Thermal Lens Size Explained: Why 19mm, 25mm, 35mm, 50mm and 75mm Lenses Completely Change What You See

Thermal Academy · Core Curriculum

Two mates buy their first thermal scopes the same week. One reads that “more magnification is better” and buys a big 50mm. The other likes the idea of a compact unit and buys a 19mm. A fortnight later they compare notes, and both are quietly frustrated.

The 50mm owner can’t find anything. Sweeping a paddock for rabbits, his view is so tight and zoomed that he’s staring down a soda straw — by the time a rabbit drifts into frame it’s already gone, and scanning gives him a headache. Meanwhile the 19mm owner can see the whole paddock at a glance and finds animals instantly — but when a fox stops at 200 metres, it’s a warm smudge he can’t confidently call as a fox rather than a cat or a small roo.

Here’s the thing: neither of them bought a bad optic. They bought the wrong optic for their use. The 50mm is a superb long-range identification tool and hopeless for close scanning; the 19mm is a brilliant scanner and hopeless for long-range ID. Same money, opposite problems — and the whole difference is the lens on the front.

Lens size is one of the most consequential choices you’ll make when buying a thermal, and one of the most misunderstood. This guide explains what those numbers — 19, 25, 35, 50, 75 — actually mean, what they change, what they don’t, and how to pick the one that fits the way you hunt. Above all, one idea runs through everything that follows.

Get that one distinction straight and every confusing spec-sheet decision suddenly makes sense. A bigger lens does not give you a “better” picture. It gives you a different view: more magnification and reach, traded against a narrower field of view and slower target-finding. Which trade is right depends entirely on your country, your quarry and your style — and by the end of this you’ll know exactly where you sit.

What Does Lens Size Mean?

First, a clarification that trips up almost everyone. When a thermal scope is described as “35mm,” that number is the focal length of the objective lens — not its physical diameter, and not a measure of how “big” the unit is. Focal length is, roughly, the distance the lens needs to focus incoming infrared to a sharp point on the sensor, and it’s the single number that sets how magnified and how wide your view will be.

If you’ve used a normal camera, you already have the intuition. A 24mm camera lens is a wide-angle — it takes in a whole landscape. A 200mm telephoto pulls a distant bird in close but sees only a tiny slice of the scene. Nobody argues about which is “better,” because it’s obviously the wrong question: they’re different tools for different shots. Thermal focal lengths work exactly the same way. A 19mm thermal is your wide-angle; a 75mm is your telephoto; 25, 35 and 50mm are the steps in between.

How Thermal Lenses Work

Thermal lenses have to do something an ordinary camera lens never does: focus heat. The long-wave infrared your target emits behaves like light in some ways — it can be focused by a lens — but ordinary optical glass is completely opaque to it. Point a glass lens at a warm animal and no infrared reaches the sensor; it’s like trying to see through a brick.

So thermal objectives are made from germanium, a silvery, metallic-looking element that is transparent to long-wave infrared the way glass is transparent to visible light. Germanium is expensive and heavy, which matters: a bigger, longer lens needs more of it, so longer focal lengths make an optic pricier and front-heavy. That’s a real-world consequence of lens size that has nothing to do with the image and everything to do with what you carry.

Germanium focuses the target’s infrared onto the sensor, just as a camera lens focuses light onto film. Ordinary glass blocks infrared entirely — which is why thermal optics can’t use it, and why they cost what they do.

Why Bigger Lenses Zoom More

Here’s the part that makes lens size feel like magic, without any maths. A longer focal length spreads the incoming scene across the sensor more, so any given object lands on more pixels — and more pixels on the target means it appears bigger and more magnified. Think of it as the lens projecting a larger picture onto the same small sensor: you’re seeing a more enlarged slice of the world.

Double the focal length and, very roughly, you double the base magnification. A 50mm shows a target about twice the size a 25mm does; a 75mm bigger again. This built-in magnification — before any digital zoom — is called base magnification, and it’s the headline benefit people chase when they “buy the big lens.” As a rough feel, base magnification climbs with focal length: a 25mm might sit near 1.5–2×, a 35mm around 2.5×, a 50mm around 3.5×, a 75mm higher again (the exact figure depends on the sensor and display too). It’s genuinely useful: more magnification means more detail on a distant animal, which is what lets you identify it.

