give a permanent record of your field of vision so your optometrist can see whether there are any changes as you get old.