As iPhone users this may be common knowledge to you but it was something new to me. I only have an iPhone for photos. (I know, not a real camera but its what I have) The below settings work on iPhone and iPad.
I've always used two fingers to zoom when using the camera. However I found out there is a Magnifier option and I think it works well as for me it has better size control than my two fingers I've been using.
To turn it on go to:
Settings>General>Accessibility. Then turn on the Magnifier. You can then close the Settings application.
To use the Magnifier, open your Camera then tap on the Home button three times quickly. This will allow a "slider bar" to open at the bottom of the screen. The slider can then be adjusted to get the proper magnification or zoom. This works in portrait or landscape orientation although the slider stays in the same location on the screen.
To turn off the Magnifier either close the Camera or tap the Home button once.
I haven't checked to see what difference in resolution there is between using Magnifier and the two finger zoom method.
Good luck all.