Measure visual acuity with the crowded symbols (50% and 25% spacing) or with crowded numbers (50%, 25% and 12.5% spacing) at 40cm distance and at the distance that the child prefers to use.
When the child has learned to read, reading is tested with texts that correspond with the child's reading ability:
2. Optimal size
Based on the measurement of the optimal text size you can calculate the magnification that the child needs to read the texts used in his/her classroom. Test the magnifying devices with that magnification and compare them with the child's present devices. If the child prefers more magnification than your calculation suggests, the child may be viewing with a different part of retina (PRL) than that used during visual acuity tests (for example, when the central island of vision is so small that individual symbols can be seen but text does not fit well within it the child may choose to use a different part of the retina which affords less clarity but a wider area of view).
3. Reading speed
4. Reading comprehension
Minute losses of the central visual field may disturb reading so that a beginner in reading may be diagnosed as dyslexic when reading errors are caused by loss of visual information due to damage to the anterior visual pathway. If, for example, there is such an area of loss of vision, a scotoma, to the right to the fixation point that the fourth letter in each word disappears in a text of a certain size, the child has an "unusual type of reading difficulty". The errors change their relative place in smaller texts toward the end of the words and in larger texts toward the beginning of the words. The errors disappear when the letters are so large that only a part of a letter is not visible. These regular "atypical" errors in reading should be noticed by the tester.
If the preferred retinal area is located above the central scotoma, planning of the eye movements uses either this new preferred retinal locus or the anatomic fovea as the centre, in relation to which the co-ordinates of the eye movements are determined. In both cases there are often irregularities in the fast eye movements, saccades, with corrective movements both horizontally and vertically. These can be best seen in recordings of eye movements or by watching the eye movements with the very expensive scanning laser ophthalmoscope (SLO), but they can also be observed with an inexpensive technique: use an age appropriate text as a transparent copy (overhead acetate) through which you can see the child's eye movements during reading. Make sure that you have a white or light, evenly coloured clothing on you as the background of the text. If the saccades are irregular, it is often helpful to change the reading technique so that the child holds the head and the eyes still and moves the text instead. If the child has great difficulties in holding fixation, the text can be shown word by word always appearing at the same location on a computer screen (where there are computers) to avoid all oculomotor demands. The child uses the arrow keys to go forward or backward on the text.
Finally, ask the child about distortions of straight lines and about blurred letters or disappearance of some letter(s). This further depicts the quality of the central visual field for reading. Depending on the quality of the central visual field and the quality of oculomotor functions you may suggest another technique for reading. Later when the demand on reading increases, you may need to re-evaluate the magnification used and change the type of low vision device or illumination.