iOS: For whatever reason, the Photos app on iOS only allows you to zoom in so far. An odd little bug caused by rotating an image allows to circumvent that restriction and zoom in as much as you want. To get nearly unlimited pinch-to-zoom potential, you'll basically just need to rotate a photo 90 degrees:
- Open up any photo in the Photos app
- Tap Edit
- Tap the rotation button to rotate the image 90 degrees
- Tap done
Now, go ahead and zoom in as much as you want. The bug stops working if you exit the Photos app, but if you really need to get a super close look at something, this is one way to do so.