Font
size and biological diversity
(Font
size matters)
Font size is an interesting variable. My colleagues and I
have been measuring (calculating) which font size is optimal for each student
for a couple of years now. This is an essential component of the protocols we
have developed and has to be worked out before we consider other factors.
Most people will think of font size in the context of
reading glasses; trips to the optician and deciding which of those lines of
letters you can read clearly, or in low vision problems, where a person needs a
larger font because of degrading eyesight from issues such as macular
degeneration, or cataracts.
Another issue is the tendency to associate larger font
size with poor academic performance, low intelligence or ageing. These associations unfortunately leave a
negative feeling and resistance towards the use of large size fonts, when they
may be needed by people in the main stream population.
The inverse, the attitude to the use of small fonts, is
that as we progress through the levels of education, the texts we have to
encounter get increasingly smaller. In higher education there is an assumption
that intelligent people can read small fonts. The decision makers (academically
successful usually) in society tend to find small fonts easy. This use of small
fonts, in itself of course as acts a ‘font ceiling’, restricting access to the
higher levels of education to those who can cope with the small font sizes.
Into this set of prejudices and assumptions appears the
issue of wearing glasses. Too often, people assume that ‘the correct pair of
glasses will enable a person to read small text. For a young person you can
extrapolate this to the situation many schoolchildren find themselves in.
·
They go to an optician because of
difficulties they experience with their eyes when they are reading,
- · get a pair of glasses,
- · go back into class
- · Find that they still have problems, often still very severe when they try to read.
They then have two choices.
- Carry on wearing them and get called ‘stupid’ because they still find reading slow, difficult and possibly painful.
- Stop wearing them, and get told off because ‘if you had your glasses you would be able to read’. The teachers regarding them and often making it quite clear, that they believe the child is ‘lazy’, ‘uncooperative’ not interested in learning and probably a trouble maker.
- But a bigger font size might be all they need!
What I will do now is to try and look at the
issues of ‘size’ diversity in the human visual system and start the consideration how
this diversity could impact on the ‘font size needs’ of the population.
I will ignore, focussing, assuming that an optician has
ensured that the physics of focussing has been dealt with. If the person needs
glasses they have them (unfortunately this is often not true).
Physical
size/dimension variations between people.
The retina.
The light sensitive cells at the back of
the eye are arranged in a hexagonal arrangement (like honey comb). There are two types.
a. Rod
cells... tiny cells used for night vision. Not used in reading.
b.
Cone
cells. larger cells used in reading
c. This
picture shows a part of a person’s retina. The red and green cells are the main
ones mainly involved in collecting the image when reading. The blue cells have
a role, but there are very few of them and probably mainly concerned with
getting the image of the word at the fovea.
d.
Notice that the ratio of red to green
cells is unequal and they are clumped. This distribution is thought to be
controlled by a similar mechanism to that controlling the stripes on a Tiger. Each person is different.
The cone cells at the centre are smaller
than those at the edge and packed very closely.
Right at the centre (the fovea) they are
extremely small... There are no rod shaped cells here. Actually the cone cells
here are so small they are similar in size to the tiny rod cells.
The diameter of the cone cells and ‘proximity’
or tightness of packing is one important variable which controls how ‘coarse’
the image will be. . The smaller the cone
cells and more closely packed, the more detail.
A bit like the number of megapixels in a
digital camera;
The
more the better.
The fovea itself, with its tiny close
packed cone cells, at the centre of the retina, is the reason why the eyes move
quickly from object to object in a visual scene.
In the rest of the retina the cone cells
are much larger, and further apart from
each other (more numerous rod cells in between them), getting larger towards
the edge of the retina.
The data from the each foveal cone cells
is treated independently whereas the data from the rest is processed in groups.
1) So
the way they are wired, the size of the groups of cells, together can vary from
person to person.
2) The
size of the cone cells in the centre of the fovea can vary from person to
person.
3) The
width of the fovea itself (the zone of small cone cells in the middle of the
retina.
4) The
size of the cone cells as we move to the edge of the retina.
5) The
ratio of red to green cells varies dramatically from person to person. Some
people have many times as many red as green and vice versa.
If we could give a numerical value to each of the five
‘size variables’ above we can see that there is a dramatic range of
combinations in design and in a way the number of ‘megapixels in your cameras’...
For
example if we consider the width of the Fovea, the central area of
small close packed red and green cone cells.
If a person’s fovea is smaller than the average,
then the
smaller fovea will
a. ‘Process’ fewer letters per fixation.
b. More pictures will be needed to get through a
sentence. Each ‘picture takes about the same time, about a third of a second.
c. It will take longer to get through the
sentence.
d. Working memory will be compromised.
Beginnings of sentences may well be forgotten by the time
the person has taken enough photographs to get to the end.
There is also what you may call a double whammy
here. It is thought that if you have developed a smaller fovea then the cone
cells in your fovea will be bigger than for those people with a bigger
fovea!
Eye movement during picture taking
An additional complication is that if the eyes are
absolutely steady during a fixation then they stop working completely. They
have to be constantly wobbling at very low amplitude so that the image is
moving across the edges of the cells. It is the switching on and off of the
cells which gives rise to the data that is computed by the brain to create the
image. It sort of has to scan the image, the words.
This movement is a consequence of
a. Muscle contraction in the neck or movement of the
object being viewed
b. Changing
contraction /muscle tone in the six muscles which otherwise move the object on
to the fovea. The eye muscles have the job of micro movement of the image on
the retina.
c. All
this muscle control is being managed by the cerebellum in the brain and
feedback from the muscles themselves and feedback from the computation of the
image on the retina. (A link to dyspraxia?)
The amplitude and frequency of these micro movements
needs to be appropriate to the size of the cells collecting the data. If there is a problem of muscle tone
management in the eye movement muscles this will change this micro movement in
terms of amplitude and possibly frequency.
Essentially all of these variables contribute to not
whether you can see a word, but to how many milliseconds it will take for your
system to collect enough data to identify the icons/ letter/word you are
looking at and match that the phonics/sounds associated with it. That time will
depend in addition on what patterns have been seen and identified before and
how often they have been seen and identified in addition in the context of
language, the syntax from the sequence of sounds which will affect the amount
of visual data needed to identify each word.
The reading process (visual data collection and
computation, in my world) requires an integrated system affected in its
functioning by all these variables and most likely more.
If one is compromised, such as the contribution of head/
neck movement musculature, this is almost certain to compromise/ limit the
reading process. Many people with whiplash or upper body trauma find their
reading performance diminishes.
I could go on much further, but I will stop here. I hope
that you can be aware then that there are considerable variations in the ‘sizes’
of the components of the visual system which influence what size of font will
work best for you. Most people reading this blog will read best on around a
font 12 or smaller, but in the general population it looks like over half of
the people need a font greater than 14 to read fluently for long periods. If you need a bigger font copy it onto a word doc.
I think this needs a mind map! Any offers?
I will develop these explanations further. For example
what is called ‘crowding’ in the next blog.
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