Thermal scope lens size is the specification buyers compare most and understand least. The question arrives as a list — 25, 35, 50 or 75 mm — with an assumption attached: bigger must be better. A longer objective does reach farther, and it pays by showing you a narrower slice of the world. What complicates the choice is that the millimetre figure does not set your field of view on its own. Two scopes in our current line carry the same 35 mm objective and publish fields of view of 12.52° and 7.53°, because the detector behind the lens differs in size. Here is the engineering behind that, with the numbers published for the ATN ThOR 6 640x512 | 2-16x.
Focal length and detector size together decide your field of view. Longer lens, narrower view and more reach; larger detector behind the same lens, wider view. Digital zoom changes neither.
What thermal scope lens size actually means
It means the focal length of the germanium objective in millimetres, printed on the spec sheet as a line like «35 mm (Ge); F/1.0». Thermal focal length is not the tube diameter and not the magnification; it is the distance from the lens to the point where it focuses infrared rays, and that distance decides how much of the scene lands on the detector. One convenience follows from the F-number: at F/1.0 the clear aperture equals the focal length, so one figure gives both.
How focal length sets field of view, step by step
Field of view is geometry between two fixed quantities: the size of the detector and the focal length in front of it.
- The detector is a rectangle of fixed physical size. These scopes use a 12µm VOx uncooled focal plane array, so pixel count sets physical width: a 640×512 array is wider than a 384×288 one at the same pitch.
- The lens projects the scene onto that rectangle. Think of the array as a window frame that cannot be resized: the objective decides how much of the landscape is painted inside it.
- The angle follows from the two. Horizontal field of view equals twice the arctangent of half the detector width divided by the focal length. Nothing else enters it.
- So the two inputs pull opposite ways. Longer focal length narrows the angle and spreads more pixels across a distant animal; a larger array behind an unchanged lens widens it again.
- ATN publishes the result, not the arithmetic. Every scope card states field of view in horizontal by vertical degrees.
What thermal scope field of view does for you
It decides how quickly you find things and how easily you keep them. A wide angle sweeps a tree line in a few passes instead of a dozen, keeps an animal in the picture when it moves across you rather than away, and gets you back on target faster after recoil. A narrow angle does the opposite in exchange for resolution at distance: the same animal covers more detector pixels, so its outline holds together farther out. One is a searching advantage, the other a shooting advantage.
25, 35 and 50 mm side by side
Read the table in pairs rather than top to bottom. All four scopes are rated ≤15mK, so sensitivity is not the variable here.
| ATN ThOR 6 scope | Lens | Field of view (H×V) | Magnification | Detection range |
|---|---|---|---|---|
| 384×288 | 2.5-20x (TIWST6325A) | 25 mm (Ge), F/1.0 | 10.53° × 7.91° | 2.5-20× | 2300 m |
| 384×288 | 3.5-28x (TIWST6335A) | 35 mm (Ge), F/1.0 | 7.53° × 5.65° | 3.5-28× | 2750 m |
| 640×512 | 2-16x (TIWST6635A) | 35 mm (Ge), F/1.0 | 12.52° × 9.41° | 2-16× | 3100 m |
| 640×512 | 3-24x (TIWST6650A) | 50 mm (Ge), F/1.0 | 8.78° × 6.59° | 3-24× | 3650 m |
Rows one and two hold the detector constant: 25 mm to 35 mm costs roughly a third of the horizontal field and returns 450 m of rated detection. Rows three and four repeat that on the larger detector — the 35 mm vs 50 mm thermal comparison people ask about most — and the proportions hold. Now read rows two and three: identical 35 mm objectives, fields of view of 7.53° and 12.52°.
The 75 mm question: what a long objective buys and costs
Seventy-five millimetres is not a model but a class — the long-focal end of thermal optics. No ThOR 6 riflescope in the table above wears an objective that long; 50 mm is the longest there. The physics of the class is that table extended: going longer narrows the angle and puts more pixels on a distant heat signature, which is what a thermal lens for long range is for. It also lengthens the instrument, adds germanium and weight past the mount, and raises the price faster than the millimetres rise.
What it does not do is remove the other limits. Detection distance is capped by detector sensitivity and by the air in between, not by the objective alone; humidity and fog absorb in the long-wave band regardless of aperture. A very long lens also leaves a view narrow enough that finding the target becomes the slow part of the night. Anyone working at that range is better served by the longest objective in the line plus a separate wide-angle scanner.
Terms worth getting straight
- Focal length. The millimetre figure on the lens line, and the only term here that describes optics rather than processing.
- Field of view. Published as horizontal by vertical degrees — 12.52° × 9.41° on the ThOR 6 640 2-16x. Degrees compare between any two optics; «wide» does not.
- Optical magnification. The figure in a range like 2-16×. It depends on focal length and detector size together, not on focal length alone.
- Digital zoom. On the ThOR 6 640 2-16x it runs 1×, 2×, 4×, 8×. It crops the detector image and enlarges it, the way enlarging part of a photograph does — useful for placing a reticle, never a way to widen the view.
