My iMac is also the Mid 2011 model. I have it running macOS Sierra 10.12.6 currently. The iMacs from that year can be upgraded to macOS High Sierra 10.13.6 at most. Apple recommends to upgrade your devices to the maximum that is supported. Generally, the added features do not make old machines crawl, luckily, if you maintain the software in a healthy state, and keep a free space reserve on the startup hard disk drive.
How to upgrade to macOS High Sierra - Apple Support
which includes a Mac App Store link: Get macOS High Sierra.
Note that some 3rd party software will have not only minimum system requirements (like your photography software), but may also have less documented maximum system requirements (won’t run beyond a certain OS version). Before upgrading, check your essential software and their websites for supported OS versions. If you “absolutely depend” on some software that won’t run beyond OS X El Capitan 10.11, then by all means use that instead.
The ‘Handoff’ feature is a cool gimmick for newer Macs and iOS devices; it is hardware dependent from a later date. It is not something you will loose by upgrading the OS. The feature is explained here: Use ‘Continuity’ to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
