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Thermal imaging specifications often list focal length as a simple number: 9.7 mm, 25 mm, 35 mm or 50 mm.
It is easy to read those numbers as a basic ranking. A 50 mm lens sounds more powerful than a 25 mm lens, while a 9.7 mm lens may look like the entry-level option.
That is not how thermal optics work.
Thermal lens focal length mainly determines how much of the scene is projected onto the detector and how much angular detail each detector pixel covers.
A shorter lens usually gives a wider field of view. A longer lens usually concentrates the detector on a narrower angular area and can place more pixels across a distant target.
But that comparison is only straightforward when the detector size remains the same.
Focal length is an optical property of the objective lens, usually expressed in millimeters.
In a thermal imaging system, the objective lens collects infrared radiation from the scene and projects that scene onto the thermal detector.
Changing focal length changes the angular area of the scene that reaches the detector.
With the same detector:
Neither configuration is automatically better. They solve different observation problems.
Field of view, or FOV, describes how much of the scene the detector can see at one time.
For a simplified optical system, horizontal field of view can be approximated from detector width and lens focal length:
The important point is not the formula itself. It is the relationship inside it.
If detector width stays the same and focal length increases, field of view decreases.
Teledyne FLIR provides a useful real-world example with its 640×480 A615 thermal camera. Using the same detector, a 6.5 mm lens provides an approximately 80° horizontal field of view, a 24.6 mm lens provides about 25°, and an 88.9 mm lens narrows that to roughly 7°.
The detector did not change. The lens did.
The full specifications are available on the
FLIR A615 thermal camera
technical page.
The LDTI006 uses a relatively short 9.7 mm thermal objective.
Its published thermal specifications include:
| Specification | LDTI006 |
| Detector resolution | 320×240 |
| Pixel pitch | 12 μm |
| Focal length | 9.7 mm |
| Field of view | 18.1° × 13.6° |
| NETD | <40 mK at 25°C |
| Refresh rate | 50 Hz |
The 18.1° horizontal field of view gives the user considerably more environmental coverage than a narrow-field long-focal-length thermal optic.
That can be useful when scanning an area, locating a heat source quickly, or observing at relatively short and medium distances where situational coverage matters.
A wider field also makes it easier to keep a moving object inside the image because more of the surrounding scene remains visible.
The tradeoff is that the same target occupies fewer detector pixels than it would with a longer focal length under otherwise comparable conditions.
Our
LDTI006 Thermal Imaging Guide
covers the detector resolution, NETD, refresh rate and laser ranging functions in more detail.
The current complete product specifications are also available on the
LDTI006 50 Hz Thermal Monocular with Laser Rangefinder
page.
A longer focal length projects a smaller angular area of the scene across the detector.
If detector resolution and pixel pitch remain unchanged, this means each detector pixel corresponds to a smaller angle in the real world.
The result is useful at distance because the same target can cover more detector pixels.
A simplified way to express this is through instantaneous field of view, or IFOV.
Smaller IFOV means each detector pixel covers a smaller angular portion of the scene.
With the same 12 μm pixel pitch, increasing focal length from 25 mm to 50 mm roughly halves the angular area represented by each pixel in one dimension.
That is one reason longer thermal lenses are commonly associated with improved detail on distant targets.
The LDTI008 series is particularly useful for understanding why focal length should never be compared without looking at the detector.
The current series uses three different combinations:
| Model | Detector | Pixel Pitch | Lens | FOV |
| LDTI008-225LRF | 256×192 | 12 μm | 25 mm F1.0 | 7° × 5.3° |
| LDTI008-335LRF | 384×288 | 12 μm | 35 mm F1.0 | 7.5° × 5.7° |
| LDTI008-650LRF | 640×512 | 12 μm | 50 mm F1.0 | 8.7° × 7° |
At first, this table may seem to contradict the usual rule.
The 50 mm model actually has a wider published horizontal field of view than the 25 mm model.
The reason is that the focal length is not the only thing changing.
The detector becomes physically wider as resolution increases, because all three versions use the same 12 μm pixel pitch.
The physical detector width can be estimated from horizontal resolution multiplied by pixel pitch.
| Detector | Pixel Pitch | Approx. Active Width | Lens |
| 256 pixels | 12 μm | 3.07 mm | 25 mm |
| 384 pixels | 12 μm | 4.61 mm | 35 mm |
| 640 pixels | 12 μm | 7.68 mm | 50 mm |
The 640×512 detector is physically much wider than the 256×192 detector.
That larger detector width offsets the increase in focal length enough for the 50 mm version to retain a slightly wider published field of view.
This is an important lesson when reading thermal specifications.
“Longer focal length means narrower FOV” is only a valid direct comparison when detector dimensions are held constant.
When comparing different detectors, focal length, detector size and pixel pitch have to be considered together.
Although the 50 mm model retains a useful field of view because its detector is larger, each individual pixel still covers a smaller angle than it does with the shorter lenses.
All three LDTI008 detectors use 12 μm pixels.
Using the simplified IFOV relationship, the approximate angular size per pixel becomes:
| Focal Length | Pixel Pitch | Approx. IFOV |
| 25 mm | 12 μm | 0.48 mrad/pixel |
| 35 mm | 12 μm | 0.34 mrad/pixel |
| 50 mm | 12 μm | 0.24 mrad/pixel |
A smaller mrad-per-pixel value means each detector pixel covers a smaller angular area.
That allows a target at a given distance to occupy more pixels, provided it remains within the field of view.
This is one reason the 50 mm configuration is better suited to preserving detail on distant objects than the 25 mm version, even though the larger detector prevents its overall FOV from becoming extremely narrow.
