Crook in Cinderford Jones in Coleford and Ida Gywnn in Gloucester for starters

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Ἀριστοτέλης A Gloster & Hereford Boy in the Forest of Dean ><((((*>

Hi David,
the majority of such studios within the county of Gloucestershire in those times were probably within the big areas of population, and therefore wealth, so they would be in the city of Gloucester, fashionable Cheltenham, and also the major port of Bristol. As Slowhands said, there were a few in the Forest as most towns had a photographer, if you search "photographer" within this forum you'll find detailed posts about the ones he mentioned, and others too such as John Wm Porter in Coleford, and my own ancestor Harry Beard in Longhope. I'm sure Lydney and no doubt Monmouth also had them too. Some of these were "only" part-time photographers as there was insufficient trade to emply them fulltime, often their main profession was their chemist/druggist shops, presumably because it gave them knowledge and access to the necessary chemicals etc.
That said, most if not all the above would advertise their name and studio on the frame or mounts of their photos, or on the back of the image itself, which presumably isn't the case with your photo ?.

Do you have any more information at all as to where your photo may have been taken, eg in the Forest or not ?.

This website gives some more suggestions of studios.
http://www.earlyphotographers.org.uk/Gloucestershire.html

This webpage has links to the main advertising Trade Directories for the Forest area, searching them will give links to photographers especially those mentioned elsewhere in this forum.
https://freepages.rootsweb.com/~cbennett/genealogy/index.htm

hope this helps, J