Various viewers and editors that display step images provide controls that allow you to change the size of the baseline or checkpoint image relative to the browser window.
The following controls are provided:
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Every time you click the control , Eyes Test Manager zooms in to show more details in the image. You can also do this by double-clicking on the image. 
Every time you click the control , Eyes Test Manager zooms out to show more of the image.
Click this control to show the image in its natural size (no zoom).
Click this control to adjust the zoom so that the entire image is visible.
Click this control to adjust the zoom so that the smaller dimension of the image will fill the screen (you may need to scroll the image in the other dimension to see it).
The , and controls function as a three way toggle, in that as you click the control you cycle through the three available zoom options.