The three current configurations and their complete specifications can be compared on the
LDTI008 50 Hz Thermal Scope
product page.
A longer lens cannot replace detector resolution, and a higher-resolution detector cannot completely replace optical focal length.
Detector resolution determines how many thermal sampling points are available.
Focal length helps determine how those pixels are distributed across the scene.
This is the same system-level relationship discussed in our previous article:
Thermal Sensor Resolution vs Display Resolution
.
A 640×512 detector with a wide lens and a 384×288 detector with a narrow lens cannot be compared by resolution alone because the number of pixels placed on a target may be very different.
Optical focal length affects the image before it reaches the detector.
Digital zoom happens afterward.
A longer thermal objective changes how the scene is projected onto the detector and can place more native detector pixels across a distant object.
Digital zoom crops part of an image that has already been captured and enlarges it on the display.
| Longer Focal Length | Digital Zoom | |
| Occurs before detector? | Yes | No |
| Changes optical FOV? | Yes | No, it crops the existing image |
| Can place more native pixels on target? | Yes, under comparable detector conditions | No |
Optical focal length changes what reaches the detector. Digital zoom enlarges what the detector already captured.
Not automatically.
If the same detector and pixel pitch are used, doubling focal length approximately halves the angular size represented by each pixel.
That is a meaningful increase in angular sampling.
But real thermal products often change several specifications at the same time.
Detector resolution, detector dimensions, image processing, NETD and lens aperture may all differ.
For that reason, focal length should not be turned into a simple performance multiplier.
Focal length and F-number describe different properties of the lens.
The focal length determines field of view and angular sampling together with the detector.
The F-number describes the relationship between focal length and effective aperture diameter.
For thermal systems, the optical design has to transmit enough infrared energy to the detector while maintaining the required field of view and image quality.
This is why the LDTI008 specifications list both 25 / 35 / 50 mm focal length and F1.0. They are not duplicate specifications.
A wider field of view makes it easier to search an area because the user sees more terrain in each frame.
The observer does not need to move the device as much to cover the same area.
This is useful when:
The 9.7 mm LDTI006 is a good example of this type of wide-field configuration.
As observation distance increases, a target occupies a smaller angular area.
A longer lens can place that smaller angular target across more detector pixels.
This improves the amount of spatial information available to the processing system.
The tradeoff is usually less environmental coverage for a given detector size.
The user therefore has to balance:
Longer focal length is often associated with longer detection range, but the relationship should not be simplified too far.
Detection performance also depends on:
The current LDTI008 specifications illustrate this well. The 25 mm, 35 mm and 50 mm models also change detector resolution from 256×192 to 384×288 and 640×512.
Their different published detection distances therefore reflect different complete system configurations, not focal length alone.
This distinction is important whenever two thermal products are compared.
If two products use different focal lengths, do not stop at the lens number.
Check these specifications together:
Looking at those values together provides a much more useful picture than asking whether 50 mm is simply better than 25 mm.
| Lens | Typical Character | Main Advantage | Main Tradeoff |
| 9.7 mm | Wide field | Fast scanning and surrounding awareness | Fewer pixels on a distant target |
| 25 mm | Narrower angular view | More target detail than a short lens with comparable detector | Less area visible at once |
| 35 mm | Intermediate long-range configuration | Smaller angular pixel coverage | Requires more deliberate scanning |
| 50 mm | Long focal length | High pixels-on-target potential at distance | Usually narrower FOV if detector size is unchanged |
This table describes general optical behavior, not a direct ranking of the LDTI006 and LDTI008 models. Their detectors are different, so the published field of view and range performance have to be read from each complete specification.
Thermal lens focal length is not a quality score.
A 50 mm lens is not automatically better than a 25 mm lens, and a 9.7 mm lens is not automatically less capable.
Each focal length changes how the available detector pixels are distributed across the scene.
Shorter lenses prioritize coverage. Longer lenses prioritize angular detail.
Detector size can change that balance significantly, which is why the LDTI008 50 mm model can retain a useful 8.7° horizontal field of view while also using a longer focal length.
The correct way to compare thermal optics is therefore to read focal length together with detector resolution, pixel pitch and field of view.
You can browse the current
LinduNV Thermal Imaging
range for additional thermal system configurations.
Thermal lens focal length describes an optical property of the objective that helps determine how much of the scene is projected onto the detector. Together with detector size, it determines field of view.
Only when detector dimensions are held constant. If a longer lens is paired with a physically larger detector, the larger detector can offset some or all of the field-of-view reduction.
Not universally. A 50 mm lens can provide finer angular sampling and more pixels on a distant target under comparable detector conditions, while a 25 mm lens can provide a wider view and easier scanning.
A short focal length generally provides a wider field of view. The LDTI006 uses a 9.7 mm lens and has a published field of view of 18.1° × 13.6°, which provides relatively broad scene coverage.
The detector also becomes larger. The 25 mm model uses a 256×192 detector, while the 50 mm version uses a 640×512 detector. Because both use 12 μm pixels, the 640-pixel detector is physically much wider and offsets the longer focal length.
No. Focal length changes the optical image before it reaches the detector. Digital zoom crops and enlarges detector data after the scene has already been captured.
It can improve pixels on target and angular detail, but detection distance also depends on detector resolution, pixel pitch, NETD, target size, thermal contrast, atmospheric conditions and image processing.
Compare focal length together with detector resolution, detector size, pixel pitch, field of view, NETD and the expected observation distance. Focal length alone does not describe the complete thermal imaging system.
