Some thermographers are surprised to learn that many wide angle lenses can be used for high magnification work. After all, the wide angle lenses have a larger IFOV and IFOV footprint, so how can they be used for small targets effectively?
Here is a typical example for a FLIR E60 camera (320 x 240 resolution):
| Lens FOV | IFOV (mrad) | IFOV footprint at 19.7” Working Distance |
| 25° x 19° | 1.36 | 0.03” |
| 45° × 33.8° | 2.59 | 0.05” |
The specs above don’t indicate that a wide angle lens is any better, in fact it has a larger footprint. So what gives?
Take a look at the thermograms below:
 | Here is a thermogram of a ruler taken with the standard 25° lens at its closest focus distance of 9.2”. Note how the scribed lines on the 100ths scale are fuzzy and merge together. The horizontal FOV is about 3”. This indicates that items this small cannot be resolved using this lens. |
 | I have enlarged the 25° image to illustrate the blurred lines on the scale. |
 | Here is the same ruler taken with a 45° lens. The scribed lines can clearly be seen and differentiated. The horizontal FOV is about 1.5”. What’s the secret? The 45° lens can be focused much closer to the target (1.2”). Hence it effectively can resolve smaller targets. |
Does a wide angle lens work as well as a dedicated close up lens? No, of course not. You might notice that the sharpest areas are in the middle of the image. A dedicated optic for close up work would be designed to reduce this effect while achieving higher magnification.
But as a dual purpose optic, when you need to do some close up work, a wide angle lens is pretty handy.
Here are some images from a FLIR P640 for comparison.
