Since you're using this with one hand, tapping anywhere other than the area where your thumb naturally rests would be uncomfortable.
Also, and more importantly I think, what if you wanted to zoom in to a position near the top of the screen? You would have to tap near the top of the screen and move your finger up, causing your thumb to go off the screen. So you wouldn't be able to zoom in to an area near the top of the screen or zoom out from an area near the bottom. If such a large segment of the screen is unusable for this feature, you might as well just have it zoom to the middle, and make the user center the place they want to zoom to on their own.
Since you're using this with one hand, tapping anywhere other than the area where your thumb naturally rests would be uncomfortable.
Not any more uncomfortable than single tapping any other part of the screen one-handed. Of course this depends on screen size...
what if you wanted to zoom in to a position near the top of the screen?
Point well taken.
Assuming that the content of interest is a good centimeter or so in from the edge, you could map that point-to-edge distance to cover the remaining zoom scale. If it's inside that centimeter, zooming into the middle is useless anyway.
Not any more uncomfortable than single tapping any other part of the screen one-handed. Of course this depends on screen size...
You're right, this would probably be perfectly fine on the iPhone, though it could be annoying on something like a Galaxy S3, especially for someone with small hands.
If it's inside that centimeter, zooming into the middle is useless anyway.
True, but if it always zooms to the middle, the user knows that they need to center the point that they want to zoom into before they actually zoom.
