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Processing every video frame through Vision-Language Models (VLMs) adds significant inference cost and limits their ability to comprehend real-time video. Existing approaches either sub-sample frames uniformly, ignoring content and missing important events, or prune visual tokens inside the VLM to cut per-call cost while still invoking the model on every sampled frame. We introduce FOCUS, a training-free three-layer framework that reduces the number of invocations and steers each one towards what matters. Temporal gating measures inter-frame dissimilarity in the patch-embedding space of a smaller VLM from the same family, triggering burst captures only when the scene changes in a semantically meaningful way. Patch-level dissimilarity is then accumulated across the burst and fused with a top-down attention prior into a spatial focus map that encodes where motion and semantic relevance coincide. That map calibrates the VLM's value cache during generation, progressively amplifying vision tokens in task-relevant regions and attenuating off-target ones, without removing any token from the context. The loop closes after each response: text-to-vision cross-attention is extracted from the decoder and fed back as the prior for the next burst, letting the system converge on semantically important regions over successive observations. We evaluate FOCUS on the 500-video EgoSchema subset in streaming mode, with a single Qwen3-VL-8B backbone shared by every method. It reaches 74.8\% accuracy using 25.4 VLM calls per clip, against 74.6\% with 55 calls for uniform sampling and 74.2\% with 55 calls for BOLT. On egocentric video captioning across four domains, it matches the best baseline's quality while issuing 57\% fewer VLM invocations than uniform sampling.