Revan,
I think this is a terminology issue. You stated you wanted to see "smoke" leaving a chimney. As has been stated, Longwave thermal cameras are useful for seeing through "smoke". What you are actually wanting to image is the thermal energy leaving a chimney and this can be imaged with a thermal camera. It could be argued that there is always heat accompanying smoke coming out of a chimney but I think your specific request to see "smoke" caused some confusion.
Whether a Lepton core can image such thermal energy is actually a little hard to answer. The Lepton3 cores come with a wide angle lens so the minimum resolvable target at a stated distance applies. Heat emissions out of a chimney should be DETECTABLE at around 50m but I would not want to say they would be seen at 100m. I have no suitable chimneys around me to check.
One thing to be aware of, the sky is very cold to a thermal imaging camera. This can force quite wide thermal imaging spans in a fully automatic camera. Such wide spans can be detrimental to imaging some temperature differentials. It is better to manually select the temperature span of, for example, 10C and then set the centre temperature to that of the emission coming out of the chimney. You may need to set a wider span in order to gain the best image of the emissions.
This sort of imaging is normally the realm of industrial thermal cameras and a resolution of 320 x 240 pixels with 24 Degree, or less, FOV lens is preferred. The Lepton series of cores are at the bargain basement end of the market. You will not achieve impressive imaging with such a core, but if "detection" is your intention, it may work well enough for you if a Lepton3 is used.
You have not detailed why you wish to image the thermal emissions out of chimneys etc. Is it a project to study the thermal air behaviour, vorticies etc ? If so, that is beyond the capabilities of the Lepton3 and its wide angle lens. To study heated air interaction with the surrounding atmosphere and vorticies etc, I would use a high performance 320 x 240 pixel thermal camera fitted with a telephoto lens. The telephoto lens would provide the confinement of view needed to extract decent thermal scene detail with only 320 x 240 pixels. The telephoto lens FOV would be chosen to suit the distance from the target and required detail in the image. I would expect to use a 7 Degree FOV lens to look at a chimney on a house. That is a sixth of the FOV provided by the Lepton3.
On the 'cheap' telephoto lens front, there are none
Thermal camera lenses are made from materials that are inherently expensive. Germanium, IR Chalcogenide Glass and ZnS being commonly used. Telephoto supplimentary afocal lenses do come up for sale on eBay occasionally but they are by no means "cheap". New supplimentary lenses from FLIR and FLUKE cost thousands of US Dollars.
If you have a project that needs to image the thermal emissions from chimneys etc, it might be better to hire a suitable professional thermal camera and telephoto lens. It will be cheaper than buying equal performance, even used.
Fraser