The same animal, same sensor. The longer lens magnifies it so it covers more pixels — which is exactly what you need to identify a target at distance. But, as the next section shows, that magnification comes at a price.

Why Bigger Lenses Show Less

Every bit of magnification you gain from a longer lens is paid for with something you lose: field of view — how wide a slice of the world you can see at once. It’s the flip side of the same coin, and it’s the single most misunderstood idea in thermal buying.

Because a longer lens enlarges the scene onto the same sensor, it necessarily shows you a narrower piece of it. On a 640×512 sensor, a 19mm lens takes in about a 23° span — a whole paddock at a glance. A 50mm sees roughly 9°, and a 75mm barely 6° — a keyhole. That narrow view is wonderful for studying a distant animal and miserable for finding one, because there’s so little country in frame that targets slip past unseen and every heartbeat of your own movement throws the picture around.

All five lenses on the same 640×512 sensor, drawn from one eye. The 19mm floods you with situational awareness; the 75mm is a narrow, high-magnification keyhole. Neither is “more” — they’re opposite tools. (Field of view is a big enough topic that it gets its own Thermal Academy guide.)

What the Field of View Actually Feels Like

Degrees are abstract, so here’s what they mean where it counts — on the ground, at a real distance. Field of view fans out with range, so the further you look the more country each lens takes in. At 100 metres on a 640 sensor, the gap between focal lengths is stark:

Read that middle column as your scanning window. With a 19mm you can watch a 40-metre-wide slice of country at 100m and catch movement almost anywhere in it; with a 75mm you’re peering through a 10-metre letterbox, and a fox can be standing just outside it, completely unseen. It’s also exactly why long lenses feel “jumpy” — at 100m a 75mm shows only 10 metres of ground, so a few degrees of hand-wobble swings you metres across the paddock. This is the practical heart of the whole trade-off: not an abstraction, but how much world you can actually keep an eye on at once.

Comparing Every Common Lens

With the trade-off understood, here’s what each common focal length is genuinely good — and bad — at. There’s no ranking here, only fit.

19mm — the scanner

Excellent for: scanning wide country fast, thick bush and dense vegetation, close-in pest control, use from a moving vehicle, and anywhere situational awareness beats reach. Its wide field of view means targets are easy to pick up and easy to keep in frame, and it’s forgiving of a shaky hand. Poor for: identifying or engaging anything at distance — there simply aren’t enough pixels on a far-off animal to call it confidently. In the Pixfra range this is the compact Arc 19mm — a pocketable scanner.

25mm — the all-rounder

A genuine jack-of-all-trades. It keeps most of the 19mm’s easy, wide scanning while adding useful magnification, which makes it a superb first monocular and a great “walk-around” optic for mixed close-to-medium country. If you’re unsure and you scan more than you shoot, 25mm is hard to regret. Pixfra offers it in the Arc and Sirius lines (and, cleverly, paired with a 50mm in the dual-lens Sirius S650D). Think of it as the 35mm’s more scan-friendly sibling: you give up a little reach for a noticeably easier, wider picture — a happy trade if most of your work is finding animals rather than reaching out to them.

35mm — Australia’s sweet spot

For a very large share of Australian hunters, 35mm is the answer. It strikes the best all-round balance we have: enough base magnification to identify foxes, pigs and rabbits out to sensible, ethical ranges, while keeping a field of view (around 12.5° on a 640 sensor) wide enough to still scan comfortably and find game without a tripod. It’s the lens that does 80% of jobs well and none of them badly — which is exactly why it’s the most popular choice and shows up right across the range, from the Arc and Sirius monoculars to the Pegasus 2 riflescope. If you want one lens and hunt varied country, start here — you’ll spend far more nights glad of the balance than wishing for the extremes.

50mm — the reach

A long-range specialist. The 50mm delivers the magnification to identify and engage animals well out into open paddocks, and it’s the natural choice where your shots are long and your country is wide. The trade is a noticeably narrower field of view (around 9°), which means scanning is slower, targets are harder to pick up quickly, and it demands better technique — a steadier hold, ideally a rest, and more patience. Reward for that discipline is genuine reach. Pixfra fields it in the Sirius and Pegasus 2 lines. One practical tip: pair a 50mm with a habit of resting the optic — a fence post, shooting sticks, a pack — and its “hard to hold” reputation largely disappears, leaving just the reach.

