Make your eyesight worse if using pc/tablet for long time everyday?

How bad is it to your eyes if you use iphone/tablet/pc for long time everyday? I use my iphone for 8hrs a day or more playing games etc… Can it for example make your eye from -07.00 to -07.50 because of stressing your eyes too much? Can anyone certified answer this question please thanks

No. That's a myth. A very popular myth, but a myth. There are some very poorly designed studies (so poorly designed that the results are not believable) that suggest that close work will make you get nearsighted sooner, but even those studies didn't find that you get more nearsighted. It's your genetic destiny. Your genes control how nearsighted you will be as you grow, just as they control how big your nose gets.