I’ve heard people claim that, just as magnification moves the text leftwards on the CSF (that is, it gets larger), illumination moves it downwards.
At first glance it seems reasonable. But it’s not true. Sure, things look more ‘contrasty’, more black & white when you get a good light on them, so it’s easy to think the contrast has improved. But that’s not how contrast works.
Contrast Doesn’t Change With Illumination
When we’re talking about a text document, the contrast indicates the brightness of the print compared to the brightness of the background paper. For example, in this picture, the paper has a reflectance of 50% (not great), while the print has a reflectance of only 10%. If we increase the illumination one-hundredfold, it makes the paper brighter, but it makes the print brighter too. They still have the same proportionate relationship — which is the Contrast.
Where does this leave our CSF?
Our two main tools with low vision aids are magnification and illumination. So, how do we represent the effect of illumination on the CSF?
Different Illumination Means a Different CSF
The answer is — we don’t. A CSF is measured at one constant illumination level (for a standard CSF it’s measured at a close to optimal level of illumination). If you change the illumination level, you get a whole new CSF line. And that’s where things get really interesting, because in the real world we’re all dealing with brighter and dimmer situations all the time.
Here’s an illustration of how the CSF of healthy eyes changes over a range of illumination levels: