For analyzing optical problems involving the eye, what quantity is fixed (for seeing something in focus)?

The distance of the image is fixed.

Explanation:

Eye diseases like macular degeneration, glaucoma, and cataracts, can cause problem in seeing properly. Symptoms change in huge amount among these problems, so keep up with your eye exams. Some change in the seeing power can be hazardous and need immediate medical care.

For detecting the particular problem in the vision or the problem your eye is going through, the thing that is to be kept fixed and not changed at all is the distance at which an image is kept which is to be focused at to see the problem.

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