As has been well covered by Vipitis, traditionally a high cost item on the BoM was the Germanium lens assembly. The lens elements were so expensive to produce that the number used in a thermal camera lens block was kept to a minimum. Zoom lenses were considered an expensive luxury and found mainly in military systems where cost was less of an issue. More Germanium lens elements tended to mean higher cost and greater weight.
As already stated, and seen, it was often a multi magnification lens block that was found in military cameras. There was not the need for a continuous zoom as commonly found in a consumer SLR VL camera. There was no ‘pretty picture’ requirement, only a hunt, identify and, if necessary, kill requirement. Two or three magnification levels provided all that was needed for such and simplified the optical design. True continuous zoom lenses can be optically complex. Telephoto lens attachments are simpler.
With the advent of cheaper and more easily produced Chalcogenide IR Moulded Glass lenses there would appear to be an opportunity to build affordable continuous zoom lenses for thermal cameras. The industry appears to prefer interchangeable prime focus lenses and supplementary lenses that mount in front of the primary. Interchangeable primary lenses offer the advantage of being faster than a zoom or a combination of primary and supplementary lens.
Multi magnification lenses would appear to remain a mainly military use technology. Modern thermal cameras use advanced interpolation to mimic a true continuous zoom function using electronic zoom principles. The IFOV remains the same however so it is a compromise solution as you would expect.
With regard to autofocus. My PM695 cameras have this built into them. Whilst the hardware is not a great additional expense, it was usually provided only on remote manual focus cameras that already contained the required motorised lens linear drives. Such cameras were already expensive because of motorised lens drives. The cheaper alternative to auto focus was fixed focus lenses. They offered the benefits of lower complexity, lower cost, ease of use and great depth of field. They could not match a manual focus lens in terms of image sharpness at several distances but then F1.0 manual focus lenses have quite a shallow depth of field necessitating regular manual focus adjustments. Users often do not like to have to keep revisiting the focus on their cameras. Auto focus was intended to provide the user with the benefits of sharp focus at all distances whilst also handling the tedious need for regular adjustment of focus due to shallow DoF. It does work but the motorised lens adds noise, weight, bulk and, importantly, power consumption to the camera design. The technology has improved over the years though, thanks to improvements in auto focus VL camera lens mechanisms.
Auto focus remains a luxury fitment on a thermal camera and so is found only on the more expensive models. While users continue to accept the limitations of fixed focus lenses and inconvenience of adjusting manual focus lenses, the manufacturers have little incentive to change this situation.
Fraser
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