Zoom an image with respect to a given image
Since you want the image size and resolution to remain constant, the term optical magnification seems apt there.

You could then say


  Please use 20x optical magnification to identify said feature.


Please be careful that the term magnification is different from zoom.

Two different cameras with the same 5x optical zoom might produce images with different optical magnification depending on their optics.

Zoom refers to the ratio of the max to min focal length of your camera, whereas, 
Magnification refers to the ratio of projection size to the real subject size.

Given a particular magnification, you could use your camera focal lengths and apertures to come up with at zoom metric for that particular camera to produce the desired results.

See this for a better explanation.