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Two of the factors that affects the magnifying loupes I use are two concepts- depth of field, and focal length.
I'm a dentist, so we spend a lot of time and $$$ on glasses/magnifiers.etc.
The focal length for a dentist is approximately the distance from the dentist sitting in their chair, looking down to the patient's teeth as they lay in the chair. At that distance, everything should be in focus. For someone reading a book, it would be from the person's eye level sitting up, down to the book, on the table. You want a focal length where you are comfortable.
With that approximate length in mind, then the depth of field comes into play. With lower magnifications, there is a deeper depth of field, so let's say you took the book off the desk and sat it in your lap (further away), you could still read it in focus. With a deep depth of field, you could also pull the book closer to your face and it would still be in focus.
As you increase the magnification 3x, 5x, etc...that depth of field decreases, so you need to pay more attention to keeping the book right at the desk height (or whatever height you read at- focal length) to keep it in focus. You get the higher magnification, but your leeway decreases in the depth of field. Put the same book on your lap, or closer to your face- and it will go out of focus quicker.
Lately, I've had to go to reading glasses myself, and have noticed that although I like the higher magX glasses, the lower ones are better for wearing around the house. I can still look up with a 1x set and see whats on tv, or walk down a flight of stairs. With the higher mag levels, the room gets pretty blurry. Obviously, you aren;t supposed to wear them walking around the house/ descending stairs/etc... but I'm lazy
If you are going in for glasses at a supermarket or something- grab a magazine and sit down with the book in your lap (or on a table) and experiment with the two concepts. If the book readable at a comfortable focal length for your needs? - then move the book away from you and toward you to see you deep the depth of field is.
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