I find the data fields on the Garmin 530 to be perfectly legible when in the riding position and wearing spectacles designed for that distance (in my case bifocals). Nothing wrong with the font or the size. I started off with 7 fields per screen but that made the size of the fields too small to see at a glance - and I don't want to be fixed on my screen when moving, so now I use 4 data fields (layout 4A) per screen for screens that I want to use on the move and anything that I'm only interested in when stopped , I will use a screen with more data fields. Reading the smaller data field labels is not so easy but manageable when I know what the options are, so to speak.
As others have said - this is not an issue with the Garmin but an issue with eyesight. The Garmin's font style and display size (especially at 4 data fields or fewer per screen) are quite clear to anyone not needing spectacles or wearing spectacles that correct for the distance. If I'm not wearing the right glasses for the distance I'm looking at, then I ain't going to be able to see well, whether that's my Garmin, my computer screen, my phone , my TV or the moon.
My eyesight is not brilliant - I have bifocals for distance and near and I have a third prescription (second pair of specs) for a bit further than near - such as and specifically for computer screens. One eye is "fixed" at far sighted due to cataract surgery. The other is very short sighted and deteriorating due to another cataract - to the extent that my distance lenses now give it clarity at about a foot and not much further whilst the near lens gives clarity at about 8". Despite this, the other eye can focus satisfactorily with glasses and I can still see my Garmin display (as well as the road ahead etc.). I have some prescription sunglasses for distance only; I can ride perfectly well with them on but reading the Garmin, my phone or my watch is difficult and I usually have to change glasses to do this.
