The GN4 version since the version 2.2.2561.x generates the thumbnails and previews also in the hi-res version for the following colorspaces:

•Grayscale: 8 bit per pixel

•RGB: 24 bit per pixel

•CMYK: 32 bit per pixel with or without alpha channel.

For the images with a different or not-supported colorspaces e.g. LAB, RGB 48/92 bit per pixel, etc) the parser will use the thumbnail instead of hi-res image. This may lead to the preview out-of-focus.