Honestly, the 50mm is fine for a start, but I had issues with my copy when I first got into portraiture on my 90D. The bokeh on the Canon EF 50mm f/1.8 STM was a bit nervous for my taste, and unfortunately, the focus hunting in low light really ruined a few shots during a sunset session I did last year. It felt cheaper than I expected. On an APS-C sensor like yours, that 50mm acts like an 80mm which is a classic length, but the quality just isnt quite there for professional-looking graduation shots. If you can stretch the budget or find a used deal, check these out:

• Canon EF 85mm f/1.8 USM for much smoother background compression.
• Canon EF 50mm f/1.4 USM if you want slightly better light gathering than the f/1.8. I really wish the cheap 50mm performed better since its well under $300, but the chromatic aberration is just annoying... definitely watch out for those purple fringes in high contrast areas.

Building on the earlier suggestion, I totally agree the 50mm can hunt sometimes... kinda annoying tbh! If you want a technical beast for that 90D sensor, find a used Canon EF 85mm f/1.8 USM. Since the 90D is a crop sensor, that 85mm acts like a 136mm lens which gives incredible background compression! The f/1.8 aperture and the fast USM motor is just amazing for portraits.

Unfortunately, I've had issues with the cheaper 50mm primes on that 90D. The high-resolution sensor basically highlights every lens flaw, and the Canon EF 50mm f/1.8 STM just wasn't as sharp as I'd expected. Chromatic aberration is also quite severe in backlit graduation photos. A used Tamron SP 45mm f/1.8 Di VC USD is a much better investment... the optics are superior for that body and wont disappoint.

Can confirm

> Can confirm Regarding what SandraVox said about "Can confirm"

• I totally agree that the basic 50mm has some real flaws when you put it on a high-res body like the 90D. It definitely feels its age there. That said, I’m gonna disagree slightly on the suggestion to go third-party for such a big event. In my experience over the years, I've found that native glass just handles autofocus more reliably. I've tried many different setups, and while those other brands are sharp, they sometimes struggle with consistency on Canon DSLRs... kinda scary for a graduation. For a big day where you only get one chance to get the shot, I’d prioritize the safety of native gear. I’d rather have a slightly less sharp photo that is actually in focus than a perfect one where the camera missed the mark. Honestly, reliability is everything when you're the designated photographer.

> Is the 50mm one good or do I need something else... Late to the party but honestly, you might want to consider just sticking with a prime from Canon. Be careful with those third party brands because they can be super hit or miss with autofocus accuracy, especially on a newer body like the 90D. I would suggest looking for a used professional grade lens from Canon instead of the basic plastic ones. Just make sure to get something with a solid build and a fast motor. I do a lot of DIY portrait stuff for family and having gear that doesnt hunt for focus is the most important thing when your sister is walking across that stage. Just get any professional grade prime from Canon and you cant go wrong tho.