I am creating a application using Game Maker 8 and it has a zoom feature, where you can zoom in and out using the scroll wheel. For now, I am using a simple formula of adding or subtracting 10% of the zoom. It works well at small scales, but it takes to much time when you zoom in highly.
I tried to look into know applications and see how they do the zooming, but it seems they use a set of predetermined zoom factor that you scroll to - that is, you always go from 100 to 75, 50, 33, 25 and 12,5%. As far as I know there is no formula that makes those numbers in order.
Should I just make it a pretermined set of numbers or should I use some formula? What other applications are doing?