raster package: RGB tif files

I want to use package raster to apply a linear function
to each band of an RGB tif file.

I thought that
 > st <- stackFromFiles("SDIM1246_L1.TIF")

would make a raster stack with the 3 bands, but
 > nlayers(st)
[1] 1

Then I've tried:

a  <- raster("SDIM1246_L1.TIF")

and str(a) indicates that there are 3 bands, but, then, how
can I use calc() on each of them?

Thanks

Agus



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