75mm — the specialist

The telephoto end. A 75mm is for open-country, long-range observation and identification — think spotting and studying animals at extreme distance, often from a fixed position. Its field of view (around 6°) is a keyhole, so it’s genuinely difficult to use handheld; every tremor is magnified, which is why long lenses “feel shaky.” It rewards a tripod or solid rest and a specific, distance-focused job. It sits at the specialist end of the market beyond Pixfra’s current lineup, and for most hunters it’s more lens than the country calls for.

Field-of-view figures are for a 640×512, 12µm sensor; magnification is relative, not absolute (it also depends on sensor size and display). The pattern is what matters: reach up, awareness down, as the lens grows.

Same Sensor, Different Lens

The image at the top of this guide is the whole idea in one picture. It’s a single 640×512 sensor and a single animal — the only thing that changes across the five frames is the lens. At 19mm the animal is a small figure in a wide, context-rich scene; at 75mm it fills the frame while everything around it is cropped away. Same sensor, same target, five completely different experiences.

This is worth sitting with, because it’s the opposite of how most people picture “buying a better thermal.” You are not making the picture sharper or cleaner by going up in lens size — the sensor and the processing behind it are unchanged. You are choosing how the world is presented to you: floodlit and wide, or narrow and magnified. The lens is the framing, not the quality.

That reframe is worth holding onto when you shop, because it flips the usual question. Instead of “which lens gives the best image?” — a question with no answer, since they all give the same image quality — you start asking “which framing suits the way I hunt?” That question does have an answer, and it’s a much easier one to get right. The rest of this guide is really just about helping you answer it.

Lens Size vs Digital Zoom

“Why buy a bigger lens when the scope has 8× zoom?” — a completely fair question, and the answer is one of the most useful things in this guide. Lens magnification and digital zoom are not the same thing, and confusing them costs people money.

A longer lens is optical magnification: it puts the target onto more sensor pixels, so there is genuinely more detail to see. Digital zoom does the opposite — it takes the pixels the sensor already captured, crops into them and enlarges them on the display. No new detail is created; you’re just making the existing (and increasingly blocky) pixels bigger. Digital zoom is handy for a quick closer look, but it can never replace the real detail a longer lens delivers.

Optical (lens) magnification lands the target on more real pixels; digital zoom just enlarges the pixels you already had, so it turns blocky. This is why a 50mm optic beats a 35mm digitally zoomed to “look” the same. (A fuller optical-vs-digital-zoom guide is coming.)

Lens Size vs Resolution

Lens size never acts alone — it works with the sensor’s resolution, and you have to think about the two together. Magnification decides how big the target appears; resolution decides how much detail there is to fill that magnified view. A long lens on a low-resolution sensor magnifies a coarse image into a bigger coarse image; a short lens on a high-resolution sensor packs detail into a view that may be too wide to use it at range.

Sensor and lens are a team. The magnification a lens provides is only as useful as the resolution and detail there is to fill it — which is why matching the two to your range matters more than maxing either one.

Lens Size vs Pixel Pitch

Here’s a subtlety that catches out even experienced buyers: the same focal length can give a different field of view on two different sensors, because field of view depends on the sensor’s physical size — and that is set by its resolution multiplied by its pixel pitch.

Field of view is, in plain terms, the sensor’s physical width divided by the focal length. A 640×512 sensor at 12µm is 7.68mm wide; the same resolution at 17µm is larger, at 10.88mm. Put an identical 35mm lens on each and the larger 17µm sensor sees a wider view, while the smaller 12µm sensor sees a narrower, more magnified one. So when you compare two “35mm” optics, check the pitch too — because 35mm on a 12µm sensor is not the same view as 35mm on a 17µm one. It’s one more reason the number on the front never tells the whole story on its own.

Lens Size vs NETD

It’s worth stating plainly: a bigger lens does nothing for sensitivity. NETD — the sensor’s ability to detect tiny temperature differences — is a property of the sensor and its electronics, not the optic bolted in front. A long lens magnifies a low-contrast smudge into a bigger low-contrast smudge; it can’t pull a target out of the thermal noise that a sensitive sensor and good processing would.

This matters because it’s tempting to think “more lens = more capability.” Reach is only one ingredient. If the sensor’s sensitivity is poor or the image processing is weak, no amount of focal length rescues the picture. The lens sets the framing and the magnification; the sensor and the software still decide how clean and readable what’s inside that frame actually is. Lens size is a powerful lever — but it is not the whole machine.

