In the world of monoculars and scopes, the size of the front lens is one of the most visible specs. We often see models that differ only in this number: WILD 35mm vs WILD 50mm, for example. What really changes, and which should you choose?
What is the purpose of the objective diameter?
In traditional daytime optics, a larger lens helps capture more light. In thermal imaging, it doesn’t work that way; the lens, depending on the sensor and microbolometer it uses, provides optical magnification and the FOV.
The objective diameter directly affects three critical factors:
- Native magnification: The larger the lens diameter (while keeping the same sensor), the device will have more base optical magnification. A 50mm model will always "zoom in" more than a 35mm one.
- Field of view (FOV): A larger lens reduces the panoramic field of view. You’ll see farther, but with less width of terrain.
- Light aperture (F value): It’s not just the diameter that matters, but its transmission capacity. Always look for lenses with an aperture F1.0 or lower, which allow all thermal radiation to reach the sensor cleanly.
Practical comparison: 25mm, 35mm, or 50mm?
| Feature | 25mm lens | 35mm lens | 50mm lens |
|---|---|---|---|
| Base Magnification | Lower (Ideal for short distances) | Medium (Ideal for medium distances and a more versatile lens on a 384 sensor) | Higher (Ideal for long distances) |
| Field of View | Wide and panoramic | Balanced and panoramic | Narrower (Tunnel effect at short distances) |
| Weight and Size | Light, compact, and ergonomic | Light, compact, and ergonomic | Heavier and bulkier |
Don’t think the 50mm is "better" just because it’s bigger and more expensive. If you hunt in dirty environments, woods, or do stand hunting at less than 80 meters, a 35mm or even 25mm unit will be much more comfortable for spotting animals thanks to its wide field of view. Reserve the 50mm exclusively for high-mountain stalking or if you need to identify precisely at more than 200 meters.
Still unsure which diameter to choose? Explore the detailed specs in our catalog of Thermal Monoculars by Lens Diameter.