Sherlock Holmes isn’t always right! The classic image of Sherlock is him examining some object for clues as in the picture of Basil Rathbone playing Holmes. Most people subconsciously copy Holmes when using a magnifier. Unfortunately in this instance he is wrong.
The proper way to get the best results with a magnifier is like this.
The closer you hold the magnifier to your eye the better image you get, it is bigger and there is a bigger field of view. The magnifier in the picture is ideal, it is self illuminated and the housing means it is resting on the page at the correct distance. The person is also wearing their reading glasses, if you don’t wear reading glasses it will be slightly out of focus and you will have to hover the lens above the page.
If the magnifier is a simple design which you just hold and find the focus point by moving it back and forth it doesn’t matter if you are wearing your glasses or not.
The other major error is that people move the magnifier across the page. Normally you read a book by holding it still and letting your eye rove across the page. With a magnifier you should not move the magnifier. You move the page under the magnifier. This sounds and feels completely wrong initially but as you practice it gets easier and reading will be more fluent and faster. You need to stick with it and it will become your preferred way of reading.