Much depends on what you mean by 'don't lie'.
There are relatively few sensor manufacturers in the commercial arena (ULIS/Lyred, FLIR, SEEK, guide IR, iRay, GST, DaliT).
They are the only people who can (sensibly) perform a true NETD measurement, and will do it at the best conditions of sensor biasing and integration times etc.
They will always use f/1.0 all-Ge lens with high transmission coatings. They can also have some very impressive 19" rack driver electronics.
While some of these companies do build modules and cameras, there are many more people making cameras from sensors or modules with varying degrees of skill starting from a simple screwdriver operation upwards. For them, they will usually just quote the NETD of the sensor as supplied. People buying are also likely to want to know 'the NETD' and with proof - ie the sensor datasheet.
One question you may be trying to ask is 'is the NETD as running in the camera the same as in the sensor datasheet'.
It is probably fair to say, not often !
Any of the usual requirements of an end product camera (dynamic range, temperature variations, power and space, response time, frame rates, video timings) may prevent the use of the 'ideal' NETD settings of the sensor at test.
Using a different lens is not relevant at this point.
The other question is 'what temperature differences will I be able to see'? Here you have a load more variables thrown in and this is where 'NETD' is less relevant and the 'MDTD' is more what you want to know about.
For that measurement the lens aperture, image processing, display, colour schemes come into play.
As an example, I can examine the use of a ULIS 03191 sensor in e2v Argus4 cameras.
Quoted NETD is '<120mK' for the usual f/1, 30Hz, 300°K
For a start the typical sensor was testing more around 65mK and later dropped to 55mK. The spec only ever changed to <100mK, just the yields went up !
The 'fire' version (P7130, f/1) had 3 gain modes with different MDTD (60mK, 100mK and 300mK).
The 'security' version (P7250 f/1) had a lower dynamic range, allowing a better MDTD (45mK)
The same settings with a narrower field of view f/1.3 lens came out at MDTD 60mK of course.
The next irony is that the same die in the ULIS 0336 sensor had a better NETD (<80mK) but, being without a peltier, could not be kept at 300°K and so gave worse camera MDTD's even with the same lens / software / display!
So were any of those specs lying ? Possibly when sales insisted we came up with a way to claim an NETD of 50mK, as the customers could not understand why MDTD mattered and NETD did not. Tenders were being rejected by pen pushers for 'NETD not <50mK' although some thought the higher number meant better
Bill