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?

Question

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?

In: Biology

Answer 1

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.