Thermal optics

• Sensor resolution is the hard ceiling. A 640×480 core resolves detail a 384×288 core simply never captured. Digital zoom on a 384 core magnifies pixels, not information. If you are identifying targets rather than just detecting heat, pay for the 640.
• NETD is the number nobody advertises. Noise Equivalent Temperature Difference, in millikelvin, is the smallest temperature difference the sensor can distinguish. Lower is better. Under about 25mK is genuinely good and shows up as usable contrast on humid nights when everything is near ambient.
• Refresh rate: 50Hz or don't bother for hunting. 30Hz smears badly when you pan or when the target moves. For static observation it's fine. For anything you'd shoot at, 50Hz is worth the premium.
• Detection ≠ recognition ≠ identification. Manufacturers quote detection — the range at which a heat blob appears. Recognition (it's an animal) is roughly a third of that. Identification (it's a hog, not a calf) is roughly a fifth. Do that arithmetic before you believe a 2,000-yard claim.
• Germanium optics drive the price. The lens is germanium, not glass, and it is a large part of the cost. A bigger objective on the same sensor buys real range, which is why an XP50 costs what it does versus the same core behind a smaller lens.
• Check your state's night-hunting law first. Thermal legality for hunting varies enormously by state, species and public vs private land. Verify before you buy, not after.

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