How do I chose the right "correction eyepiece" when I need to go past the furthest left on a Nikon viewfinder?

Question

I want to buy a Correction Eyepiece to correct my view. and I am hesitated between (-2.0) the minus diopter and (+2.0) the plus diopter and I don't know what to choose.

I've got my Nikon camera diopter adjusted to the maximum left and I need just one more tick or two to get a neat view.

After I consulted my ophthalmologist, he advised me to choose the minus (-2.0) one. But when I test it with him he got more confused :) — believe it or not. Does it need more minus in addition to the minus (cumulative), or is the correction is to decrease the values? by adding a plus value to decrease the error distance between the two values my eye prescription and the value it should be (in opposite to counteract that)?

So I need your advice. Please find attached eye medical receipt:

I want to make sure that I choose the correct one, please. Is it the minus or the plus?

Please Read My Profile · Accepted Answer

This prescription is for -1.25 diopters (under "sphere"), plus some astigmatism ("cylinder" and "axis"). You won't find an off-the-shelf corrective piece to deal with astigmatism — usually we just ignore that.

The add-on corrective eyepiece is designed so the nominal number is the result when used in combination with the existing adjustment in its neutral position, which will be -1 (not 0). So, the -2 diopter seems right; you can then set the adjustment in the built-in diopter a couple of clicks to the positive, so your end result is around your prescription of -1.25.

On rereading several times, I see the source of your confusion.
The prescription tells you the correction you need. You're thinking that maybe it tells you the error of your eyes and you need the opposite to counteract that — nope. Just get a corrective lens that matches what's written.

Michael C · Answer

You need the minus two (-2) diopter eyepiece.
If you read the link at Nikon, the corrective eyepieces are not cumulative. The OEM eyepiece on Nikon cameras with a diopter adjust wheel are -1.0. Adding the -2 diopter eyepiece brings it to -2 (and not -3). From the link for the -2 eyepiece above:

"The eyepiece’s diopter value is, when combined with the viewfinder’s
minus one value, the end-result value. No combination of the camera’s
base value of minus one and a given accessory diopter’s value is
required to arrive at the final desired value."

Digital Lightcraft · Answer

Agreed - my prescription is aprox -2, and the -2 eyepiece works for me.

How do I chose the right "correction eyepiece" when I need to go past the furthest left on a Nikon viewfinder?

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