Oh man congrats on the a6400!! You are gonna have a total blast in Hawaii! Honestly, the 18-135mm lens is the absolute king for travel especially on a budget. I literally refuse to travel without my Sony E 18-135mm f/3.5-5.6 OSS because it just works every single time and I trust it completely. I remember my first big trip to Kauai... I was so nervous about getting the perfect shots that I brought three different lenses. Big mistake. I was trying to switch to a long zoom on a boat tour near the cliffs and almost dropped my camera right into the salt spray because the boat rocked! It was terrifying and I probably got salt air and dust inside the body too... never again. Since then, I just use the one zoom so I never have to open the camera up in the elements. The image stabilization on that Sony lens is fantastic for keeping things sharp when youre on the move or even if your hands are a bit shaky. Plus it fits right into your $600 budget if you find a good deal or buy it used! Quick tip tho: definitely grab a protective filter for the front glass. The volcanic sand in Hawaii is super abrasive and can ruin your lens if you are not careful. You wont believe how amazing the photos look!! Its basically the only lens you will need for those sunset beach shots and the hiking trails. Have a fantastic time!!

Yo! Had a few mins to look at the latest optics data and honestly, you picked a fantastic camera body! The a6400 sensor has amazing dynamic range for those Hawaii sunsets, but here is the technical catch: it lacks in-body image stabilization (IBIS). This is a massive detail because if you are zoomed in on a turtle or a distant surfer, every tiny hand shake makes the photo blurry unless the lens has its own stabilization system. When I first started traveling with a mirrorless setup, I totally ignored the aperture specs and just bought the cheapest zoom I could find. Big mistake! I ended up with super grainy photos during evening beach dinners because the lens couldnt let enough light in. Here is what you should prioritize for your $600 budget:

• Optical Stabilization: On Sony lenses look for OSS, and on Tamron it is called VC. Since your camera body doesnt have it built-in, this is basically non-negotiable for handheld travel shots.
• Focal Range Versatility: 17mm or 18mm is wide enough for the mountain landscapes, but you want at least 70mm for tighter shots of people or details.
• Constant Aperture: Lenses with a fixed f/2.8 are technical masterpieces because they stay bright even when you zoom in. If you want one lens that does literally everything, the Tamron 18-300mm f/3.5-6.3 Di III-A VC VXD is an incredible piece of engineering for the price. If you want better low-light performance for those indoor luaus, the Sigma 18-50mm f/2.8 DC DN Contemporary is tiny, super sharp, and fits your budget perfectly tho it lacks stabilization. Enjoy the islands!

Wait a sec, what are you actually planning to photograph the most? Landscapes or wildlife? It really changes what you should buy.

• Be careful with lenses that dont have built-in stabilization, especially for handheld shots.
• Make sure to consider the weight if you are hiking a lot, some zooms are total tanks.
• Honestly, just get any all-in-one zoom from Tamron, you really cant go wrong with their stuff.