I usually rotate all my jpg photos (using Windows Photo Viewer) after removing them from my camera's card. Is this correct? Am I losing quality for rotating them? Should I just leave them as they are and later incorporate the rotation as another retouch step - at a post-processing software like, for instance, Lightroom?
Sometimes I see websites like DPreview.com showing their camera's sample photos not rotated, thus I thought rotating them would reduce (a lit bit) their quality. It it a fair assumption?
Answer
Whether rotation loses quality or not depends on software (and its version) used and image dimensions. Images that have width or height (measured in pixels) not divisible by 8, cannot be rotated in a lossless way.
As for Windows Photo Viewer, Matt Grum has already given a stellar answer to the more specific question. For other software, you could use the sample images in his answer and try the same experiment.
Note that rotation can only be lossless for rotation by a solid angle or 180 degrees. Rotation by arbitrary angle always requires re-compression and also makes the picture softer, even if you use a non-compressed format (such as BMP) - the new pixels do not have exactly corresponding pixels on the non-rotated image and therefore have to be calculated based on weighted average of surrounding pixels.
After rotation by arbitrary angle, you might also lose some edge areas of your composition when you crop the image into a rectangle with horizontal and vertical sides.
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