Let’s consider how the CSF can help us in understanding the effects of using a magnifier on some text.
Any text has a position on the CSF defined by its size and its contrast. For instance:
Text 1 is normal article print (say N10) on your (lovely crisp black & white) reading chart you have in your consulting room.
Text 2 is large print, again on your nice high-contrast reading chart.
Text 3 is normal print (N10 again) in the local newspaper. Newspaper print is not terrible contrast, but not as good as your consulting-room reading chart.
Text 4 is large print in the newspaper, let’s say headlines.
Text 5 is also around N10, but quite pale print, perhaps like you sometimes find on bank forms or bills — very stylish from a design point of view, but not all that easy to read, even for people with normal vision. (Imagine trying to read a book with pale pastel print).
The orange line is for a person with normal vision. Notice that all the print examples are within the ‘seen’ area (which makes sense — it’s unusual to choose print that most people can’t see!)
Vision Impairment has a Bigger Impact on Low Contrast Documents
But let’s compare now with a person who has impaired vision. The purple line here indicates a fairly typical CSF plot for someone with early-to-moderate AMD.
Notice that even though the AMD patient can still read N10 print on your consulting room’s high contrast reading card (Text 1), they can’t read that same size print in the newspaper (Text 3). Remember that next time someone (especially an elderly patient) comes back saying that they can’t read the newspaper properly with their new reading glasses, but they still read well on your reading chart. If you really want to get insight into how your patient is seeing a document, you should check them with text of the same contrast (ideally the same newspaper).
However, notice also that the patient can still read the headlines in the newspaper (Text 4), so we can be sure that using a simple magnifier will let them see the smaller newspaper print. The way we show that on the CSF is that the print shifts from it’s original position (3) over to a spot more to the left — that is, the print was made bigger. In this case, the patient only needs a little bit of magnification to make it readable.
Magnification Calculations Don’t Hold True for Low Contrast Documents
Our AMD patient couldn’t read the normal newspaper print, so of course they can’t read the pale print either (Text 5). This time though, if a magnifier is going to help it’s going to need to shift the print position a whole lot further to the left (that is, a lot more magnification will be needed) before the patient can read it. And notice also, even with much stronger magnification the print will be only just within the visible zone, implying that the patient can still only barely read it. Adding even more magnification isn’t going to make it a lot better.
If the print was any paler than the Text 5 print, this patient wouldn’t be able to read it no matter how big we made it, or no matter how strong a magnifier they used.