Each viewer works slightly different.
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I’m using FPV since 15 years and still love it for its simplicity, even use it for “instant” culling (raw file + ooc jpeg !) … before I dive into PhotoLab ( more on that here… or here… )
Even back then, the app supported a wide range of RAW formats via additional codecs. However, one needs to familiarize with the app’s quirks – such as the fact that, for RAW files and their corresponding out-of-camera JPEGs, it displays the JPEG preview, or that I have to manually select the image display profile when using my calibrated monitor with other color spaces.
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Then there are other viewers that can be configured to automatically detect the active display profile in order to display the image correctly in the large preview – though not necessarily in the thumbnail view (which is quite annoying, if not downright misleading). I’ve written more about this somewhere, including print preview and the like.