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The image below shows a typical µManager GUI. The details of the Groups and Presets may vary from system to system. Select the right Magnification Preset to match the objective lens and optovar (if applicable). Make sure the camera depth is set to its maximum (usually 12-bit). Click Live to get a live image.
Click the Zoom tools to make the live image bigger or smaller. You can click the Auto button next to the live image contrast histogram to auto-contrast the image once, or you can tick Autostretch to have auto contrast on constantly.