The cheapest 640 thermal you can actually buy

What a 640 actually buys you, and what it does not

A 640x512 sensor has four times the pixels of a 320-class detector and about two and a half times the pixels of a 384x288. What that buys is frame width at the same detail: at a given magnification the 640 shows you more ground, so a sounder working a fence line fits in the picture instead of being found one hog at a time.

What it does not buy is reach. Detail on the animal comes from pixel pitch and lens focal length, and sensor size is not in that calculation. A 640 behind a 19 mm lens puts no more pixels across a hog than a 384 behind the same lens — it just gives you a wider window to find it in. We work the arithmetic through on our 256 vs 384 vs 640 page, and it is the single most useful thing to understand before spending up.

So the honest case for buying a 640 on a hog budget is scanning and situational awareness, not identification range. If you hunt one feeder at 80 yards, a 384 with a good lens is the better buy. If you drive pasture roads and glass fields where a sounder could be anywhere in 300 yards of ground, the wide frame earns its money.

Where the money goes instead on a cheap 640

Every unit here reaches 640 by giving something up, and the pattern is consistent across all five. Three things get cut first:

• Runtime. The two cheapest riflescopes here are rated about five hours and about four hours per 18650. A full night from last light is longer than that, so spare cells or a USB-C pack stop being optional.
• Published specs. The ATN ThOR 5 640 is the clearest case: no NETD, no battery figure, no IP rating and no recoil rating on the product page. The sensor headline is there; the numbers that tell you whether it survives a season are not.
• Features around the sensor. The Guide Orion 635 has the best thermal sensitivity on this page at 15 mK and the longest sensor warranty at ten years — and no recording, no Wi-Fi and optional mounts.

None of those is disqualifying. They are just the shape of the trade, and the reason a cheap 640 is a different purchase from a cheap 384. For what a thermal costs across the whole market, see how much a thermal scope costs.

How we picked

We took every product in our registry with a published 640-class detector, re-read each maker's spec page on October 17, 2026, and ranked by how little you give up to get the sensor: budget tier first, then published sensitivity, lens, rangefinding, runtime, durability ratings and warranty. Each is scored on our published thermal rubric — sensor and lens for identifying a hog (30%), usability at night (20%), battery and power (15%), durability (15%), warranty and support (10%) and value within its tier (10%). The rubric is on how we choose.

We do not test optics, and we do not print a number a maker has not published. Where a figure is missing from a product page, the table below says so rather than filling the gap.

The picks side by side

The picks in context

1. oneleaf.ai Mars MT1000LRF (35 mm)

The cheapest way to put a 640 on a rifle. A 640x512 at 12 µm and <20 mK behind a 35 mm F1.0 lens, a 2x base with a wide 12.5° x 10.1° view, selectable 60/50/30 Hz, a 1024x768 OLED, a built-in rangefinder good from 5.5 to 1,300 yd feeding a ballistic calculator with seven profiles, IP67, a 6000 J recoil rating and 50 mm and 75 mm lenses available later. The wide base is the part hog hunters feel: you see the whole group arrive. Against it, about five hours on a 3,200 mAh 18650 and a three-year warranty where the RIX and the ATN ThOR 6 line get five.

2. Guide Orion 635

If the rifle already wears a scope you trust and zero, a clip-on is the cheapest route to this sensor class. 640x512 at 12 µm, 15 mK — the most sensitive figure on this page — behind a fast 35 mm F0.9 lens, 1x so your existing zero and reticle carry over, a 1920x1080 AMOLED, IP67, a 6,000 J rating, 10 oz, two 18500 cells in the box rated to about 6.5 to 7 hours each, and a ten-year warranty on the thermal sensor. Guide's page lists no recording and no Wi-Fi, and Picatinny and bell mounts are accessories. Our clip-on vs thermal scope page covers whether that route suits you.

3. RIX Storm S6R 640 (35 mm)

The best-rounded 640 riflescope here, in the top budget tier. 640x512 at <20 mK, a 2x base with 1-4x digital, 50 Hz, a 1024x768 AMOLED, a 1,312 yd rangefinder with ±1.09 yd accuracy and a ballistic calculator, a 6000 J rating, a QD Picatinny mount in the box and a five-year warranty, all at about one pound. The two gaps are runtime — about four hours per 18650, the shortest here — and no published IP rating on RIX's S6R page.

4. Armasight Sidekick 640 Mini

The cheapest 640 you can wear rather than mount. 640x480 ArmaCORE at 12 µm, 60 Hz, a wide 24.16° x 17.37° view at 1x, an XGA OLED, IP67, 254 g, and a dovetail mini-rail in the box so a helmet mount is not a second purchase. Armasight is also the only maker on this page that publishes recognition (177 m) and identification (111 m) ranges next to its 709 m detection claim — read those as the sanity check on everyone else's marketing. The costs: <40 mK is the weakest sensitivity here, a 19 mm lens at 1x means all detail comes from digital zoom, and one CR123A runs about 90 minutes without the included extender.

5. ATN ThOR 5 640

Often the lowest sticker price of any 640 riflescope on the shelf, and the reason is that it is the previous generation. You get a 12 µm 640x480 at 60 fps, four lens and magnification options from 2-16x to 5-40x, dual-stream video and Recoil Activated Video. You do not get a published NETD, lens size, battery life, IP rating or recoil rating — ATN's own ThOR 6 pages publish all of those. If you are choosing between ATN scopes today, the newer line tells you more about what you are buying.

What to look for in a cheap 640

1. A published lens focal length. It sets detail on the animal. A 640 behind 19 mm is a scanner; a 640 behind 35 mm is a shooting optic.
2. A published NETD. Sub-20 mK reads humid Texas air and cedar far better than a 40 mK sensor. Two units here publish 15 to 20 mK; one publishes nothing.
3. Swappable batteries, and know the hours. Four to five hours per cell is normal at this price. Budget two spares or a USB-C pack per night.
4. A recoil rating in joules. 6000 J is the figure three units here publish. A scope with no rating on an AR is an experiment.
5. An IP rating. IP67 on the oneleaf, Guide and Armasight units. RIX and ATN publish none for these models.
6. Warranty length in writing. Ten years on Guide's sensor, five on the RIX, three on the oneleaf and Armasight. It is the cheapest insurance on a sensor you cannot repair.

Mistakes buyers make chasing a 640

• Buying the sensor and ignoring the lens. This is the expensive mistake. A 384 behind a 35 mm lens identifies better than a 640 behind 19 mm.
• Reading detection range as identification range. Armasight's own numbers put identification at roughly a sixth of detection. Our detection range page works through why.
• Forgetting the battery math. Four hours per cell and a six-hour sit do not meet.
• Stretching past a rangefinder to afford the sensor. Past 150 yards, knowing the distance changes the shot more than another 100,000 pixels does.
• Paying 640 money for a 640 you will use at 80 yards. Over one feeder, a sub-$1,000 scope does the job and leaves money for a light and a rest.