- Detector size and pixel pitch. 12µm across these four scopes, so pixel count is also physical size — the second half of the field-of-view equation.
- Detection range. Where a standard target still registers as a heat source. It rises with focal length, but it is not a recognition figure — see thermal scope detection range.
What to look for before you choose
Start from your longest realistic shot, not your longest imaginable one. Pick the shortest lens that covers that distance comfortably, because every millimetre past it is paid for in field of view you will miss on every other shot of the night. Then confirm the degrees on the exact SKU rather than the family — two scopes wearing the same lens can differ by five degrees. Which body carries that lens is a separate question — see our best 640 thermal scopes roundup.
Common mistakes with thermal scope lens size
The first is treating millimetres as magnification; the table holds the counterexample, where the 25 mm scope starts at 2.5× and the 35 mm 640 at 2×. The second is expecting digital zoom to rescue a lens that is too narrow — zoom crops what the objective already collected, so a narrow field simply gets larger. The third is buying the longest objective available for hunting that happens inside 150 yards, which spends real money on reach and removes the searching speed that matters there.
The lens on the ATN ThOR 6 640x512 | 2-16x
The ThOR 6 640x512 | 2-16x (SKU TIWST6635A) is the balanced point of the four. Its objective is 35 mm of germanium at F/1.0 with a manual focus ring, feeding a 12µm VOx uncooled focal plane array of 640×512 pixels. That pairing yields a 12.52° × 9.41° field of view — the widest in the table — with 2-16× magnification, digital zoom to 8× and a rated detection range of 3100 m. The combination suits mixed ground: enough angle to find animals quickly, enough reach that the lens is rarely what holds you back.
Is a longer objective worth the money?
It depends entirely on whether you will use the reach. Moving from the 35 mm ThOR 6 640 2-16x to the 50 mm 3-24x takes the promotional price from $2,295 to $2,595 — list prices are $3,625 and $4,175 — and buys 550 m of rated detection for roughly a third of the horizontal field. Where animals are first seen several hundred yards out, that is a sound trade. Where most of the night happens inside 200 yards, you have paid for a narrower picture.
Thermal scope lens size: common questions
Does a bigger thermal lens always mean a better scope?
No — it means more reach and less field of view. On the ThOR 6 640x512 | 2-16x (TIWST6635A) the 35 mm objective gives 12.52° × 9.41° and 3100 m of rated detection; the 50 mm 3-24x (TIWST6650A) gives 8.78° × 6.59° and 3650 m. Which is better depends on your shooting distance.
What is the best thermal lens for hogs?
Usually the shorter objectives, because sounders are found by sweeping ground at night and are often engaged close. A 25 mm or 35 mm lens gives the wider angle that makes sweeping fast. Take 50 mm only if your shots come several hundred yards out.
Is thermal focal length the same as magnification?
No. Focal length is a physical property of the lens in millimetres; magnification is how large the image is drawn for your eye, which depends on the detector as well. The 384×288 2.5-20x on a 25 mm lens starts at 2.5×, the 640×512 2-16x on a 35 mm lens at 2×.
Can digital zoom make up for a narrow field of view?
No. Digital zoom crops the detector image and enlarges it, so it always makes the view narrower, never wider. It helps you place a reticle on a target you have already found, and adds no detail the objective did not collect.
Why do two scopes with the same 35 mm lens have different fields of view?
Because the detectors differ in physical size. At 12µm pixel pitch a 640×512 array is larger than a 384×288 one, so the same lens covers more of the scene on it: 12.52° horizontally on the 640×512 2-16x against 7.53° on the 384×288 3.5-28x.
Does a longer lens see through fog better?
No. Water vapour absorbs in the long-wave infrared band along the whole path, and no focal length removes that. A longer objective puts more pixels on whatever signal survives, but detector sensitivity and the air in between set the ceiling.
Choosing your lens size
Pick the lens from your ground, not from the largest number on the spec sheet. Inside timber and over feeders, take the widest angle you can get; across open country, accept the narrower picture and take the reach. Read the lens line and the detector line together, because neither means much alone. For the optical material behind these numbers, see our explainer on germanium thermal lenses, or compare the ATN thermal scopes line, which lists field of view on every model.
Quick spec reference
- Featured optic: ATN ThOR 6 640x512 | 2-16x, SKU TIWST6635A
- Objective: 35 mm germanium (Ge), F/1.0, manual focus; 12.52° × 9.41°; 3100 m
- Detector: 12µm VOx uncooled focal plane array, 640×512, 50 Hz, ≤15mK NETD
- Longer lens: ThOR 6 640×512 | 3-24x (TIWST6650A) — 50 mm; 8.78° × 6.59°; 3650 m
- Shorter lens: ThOR 6 384×288 | 2.5-20x (TIWST6325A) — 25 mm; 10.53° × 7.91°; 2300 m
