Honestly, the numbers are a total headache when you're starting out. Basically, the X-Pro3 has a sensor that makes everything look a bit more zoomed in than a traditional film camera. For portraits with that nice blurry background, you generally want something between 35mm and 56mm. The lower that f number on the lens, the more blur you get. If you want the best bang for your buck under 600 bucks, you should definitely look at the Fujifilm Fujinon XF 50mm f/2 R WR. I've used it for years and it is super sharp, plus it's small enough to not look intimidating at a party. It handles portraits beautifully because it compresses the background just enough. If you really want maximum blur though, the Sigma 56mm f/1.4 DC DN Contemporary for Fujifilm X is probably the better bet for your budget. Since it hits f/1.4, it lets in way more light than the f/2 lens, which helps if the party is indoors or in low light. The 56mm focal length is perfect for headshots or upper body shots. Just keep in mind that with a prime lens, you have to move your feet to zoom in or out. It takes a second to get used to but the image quality is way better than a basic zoom. Dont overthink it too much, both of these will make your niece look great... it mostly comes down to how much room you have to step back at the venue.

@Reply #4 - good point! saw this earlier while grabbing coffee and wanted to jump in because i went through a massive learning curve when i first started doing events with my fuji gear. i used to think the biggest glass from those third party brands was the only way to get that pro look but i was totally wrong! 1. i once bought this massive lens from a budget brand because the specs looked cool on paper, but it felt like carrying a brick on my x-pro3. it actually made my hands cramp after like twenty minutes lol. 2. i eventually realized the native stuff just handles the colors and skin tones so much better. there is just something about the way the fuji glass renders light that makes everyone look amazing. 3. the one i got recently is much lighter and the autofocus is a total game changer for kids running around at parties. just stick with it tho! you are gonna have a blast once you find your rhythm. honestly that camera is fantastic and you are gonna get some killer shots at your nieces party! love that you're getting into it!

actually i was thinking... is the party gonna be indoors or outside? once brought a long lens to a house party and could only take photos of peoples noses lol. make sure to consider:

• Sigma 56mm f1.4 DC DN Contemporary Fuji X light and sharp
• Viltrox 75mm f1.2 AF Fuji X insane blur but heavy id suggest the sigma, just be careful with the viltrox weight.

Late to the party but i would be careful with wide aperture lenses for a fast birthday event. If the focus is sluggish, you might miss the big moments. IIRC some older lenses have issues with newer sensors. Make sure to look for:

• Fast autofocus motors
• Lightweight builds
• Weather resistance I think i read somewhere that heavy lenses feel awkward on that body, so definitely watch out for the ergonomics... its just safer that way.

Been thinking about this and honestly you should just grab an f2 lens and call it a day! Lugging around giant expensive glass used to make my wrist ache after an hour. Finally picked up the Fujifilm Fujinon XF 50mm f2 R WR and it is an absolute beast. It is super reliable and the autofocus is way faster than the bigger lenses, which is huge when people are moving around at a party. Here is what I learned:

• focus on the eyes, the x-pro3 is great at tracking them automatically
• dont be afraid to get close to your niece to get that blurry background Seriously, the build quality on that thing is fantastic and it fits the x-pro3 vibe perfectly. You get amazing shots without the headache of a heavy setup!