Detection, Recognition and Identification

The clearest way to understand what a longer lens buys you is through Detection, Recognition and Identification (DRI) — the three jobs a thermal does at range. Detecting that something warm is out there needs very few pixels on the target, so even a wide 19mm can detect a long way off. But recognising it as an animal, and then identifying it as a fox rather than a cat, each demand far more pixels across the target — and that is exactly what a longer lens delivers.

Longer lens → the target lands on more pixels → more detail across it → you can recognise and identify it further out. That chain is the real, practical reason to reach for more focal length: not to “see better,” but to put enough pixels on a distant animal to make a confident, ethical call before you act.

Conceptual only — not a performance claim. Real identification distance depends on target size, sensor resolution, NETD, atmospheric conditions and technique as much as focal length. The point is the direction: more lens generally means confident ID further out, at the cost of field of view.

Common Buying Mistakes

Almost every lens-size regret comes from one of a handful of predictable mistakes. Here they are, with what to do instead.

The thread running through all five is the same: people buy a number instead of buying for a use. “Biggest,” “most zoom,” “one for everything” are all attempts to avoid the real question — which is simply what you point the thing at, and how far away it usually is.

Choosing the Right Lens

So make it about the use. Ask one question — what do you do most? — and the answer falls out quickly.

Start from your dominant use, not from a spec. For most Australian hunters working mixed country the honest answer is 35mm; the edges of the tree are for people who genuinely live at one end of the trade-off.

Lens Size Myths

And the biggest one of all — “one lens suits every hunter” — is simply impossible. Because lens size is a trade-off between reach and field of view, there is no focal length that is best at everything. The 35mm is the best compromise, but a compromise is exactly what it is: it gives up some of the 19mm’s scanning ease and some of the 50mm’s reach to be good at both. Anyone selling you a “does-it-all” lens is selling you a compromise and calling it a champion.

Buying Advice

No product picks here — just a way to think about it, built entirely around how you actually spend your nights. Weight your choice to your most common situation, not the rare edge case you might face once a season.

If you scan more than you shoot, in mixed or close country — bush, scrub, pest control near buildings, off the back of a ute — lean short: 25mm, or 35mm if you also want to reach out sometimes. Finding game fast and comfortably will matter more to you than the last hundred metres of reach.

If you shoot more than you scan, in open paddocks with longer average distances, lean long: 35mm as a minimum, 50mm if your shots are genuinely out there. You’ll accept slower scanning in exchange for the magnification to identify and take ethical shots at range.

If you genuinely do both, don’t force one lens to be two. Either choose a dedicated wide monocular for scanning alongside a longer riflescope for shooting, or look at a dual-lens optic that switches between a wide and a long focal length. That’s the honest answer to “I need everything” — not a single compromise lens.

If you’re unsure, or you want one optic for varied Australian country, 35mm. It is the safest single choice precisely because it refuses to be terrible at anything — and for most people, most nights, that’s worth more than being brilliant at one thing and useless at the rest.

There Is No “Best” Lens — Only the Right One

A thermal lens doesn’t simply make things look closer. It fundamentally changes how you experience the scene — how much you can see, how quickly you find a target, and how confidently you identify it. There is no universally “best” focal length; there is only the focal length that best matches your environment, your target species and your style of hunting. Choosing the right lens is about balancing field of view, magnification and real-world usability, not chasing the biggest number.

The two mates from the start of this guide didn’t make a quality mistake — they made a fit mistake. Get the fit right and a modest lens will delight you every night out; get it wrong and the most expensive optic in the shop will sit in a drawer. So don’t ask which lens is best. Ask which lens is best for the way you hunt — and let that answer, not a number, decide.

Keep going: 12µm vs 17µm Pixel Pitch and 384 vs 640 Sensors explain the sensor side of the equation, Thermal Image Processing Explained covers why the picture inside your chosen frame looks the way it does, and Detection vs Recognition vs Identification and What Is NETD? round out how far and how clearly you’ll really see.

Frequently Asked Questions

Going deeper: for the full breakdown of what 256, 384, 640, 1024 and 1280 sensors actually change — and why the number alone never tells the whole story — read Thermal Resolution Explained.