Canon Dioptric Adjustment Lens Eg (+2)

How do these work?

Like how do dioptric adjustments work in general or how do you replace the dioptric I have on my camera now with a +2?

Let's assume your distance vision works fine with no correction, but because you are over 40 you need 'long arms' to hold a book out where you can read the print. Let us assume that the correction for your vision, so that you can read with a book at 10" rather than at arm's length is a diopter reading glass found in a drugstore rack and its lens is +2.0 So if you hold a camera viewfinder to your eye while wearing +2.0 reading glasses, you can read the text in the viewfinder like lens aperture and shutter speed.
Instead of wearing reading glasses, you can put the correction in the viewfinder eyepiece by using the +2 supplemental eyepiece lens, or by dialing +2 setting on the variable eyepiece diopter control.

Something that can vary by manufacturer is how an eyepiece lens is label. Let us assume that the manufacturer puts a =1 diopter value on the eyepiece to be used by a 20 year old who needs NO correction to his vision.

1. For a 40+ year old needing +1 diopter reading glasses, Manufacturer A might label his supplemental lens +0 (the actual strength of the combination)
   
2. For a 40+ year old needing +1 diopter reading glasses, Manufacturer B might label his supplemental lens +1 (the additional strength of the supplemental addition to the eyepiece)
   

"The maximum lens available is marked -7 dioptres. It is, in fact, -4 dioptres, but it extends an EOS camera’s built in -3 dioptres range to -7. If you suspect that you may need an additional dioptric adjustment lens, it is worth checking with your optician and establishing the exact value that you require before ordering.

Dioptric lenses are mounted onto Canon EOS DSLRs behind the viewfinder using a rubber frame that needs to be ordered in addition to the lens. These frames are designated Eb, Ec, Ef or Eg depending on the camera model. To ensure that you order the correct one, check the system map for your camera in the instruction manual. Also, note that it’s important not to confuse these rubber frames with the Eyecup Eb, Ec, Ef or Eg options that will come with your EOS DSLR camera as standard."

So it's like attaching reading glasses to the eyecup. Thx.

The camera's built-in dioptric setting has a range of -3 to +1. It means it's centered at -1. The labeling of the additional correction lenses is a subject of much confusion. Not even Canon themselves always know how they do it. The description Wilt quoted above, Canon's own description, is wrong, for example.

The correct information is that Canon's dioptric correction lenses are marked with the result you get if you combine them with the camera's own setting centered. Since the camera's center setting is -1, a lens marked +2 will shift the camera's range to 0 to +4. The physical value of such a correction lens is +3, but the marking is +2.
Canon don't even have a correction lens, at least not in the Eg range, that can take the camera's total correction to -7. Their most negative one is -4, and since the camera can add two more diopters to that, you get a range of -6 to -2 with that one.

That the correction lenses are marked with the combined result of lens and camera, with the camera's setting at -1, is easily proven by the fact that there is no -1 lens available. Well, there is, but it's the standard one, that has just a hole, no lens. But there is a 0 correction lens, which brings the range to -2 to +2.

Thank you for correcting Canon's own erroneous description!

chuckmiller wrote in post #19062463
How do these work?

Look through the viewfinder, adjust the wheel for the sharpest vision, that’s your setting. It varies from person to person; some people may have vision that exceeds the range of the built in optic adjustments in which case Canon sells a slip on accesory that mounts over the eyepiece>

chuckmiller wrote in post #19062463
How do these work?

apersson850 wrote in post #19063352
The camera's built-in dioptric setting has a range of -3 to +1. It means it's centered at -1. The labeling of the additional correction lenses is a subject of much confusion. Not even Canon themselves always know how they do it. The description Wilt quoted above, Canon's own description, is wrong, for example.

The correct information is that Canon's dioptric correction lenses are marked with the result you get if you combine them with the camera's own setting centered. Since the camera's center setting is -1, a lens marked +2 will shift the camera's range to 0 to +4. The physical value of such a correction lens is +3, but the marking is +2.
Canon don't even have a correction lens, at least not in the Eg range, that can take the camera's total correction to -7. Their most negative one is -4, and since the camera can add two more diopters to that, you get a range of -6 to -2 with that one.

That the correction lenses are marked with the combined result of lens and camera, with the camera's setting at -1, is easily proven by the fact that there is no -1 lens available. Well, there is, but it's the standard one, that has just a hole, no lens. But there is a 0 correction lens, which brings the range to -2 to +2.

Something tells me the OP might still be wondering...

Anders has it right, I wasn't asking about the built-in diopter, I was curious about the additional piece you can add via the eyecup.

Thank you.