Ah, it's largely irrelevant if your objective is larger than the thermal's ocular end as you're only staring at a tiny screen within the ocular end anyway. Also, as you'd typically be using it at night, you shouldn't have to worry about ambient bleed through since there won't really be any ambient light anyway. It's more of an OCD thing, you could just wrap something around difference between the two if you really wanted to.
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