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As far as I know a human eye has approximately 19 diopters and this could be used to calculate the focal point when the observed object is placed in infinity and incoming lights are parallel.

How can I calculate diopters needed to focus on some object close to a human eye (let's say 40 cm.) when I still need the image focused on the retina?

I see that I have two "focal points" here - on the object and one the retina. If the distance to the object changes, what formula could be used to adjust the lens so that the image is still in focus on retina?

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When lenses are close together, their diopters add. Diopters are $m^{-1}$, the inverse of the focal length. So, when I wanted glasses for bench work, typical distance 50 cm, I asked my optometrist to write me a prescription for 2 diopters more than my distance prescription. They work great!

To bring the focus from infinity to 40 cm, add 2.5 diopters to whatever focuses on infinity.

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  • $\begingroup$ Thank you. I realized that my question is slightly incorrect. To avoid editing after answering and discussions in comments, I've created a new one, more precise: physics.stackexchange.com/questions/821855/…. Could you please take a look? $\endgroup$
    – Damir Tenishev
    Commented Jul 18 at 17:03

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