With a lens mounted and a subject focused on, adjust the viewfinder diopter until the scene is sharp; I make additional menu and other setting changes based on current shooting scenarios, but this list covers my initial camera setup process. To copy this configuration means you intend to shoot as I do - including in RAW-only format.
Thus 50mm = 20 diopters and 120mm = 8.33 diopters. The total is 28.33 diopters. Now we convert back to millimeters thus 1/28.33 X 1000 = 35.3mm (the focal length of the combination). The power of the reading glasses in diopters and the adjustment of the camera eyepiece correction are the same.
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diopter adjustment on canon camera