If you have poor vision there are a number of options built into iOS and Android that make using a smartphone easier and if you are blind there is another range of options, but what if your vision is not good enough for the helping options but you are not blind?
The obvious answer is to make the screen bigger and whilst you could do this moving to a tablet rather than a standard phone you could also use a product like the RakuRaku Magnifier.
The magnifier is attached to the phone by a clamp that can work between 8 and 10mm and has a flexible double jointed arm that lets the magnifier be adjusted to the correct focal length. The whole assemble folds away behind the phone when not required. It’s a simple idea but well done and at a relatively low cost of $16, available from RareMonoShop in Japan but they will ship to the EU for an additional $7.