I’m assuming this came with the latest iOS upgrade as I only noticed it yesterday. There is now a magnifier icon which is so useful and far better than an ordinary magnifier. If you haven’t already found it it has a black background with a white magnifying glass symbol with a yellow plus sign on it.
It doesn’t just magnify to different degrees - it also lets you change contrast and brightness. But the feature that really makes a difference to me is that you can change the reading colours and this includes black print on yellow background. This means you can use it on say, a letter and have it converted instantly to an enlarged black on yellow document. You can do multiple screenshots on a long document and then read it on your iPhone - or iPad.
Hope this is useful to someone.
TwasBrillig