I'm age 97 and have had an Apple computer of one size or another for quite a few years. My previous one was a 27 inch "Lion." My eyesight is still pretty good with tri-focals for regular use and bi-focals prescribed for better computer viewing, and I have little trouble except for very small print like phone-book-size type especially when in light colors (a dumb idea when black would be much better) and I was led to believe by the advance hype that the new Apple Yosemite would be great for all kinds of viewing of on-screen text.
So as soon as the Yosemite came on market I bought (at a cost of about $2800) one of the early 27 inch Yosemite models and what a mistake that was. The text had a clearer resolution but they got that by making it so tiny that I need to hold a hand magnifying glass to the screen to read it. I had to go to the least resolution available on the scale menu in order to read it at all and I still have to do so when not online or when using the Apple Safari browser. I had trouble from the beginning and an Apple phone tech had to erase my whole factory-installed op system and download me a new one which didn't help.
After months of frustrating experimentation I finally stopped using the Safari browser altogether and after downloading the Google Chrome browser found that it helped a great deal because the Google browser has a way to adjust the the overall type font to a readable size (some of their type is permanently in BOLD) but best of all Chrome has an extension which allows one to go through a range of color modifications, the best being the High Contrast option which makes the Apple contrast option look like no contrast at all. On the Google browser it also makes the texts look as if they were all set to BOLD.
I have recently upgraded the Yosemite system to the new El Capitan which may help some, but by and large I would say the trouble I have had with Yosemite from the beginning seems to me inexcusable for an Apple product. In my view Yosemite certainly wasn't ready for prime time when I bought it and is only now useable for me with the Chrome browser. I find Safari to be useless.
For those having the same problems I did, I suggest trying the Google Chrome browser which does have a few bugs of it's own such as making some color views too dark but in my case it made considerable improvement in my use of all the computer's features when online. Unfortunately, though, when using the computer offline I still have to wrestle with almost invisible texts which defy reasonable viewing on many of the built-in apps.
Why can't Apple just give us a download option to make all the texts stand out in BOLD like Google has done with their High Contrast extension? Seems like a simple thing for them to do and would make all the difference for those who have had problems similar to mine and from the number of complaints I see on these discussion groups it seems like a significant number.