Choosing the Right Thermal Lens & Sensor for Roof-Mounted Spotting
- by Ben Van Der Veen
Choosing the Right Thermal Lens & Sensor for Roof-Mounted Spotting
If you’re mounting a thermal monocular on the roof of your ute to spot animals while driving paddocks or tracks, field of view (FOV) is everything. But it’s not just about wide or narrow – it comes down to the right combo of lens size and sensor resolution. Get it wrong, and you’ll either see too wide and not far enough – or too far but miss everything off to the sides.
Understanding Lens and Sensor Basics
- Lens size (25mm / 35mm / 50mm) affects zoom and field of view – the higher the number, the more "zoomed in" it is, and the narrower the image.
- Sensor resolution (384 vs 640) affects detail – 640 gives you more pixels across the same view, so you get better clarity at distance.
💡 FOV in Degrees vs Metres at 100m:
Think of degrees as the angle of your view, and metres as the real-world width of what you're seeing 100 metres away. A wider angle means more terrain in your image, but less detail on each target.
But here’s another thing most people don’t think about: environmental conditions and lens size both affect image quality.
- Smaller lenses like 25mm don’t let in as much thermal energy. That means the image will be softer, especially in cooler or low-contrast conditions (like after rain or when the ground temp is similar to the animals).
- Fog, drizzle, wind, and cold nights can all reduce contrast – so a bigger lens helps "pull in" more heat and detail, especially at longer ranges.
Common Roof-Mounted Thermal Combos – Pros, Cons & FOV at 100m
🔸 25mm lens with 384 sensor
- FOV: ~14.9°
- FOV @ 100m: ~26.5m wide
- Wide view – good for spotting animals nearby (within 100m).
- Limited detail – struggles in cold, fog, or rain.
- Best for: tight bush blocks, slow driving through scrub.
🔸 25mm lens with 640 sensor
- FOV: ~25°
- FOV @ 100m: ~44.6m wide
- Ultra wide – but stretched pixels mean poor clarity.
- Basically useless past 50m – great view, not much detail.
- Best for: navigation or very close surveillance – not hunting.
🔸 35mm lens with 384 sensor
- FOV: ~10.7°
- FOV @ 100m: ~18.8m wide
- Better reach, modest FOV. Struggles in poor conditions.
- Best for: medium-range scanning along tree lines or paddocks.
🔸 35mm lens with 640 sensor
- FOV: ~17.5°
- FOV @ 100m: ~30.9m wide
- Ideal balance – wide enough for scanning, enough resolution for ID.
- Works well even in colder or foggy nights.
- Best for: most roof-mounted setups – a real all-rounder.
🔸 50mm lens with 384 sensor
- FOV: ~7.5°
- FOV @ 100m: ~13.1m wide
- Tight FOV – not great for fast scanning.
- Good contrast and reach, but hard to keep animals in frame.
- Best for: slow or stationary scanning in wide-open ground.
🔸 50mm lens with 640 sensor
- FOV: ~12.5°
- FOV @ 100m: ~21.9m wide
- Razor-sharp image – long-range detection and ID.
- Narrow view makes it less ideal for sweeping large areas quickly.
- Best for: spotting across big paddocks when you know where to look.
Ben’s Take
If you're mounting thermal on a rig, you need something that can see wide enough to cover country, but still sharp enough to ID targets before they're gone.
The 35mm / 640 combo is my go-to – 30+ metres of view at 100m, and plenty of detail to spot foxes, roos, or pigs out further. It holds up in cold nights, fog, and drizzle – things that kill image contrast on cheaper or smaller gear.
Avoid the 25mm / 640 – sounds fancy, but you’ll see a huge patch of landscape and no bloody animals unless they’re licking the bumper.
Need Help Choosing?
If you’re not sure which combo suits your terrain or target species, give me a ring. I’ve field-tested these in the real world – wet nights, cold mornings, open paddocks – and I’ll help you pick gear that works the first time, not the third.
