Look up the minimum focus distance before buying a thermal camera for close work. If the camera is too close to focus, more pixels or digital zoom won't make the target sharp. And even a sharp hot spot can be too small for a reliable temperature reading.
Ask two questions, in this order
- Can the camera focus at the distance I can actually work from? If your setup leaves 10 cm (3.9 in) between the lens and the board, a camera that focuses from 30 cm (11.8 in) won't focus in that position. Check the thermal lens specification rather than the focus spec for the visible camera.
- How much of the target fits on the detector? Compare the size of the spot with the native pixel footprint at your distance. Seeing a hot spot and getting a trustworthy temperature from it are different jobs.
Worked example: a 2 mm component at 10 cm
Suppose two cameras can focus at 10 cm (3.9 in). The first has an IFOV of 3.8 mrad; the second has an IFOV of 1.7 mrad. IFOV is the angle covered by one detector pixel. For a small target near the center of the image, multiply IFOV in mrad by distance in metres to estimate the footprint in millimetres.
| Native IFOV | One pixel | Five pixels across |
|---|---|---|
| 3.8 mrad | 0.38 mm | 1.90 mm |
| 1.7 mrad | 0.17 mm | 0.85 mm |
Against a 2 mm (0.079 in) target, the first optic is close to our five-pixel planning threshold; the second leaves more room for imperfect alignment and focus. This is an illustration, not a measured result or an accuracy guarantee. The relevant target is the hot area itself: a component 2 mm wide can have a smaller hot spot. Calculate the footprint for your own distance.
When a macro lens makes sense
A macro lens can help if you regularly inspect small components and can work at the distance specified for that lens. Before buying, look for three things: working distance, spot size or spatial resolution at that distance, and whether the lens is included. A macro lens intended for work at 3 cm does not automatically solve a job that requires 10 cm of clearance. Mounts, cables and the shape of the enclosure all affect what distance you can use.
For example, HIKMICRO specifies a 0.1 m focus distance for the Mini2Plus V2. That tells you where to start checking the stock optics. It does not guarantee that every component at 10 cm is large enough to measure.
What to do if a spec is missing
Ask the seller or manufacturer for the thermal camera's closest focus distance and a native thermal image at your intended distance. If a temperature reading matters, ask how large the target must be for the stated accuracy with that lens. Don't use digitally enhanced resolution to estimate a measurement spot.
Try the 10 cm / 2 mm scenario in the selector. If you only need to find which component runs warmer, choose the option for spotting heat differences. See the electronics repair buying guide for more on choosing the camera format.
Sources
- FLIR A40/A50/A70 placement guidelines — the difference between detecting a hot pixel and filling enough pixels for specified accuracy on those cameras.
- HIKMICRO Mini2Plus V2 specifications — the published thermal focus distance for that model.
- The Thermogram selector methodology — why the selector uses five pixels as a conservative working rule.
This guide is based on the documentation above. The cover illustration is not a field measurement. The Thermogram did not conduct a hands-on test for this article.