Honestly, one thing I’ve noticed after years of shooting with the 90D is how picky the Digic 8 processor is regarding lens communication protocols. While everyone talks about glass, the technical compatibility of the AF drive motor is what usually kills street shots. I spent weeks troubleshooting why my older EF glass felt sluggish before realizing that the high pixel density (3.2μm pixel pitch!) makes even the tiniest phase-detect communication lag look like a total focus failure. My experience using the Sigma 30mm f/1.4 DC HSM Art taught me a few things about fitting third-party glass to this specific body: * Firmware mismatch: Older versions often clash with the 90D's internal 'Lens Aberration Correction' data, which can actually cause the camera to lag or lock up during high-speed bursts.
* AFMA precision: Because of the 32.5MP sensor, you absolutely MUST use a USB dock to calibrate the lens at multiple focus distances. The global in-camera AF Microadjustment just isn't granular enough for street distances.
* Mount Tolerances: I've seen slight 'mount play' with some adapted or third-party lenses that causes decentering on high-res sensors. If you don't check for firmware compatibility first, you're basically fighting the hardware. I think I'd prioritize lenses with user-upgradable firmware to handle the 90D's specific polling rate...

Honestly, if you want something that's gonna stay on your camera for the next five years, you should look at the Canon EF-S 17-55mm f/2.8 IS USM. It’s basically the "hidden L-series" lens for APS-C sensors like the 90D. While primes are great, for street photography, having that f/2.8 constant aperture across the whole focal range is a lifesaver when you need to frame a shot quickly without moving your feet. The inclusion of dedicated Optical Image Stabilization (IS) is the real clincher though; it compensates for micro-jitters that can ruin a high-resolution 32.5MP file. I've owned mine for years and the ring-type USM is incredibly fast and reliable compared to the cheaper STM motors. You do gotta be aware of "lens creep"—the barrel can slide out if you're pointing it down—but it’s a small trade-off for the internal build quality. You can usually find these used right around your $500 mark. It’s a much safer long-term bet for the 90D’s high-density sensor than some of the older glass people usually recommend. Tbh, the sharpness and color rendition are industry standard for EF-S mounts.

> having that f/2.8 constant aperture across the whole focal range is a lifesaver when you need to frame a shot quickly without moving your feet. I see where you're coming from with the versatility of a zoom, but I’d actually argue that for street on a 90D, raw optical performance and resolving power should be the main priority over focal convenience. That 32.5MP sensor is incredibly demanding; it basically outresolves most mid-range glass. If the lens optics can't keep up with that pixel density, you're just getting "soft" high-res files, which is tbh a waste of that sensor's potential. Before you commit, I’ve gotta ask: are you more focused on high-speed AF tracking for candid shots of people moving, or do you prioritize edge-to-edge sharpness for more static street scenes? Also, how much do you plan on cropping your shots in post? On a high-MP body like this, the lens's ability to resolve fine detail at the micro-level is way more important than stuff like stabilization for street work.

I would suggest being quite cautious with your choice because that 32.5MP sensor on the 90D is very unforgiving. If you pick a lens with older optics, you might notice a lot of softness that wasnt visible on a 20MP body. I have spent a lot of time looking at MTF charts on sites like The Digital Picture to see which glass actually holds up to modern high-res sensors.

• Canon EF-S 24mm f/2.8 STM: This is probably your safest bet for daytime street work. It is incredibly light and discreet, which helps you blend in. However, make sure to test the focus accuracy since these STM motors can occasionally struggle with tracking fast-moving subjects compared to dedicated USM glass.
• Sigma 30mm f/1.4 DC HSM Art: This offers a more natural field of view on your crop sensor. The image quality is technically superior to the 24mm, but you must be careful with Sigma calibration. I would highly recommend using the Sigma USB Dock UD-01 to fine-tune the autofocus specifically for your 90D body to avoid any phase-detect issues. Check out the LensRentals blog for their reports on copy-to-copy variation. It really helps to see how these lenses perform in the real world before you spend your cash.

Sigma 18-35mm f/1.8. Had focusing issues, but image quality's great for the price. Got mine used for $400.

Hey! For street shooting on a budget, check out the Canon EF 50mm f/1.8 STM. You can usually snag one used for around $125. Super sharp and leaves you cash for coffee runs!

For street, consider Canon's EF 40mm f/2.8 STM. Its "pancake" design offers discretion, crucial in potentially sensitive areas. I've seen sharp results, but always prioritize situational awareness.

Consider a used Canon EF 50mm f/1.8 STM. Smaller profile is less likely to attract unwanted attention, a safety plus!

Sigma's 18-35mm f/1.8 is popular; I see them used on 90Ds often. It's a bit wider, but super sharp. Do you need weather sealing? Canon's own EF-S lenses might be cheaper used – more plentiful. Check KEH.com; their grading is usually accurate. It really depends if you value new vs. used, and if you want to spend a little more to get that f/1.8.