nospam
In article <b-qdnUuFUvwIp8nOnZ2dnUVZ_vudnZ2d@giganews.com>, Alan Browne <alan.browne@FreelunchVideotron.ca>wrote:

PeterN
The only time you might see a difference would be if there were colors in the LAB spectrum that are not in the RGB spectrum, and those differences would rarely be noticable in a photograph.

Alan Browne
A good point - but nospam's contention is conversion differences.

Let's color the situation. The files were taken from raw, reduced in size, saved as TIFF in each RGB and Lab. Re-loaded and compared (differenced). There is not a hint of delta.

http://tinyurl.com/m35p59t RGB

http://tinyurl.com/mldetvx Lab

http://tinyurl.com/mvl6vwj Diff

nospam
there's a difference there too.

i did: image/duplicate, append -converted to the name. image/mode/lab image/mode/rgb image/calculations, original on top, converted below, subtract mode

optionally, just before the calculation step, switch between the two images while looking at the histogram palette. there's a difference. then do the calculation step.