Not the green, grassy kind but your field of vision.There are many different fields of vision that can be measured and many different ways of measuring them – it all depends what your optometrist wants to know.
Reading a letter chart gives a measure of central visual acuity, this is a recordable measurement of how well you can see small details.
Generally your field of vision relates to your ability to see movement and/or changes in the periphery of your vision.
- For the purpose of screening for disease each eye is tested on its own the eye not being tested is occluded.
- But for driving purposes it is sometimes preferred to perform a binocular field test – both eyes are used together as in normal seeing. This type of field assesses your Useful Field of View (UFoV).
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.
A simple test of the central fields uses 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.
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.
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.