I've been using this lens extensively without any sharpness issues. At long focal lengths, you'll have to factor in the need for a faster shutter speed (< 1/500-ish at 350mm) and other factors like atmospheric distortion, fog/dust haze, etc. All these factors contribute to a deterioration of image quality at longer focal lengths.

That's supposed to be a pretty good APS-C lens. Can you try it on a different camera just for the heck of it? Friend? Camera shop? The lens is noted for sharpness, so if you're having as much trouble as you say, you may want to look into a replacement or repair. 

Hi everyone, I’m reaching out to the community because I’m facing a persistent image quality issue with my Sony 70–350mm f/4.5–6.3 G OSS lens, and I’d like to know if this is normal behavior or if my copy is defective. Problem description: I’ve extensively compared the 70–350mm G OSS with my Sony 18–135mm f/3.5–5.6 OSS, using a Sony A6700, under controlled conditions: • Identical lighting and background • Same subject and position (LEGO figure, consistent framing) • Tripod or steady support • Manual focus or AF with center point • Same shutter speed (e.g., 1/200s), similar ISO (ISO 4000–6400), RAW + JPEG • OIS turned on (and also tested with OIS off) My observations: • At 135mm, the 70–350mm G OSS delivers softer, flatter images than the 18–135mm, even when stopped down. • At 350mm, the sharpness drops significantly – the center is soft, and textures (like LEGO tiles or fabric) appear blurred or smudged. • Contrast and micro-detail are noticeably inferior across all focal lengths. • The 18–135mm at 135mm (even cropped) retains better edge sharpness and detail definition. • Both JPEG and RAW files confirm the issue – this is not just JPEG processing or noise reduction. Question to the community: • Have others experienced similar softness with the 70–350mm? • Is it possible I have a decentered or optically misaligned copy? • Is there a known issue with OSS introducing softness at long focal lengths? I wanted to love this lens due to the range and portability, but currently it’s unusable for anything where image quality matters. I’m considering returning or sending it for service. Thanks in advance for any feedback or comparison results you can share.