Dating a la carte
Too many fences, rocks, and other decorations detracted from the phototgraph's subject.
Look for even, sharp corners to decide if it was round or square.
For example, early cabinet cards used the exact same process as the CDV, but were just larger. Note the size may vary up to 1/4", especially with early samples as the photographer may have cut his own card stock.
But generally they were close in size, partly due to the standard size of album slots.
From August 1864 to August 1866 photographs were taxed, which required that stamps be affixed to the photo.
This has become a great way to date Civil War era photos.