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Distant vision is blurry when you are myope, it happens when your eye is not able to focus light rays from the outside world on to the retina, which is a light-seeing layer in the eye and is essential for vision.
In myopia, light rays are focused in front of the retina. It is corrected by prescribing concave or minus powered lenses which diverge in coming light rays into the eyes and they are focused on the retina rather than in front of it.
Myopia increases up to the age of 22. For proper vision, it needs to be corrected by lenses after examination. It is probably true that your left eye is more myopic than -0.25 as your vision in that eye was still blurry when you were given a -0.25 prescription. As your vision is better and colors are brighter with -0.50 correction, then you will need a -0.50 Diopter lens. Wearing a lens of -0.50 D does not cause lazy eye.
A variation of -0.25 D in prescription is very minimal. A minimal variation can happen from one test to the other. However, if your present prescription of -0.50 D is giving you best corrected vision, than it can be worn.
Regular eye sight testing is needed for all those wearing eye glasses. A once a year test is required till the age of 18-20 if no problems with vision are encountered. Hope this information is useful. Thank you.