I have recently undertaken a number of practice visits and have noticed a couple of things about acuity charts that seem to crop up time and time again. The first is the number of practices that use a projector chart upon a flat grey metallic screen. Presumably because the lens or bulb housing has tarnished to some extent, the letters projected are no longer fully black and in a few cases I have seen the pre-reg having to dim the room lights in order for the patient to see the letters better. As Snellen notation is based upon the viewing of a high contrast target, such adaptation brings the whole accuracy of acuity measurement under question.
A second issue seems to be the variability in the application of a spherical adjustment to the final correction when using an acuity chart at less than six metres. The confusion seems to have arisen with the increasing use of electronic displays. These can be positioned at any distance from the patient, down to around three metres. The letter size can then be calibrated so that they represent the size that they would be viewed at as six metres. However, it is essential to remember that with such charts, the viewing distance still needs to be corrected for in the final spherical lens choice (for example an extra -0.33DS for a three metre viewing distance). This is before any final adjustment for the individual patient demands.
There are other systems which incorporate a set of mirrors within a microwave-oven style box which are known as short-form units and do simulate six metres – these should not require adjustment. Interestingly, I saw one such unit recently with a big sticker on it saying ‘NO ADJUSTMENT NEEDED.’ The irony is that the sticker may itself have induced spherical error requiring adjustment via the Mandelbaum effect.