Hi Jen
An excellent booklet for helping date photos is "Photographers in North-Eastern Scotland to 1914", compiled by by D Richard Torrance and published by the Scottish Genealogy Society. I got my copy from ANESFHS a few years ago for £4.50 and it has been invaluable as it lists photographers and when they worked at specific addresses as per censuses and directories.
Yes, your Edward Middleton is in it:
MIDDLETON Edward McIntosh
1881 Apprentice photographer aged 14, b. Aberdeen, s/o John M., master tailor, and Jane F. M., resident 8 School Hill, Aberdeen.
1891-1914+ 181 King Street, Aberdeen. (1893-1896 Middleton's Studios; 1896 'E. & J. Middleton'.)
PS - I too have lots of photos by Middleton, and as I have MIDDLETON in my tree, I always wondered if there was a relationship there. My Middletons came from Aberlour, Banffshire.