It is worth changing the preview size to the new Auto settings, if required. From the LR menu go to: Edit >> Catalog Settings >> File Handling (tab) Lightroom >> Catalog Settings >> File Handling (tab) on Mac For Standard Preview Size choose Auto from the drop-down list. This is the default setting if you are using LR6 for the first time and it’s particularly useful for retina and HiDpi displays but will optimize LR for any monitor. For those using LR5 or previous versions, it is recommended that the preview size is changed to the Auto setting.
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