I've been using the Tamron 70-180mm G2 f/2.8 with my a7C for two years of hiking and travel, and it's been perfect. At 810g, it's substantially lighter than Sony's 70-200 options while still delivering excellent image quality. The slightly shorter range (180mm vs 200mm) has never been an issue for me. It's weather-sealed enough for light rain, and the compact size balances nicely on the small a7C body.

Tbh, I’d be really careful about putting an APS-C lens like that 70-350mm on your a7C. You’re gonna lose a ton of resolution (drops to about 10MP in crop mode), which really limits how much you can print or crop later on. If you're hiking in dusty or unpredictable conditions, the real safety concern is swapping lenses and exposing your sensor. I’ve found that the Tamron 28-200mm f/2.8-5.6 Di III RXD is actually the most reliable "safe" bet for travel. It covers the whole range so you aren't constantly fiddling with gear on a trail. It’s surprisingly sharp and weather-sealed well enough for most mist. If you definitely want a dedicated telephoto, look at the newer Sony FE 70-200mm f/4 Macro G OSS II. It’s lighter than the older G version (only 794g) and the build quality feels way more rugged for long-term use. I've used mine in some pretty humid spots and the sealing is top-notch. Basically, don't compromise on the full-frame glass if you want the best reliability out of that a7C sensor, it's just not worth the risk of soft files or dust issues.

Wow ok that changes things. Gonna have to rethink my approach now.

Consider the Sony 70-200mm f/4 G OSS. At 840g, it's significantly lighter than the f/2.8 GM versions while still offering excellent optical quality and full weather sealing. The constant f/4 aperture is more than adequate for most daylight hiking/travel scenarios, and the OSS helps in lower light. I've taken mine through Patagonia and Nepal with zero issues.

Basically, looking at what everyone has said, it seems like the choice is between fast f/2.8 aperture, reliable native f/4 glass, or a super-zoom that covers everything. Honestly, I am pretty new to this and still trying to wrap my head around all the specs, but I have been doing some market research on how these brands compare. I am not 100% sure but I think some of the third-party options might actually be lighter because they use different materials for the barrel or maybe simpler optical designs? I read somewhere that native Sony lenses usually handle the a7C autofocus tracking a bit better, but the weight savings on the other brands is hard to ignore for hiking. Is the build quality of those lighter lenses actually tough enough for dusty trails? It feels like the market is really shifting towards these compact setups, but I am still uncertain if sacrificing a bit of build quality for a lighter pack is the right move. Does anyone think the brand ecosystem actually matters for stuff like weather sealing, or is it all pretty much the same these days?

^ This. Also, Richardfen is spot on about that crop mode. Dropping to 10MP on a 24MP sensor like the a7C basically ruins the advantage of having a full-frame camera. I have been extremely satisfied with the Sony FE 70-200mm F4 Macro G OSS II for my hiking kits. It works well because it solves the size issue technically:

• Compactness: Its about 15% shorter than the first-gen Sony f/4 because it has an extending barrel.
• Magnification: Having 0.5x macro capability at all focal lengths is huge for trail shots.
• Autofocus: It uses four XD linear motors. In my experience, these track much better than the motors in the Tamron 70-180mm f/2.8 Di III VC VXD G2. Tamron definitely wins on the f/2.8 aperture and weight-to-price ratio, but the Sony G II is technically more versatile for travel. The weather sealing feels more robust, which is vital when you're hiking in unpredictable weather. No complaints with the f/4 max aperture for outdoor use tbh.