Parallax error


Parallax for opticians:

We deal with parallax every day as opticians, though many of us don’t know the word parallax or really what it means.  It is an important topic of discussion and, as opticians, we need to be mindful of this all the time.

Wikipedia defines Parallax error as “Measurements made by viewing the position of some marker relative to something to be measured are subject to parallax error if the marker is some distance away from the object of measurement and not viewed from the correct position.”  There are also many good videos explaining parallax and how it pertains to science and measurements, astronomy,  photography and many things.  But how does this apply to opticians on a daily basis?

Parallax error shows up any time we have a scale that is at a different distance away from us than the item we are measuring.  We see the scale over the object from different points depending on our perspective.  This can give us different measurements if our head position isn’t correct.   Here is a good video depicting this.

In the past we dealt with Parallax in the lab with most of our layout machines.  When we are lining up dots on lenses and markings on machines and the lens doesn’t fit flat against the scale we have the potential for error.  With new technologies in cameras and computers we have been able to largely reduce this but we sometimes still see it.

Take an example of measuring a pd with a ruler.  If you are looking at the patients left eye with your right eye, you line up the 0 point on your ruler to her pupil.  If you looked directly over to her right eye using your right eye then you would see the scale at a different point than if you used your left eye, which is directly in front of her right eye.

Even still technology is reducing this by developing a pupilometer and now computer measurements.  Even for seg heights where making sure we are on the same height as your patients eyes.  Not looking parallel can give us error, and computers are making our lives easier here too in some cases.  Parallax, however, will always be an issue when we are dealing with measurements, scales, and people with 2 eyes.

We can see how parallax effects vision simply by looking at something and covering one eye, then cover the other.  You see how the image jumps a bit.  Try lining two points up then cover your dominant eye, you see parallax error.  We even take it into account when looking through a lens from different angles and especially with progressives.

Be mindful of parallax error and how it works, ponder it in different situations.  Any time we have objects that are not the same distance from our vuepoint we get parallax.  There are some things we can do to reduce this error, compensate for this error or eliminate it.

First thing is your viewpoint needs to be parallel with the subject.  Your line of vision must be directly in front of what you are measuring and the scale directly between the two points.

Keep your scale as close to the items you are measuring as you can.  The further away from your subject your scale gets the more error will occur.

Don’t get too close to your subject.  The farther you are away from your subject the less parallax will effect you.  You don’t want to get too far away or you won’t be able to accurately measure anything but don’t get right up in someones face.  You may feel like you can be more accurate in getting parallel but you also have to be closer as the error will be greater at closer distances.

Keep in mind that this phenomenon happens and you will recognize it when it comes up and be able to confront it.

Add a comment where you found Parallax error occurring and how you confront it.  Follow my page and you’ll get updated when I post new topics.

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