Came here to say the same thing lol. Great minds think alike I guess.

Re: "Came here to say the same thing lol...." - i have been reading through everyone's suggestions and honestly, i am a bit let down that we are mostly talking about 50mm lenses for buildings. i have had my gear for over a year now and i have learned a few things the hard way.

• the narrow field of view on the lenses mentioned so far is really frustrating when you are trying to fit a whole building in the frame.
• i had issues with my first setup because i kept having to back up into traffic just to get the shot... not ideal.
• manual focus on vintage glass is cool for some things but i found it way too slow for fast-moving street scenes. i really wanted to love the budget options everyone suggests, but they just were not as good as i expected for architecture. i eventually found a much wider lens that finally let me capture the scale i wanted, but it took a lot of wasted money on the wrong glass first. just be careful not to get stuck with something that feels like you are looking through a straw!

So, I definitely agree that adapting glass is the budget king, but you have to be realy careful with the technical execution if you're shooting buildings. If you go the DIY route with cheap adapters, you risk slight misalignments in the flange focal distance, which can lead to soft corners—definately not what you want for architecture where edge-to-edge sharpness is critical. For a street-focused setup on the R100 that stays under $300, I’d suggest looking at these modern native options instead:
* Canon RF-S 10-18mm f/4.5-6.3 IS STM - This is basically essential for buildings. It's wide enough to get the whole scene and has built-in optical stabilization.
* Canon RF 28mm f/2.8 STM - It’s a tiny pancake lens. It keeps the R100 profile low-key for street work and provides a much more natural field of view on a crop sensor than a 50mm. Honestly, if you're buying used to save cash, just do a quick DIY calibration check by shooting a flat wall to look for decentered elements. It’s a professional standard to ensure your glass is actually performing. Avoid paying for a pro "CLA" (Clean, Lubricate, Adjust) service on these cheaper lenses though—it usually costs more than the lens is worth!!!

Hey! I actually use the Canon EF 50mm f/1.8 STM with an adapter on my R100. It's super cheap – usually around $125 new – and surprisingly sharp for street photography. Not the *widest* angle for buildings, but the image quality punches way above its weight. Plus, it's tiny! Definitely worth a look if you're on a budget and prioritize image quality over super-wide shots.

Canon's EF 50mm f/1.8 STM? Under $200, and it works surprisingly well for street stuff.

For street shooting on the R100, consider adapting vintage glass! I'm happy with my circa 1980s Olympus Zuiko 50mm f/1.8. With a cheap adapter (~$20), it's surprisingly sharp. You'll need to manually focus (obviously!), but the image quality is great for the price. No complaints so far!

Before choosing a lens, consider your surroundings! A fast aperture (low f-number) helps in low light, but be aware of pedestrian safety when focusing. Sigma lenses offer good quality. Maybe consult a pro shop for advice?