Spectacle power, How do opticians decide short sighted glasses power by making one read letters on a board which is just 10 feet away? One needs to look far for that, isn’t it?

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Spectacle power, How do opticians decide short sighted glasses power by making one read letters on a board which is just 10 feet away? One needs to look far for that, isn’t it?

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You’re supposed to be 20ft away from the chart (often they’ll use as mirror to double the effective distance in a smaller room).

In terms of how your eyes focus 20ft is close enough to your “infinity” focus that they can get an accurate prescription from it.

If you have a camera with a manual focus feature [you can see the same thing where the further focus distances get bunched together closer to infinity focus](https://www.blog.jimdoty.com/wp-content/uploads/100324_DOF_scale_50mm_lens_f8_5D_1406-w8.jpg) because of how focusing works in general.