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Optical Terms Explained!

January 10, 2017

 

It is important that when we communicate with our patients, we use terms that don’t require they qualify as an optometrist! As such, we generally try to avoid the use of jargon and use more end-user friendly terms in their place.

 

However, there remain terms that are inescapably used, but applying a smidgen of etymology will help to de-mystify their meaning. As an aside, the etymology of the word etymology is as follows:

 

‘etymos’ – true/actual
‘logia’ – study of

 

Bringing this back to the world of optics, let us start with something more fundamental. The term lens is named as such, since when viewed side on, it resembles a lentil. Thus lens means ‘lentil shaped’.

 

We may also hear words thrown about such as myopia which is derived from ‘myein’ meaning ‘to shut’, and ‘ops’ meaning ‘eye’. The term means short-sightedness, however the derivation of the word relates to how one may squint in order to see a sharp image.

 

The opposing word to myopia is hypermetropia, or hyperopia which means long-sightedness. The word comes from ‘hyper’ meaning ‘over’ and ‘ops’ once again meaning ‘eye’. To explain this, here is a bit of background about the biological makeup of the eye. The eye is a bit like a camera in as much as it has an external lens (called the cornea) and an internal focussing lens (called the crystalline lens). When we are young, the crystalline lens has a very large focussing range, so if one is long-sighted, the crystalline lens compensates for this by increasing its power. When viewing a distant object, ordinarily the crystalline lens should be in its relaxed state, but in this case the lens is never fully relaxed (except when the eye is closed) hence the term ‘over’.

 

In a roundabout way, this brings us to the term that is heard occasionally: ‘presbyopia’.  In the above paragraph when describing hyperopia, I mentioned that the crystalline lens has a very large focussing range when we are young. It would be reasonable to conclude, that this ceases to be the case as we get older. To explain the effect of this, let us assume we consider an old eye that is perfect sighted for long distances. If this eye views a near object, the crystalline lens will struggle to focus. It is this condition that is called presbyopia, where ‘presbys’ means ‘old’ or ‘elder’ (same as for priest), and ‘ops’ meaning ‘eye’.

 

Finally, we look at the term ‘astigmatism’. Often we hear patients saying the word ‘sounds painful’. I remember this of a word in Music, ‘anacrucis’ which is where the music starts before the beginning of the bar, from ‘ana’ meaning ‘back’ and ‘krouein’ meaning ‘strike’, but I digress...

 

Astigmatism breaks down to ‘a’ meaning ‘without’ and ‘stigmatos’ meaning ‘a point’. The condition is where two different lens powers are required to correct the eye, along orientations perpendicular to each other. This usually comes from the fact that the cornea – the lens on the front of the eye, is slightly teaspoon shaped. The meaning of the word astigmatism comes from the fact that when you shine light through an astigmatic lens, it will come to focus on a line, not a point. Hence ‘without a point’.

 

This of course barely scratches the surface, there are other terms such as amblyopia, antimetropia, asthenopia to name but a few (and that’s just those beginning with ‘a’), but that would be for another day.

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