If you only have 50 MB free, the normal caching activity from iOS alone can chew that up in no time. The cache files will increase and decrease in size during normal usage. But, if you need to take a bunch of photos, you need a lot more free space than 50 MB in order to deal with the constantly changing system file sizes.
As others have pointed out, try restarting the device and that will free up some space, because restarting will purge some of the cache files.
With iOS 8, photos no longer go away when you trash them. They get moved into the "Recently Deleted" photo album, where they stay for 30 days. If you want to purge them now, you need to go to that album and permanently delete them manually.
Another photo setting you should uncheck is "My Photo Stream." With Photo Stream enabled, iOS creates a local duplicate of all your photos and uploads the duplicate to iCloud. If you don't care about sharing your photos, then uncheck the option and iOS will stop making duplicates of every photo.
Also, if you use iTunes Match, toggling it on and off will purge the cached songs that you've downloaded (assuming that you don't need them on your phone).
But, if you have a persistently large "Other" directory (about ~3+ GB), then you likely have a bunch of ghost files and garbage data piled up from botched app and media downloads or crashes. The only way to get rid of this stuff is to restore your device as new, and then restore your apps and data from a backup. I generally prefer to use a local backup with iTunes because it will also restore your messages. If you restore using an iCloud backup, you lose your messages.