IR Lenses performance
just go out and try
There's not much to say on lenses.
Just go out and shoot, and then see the results for yourself.
Lenses are designed and engineered to maximize performance for the visible light, but not for the infrared spectrum, as NIR wavelengths are filtered out by the hotmirror in front of the sensor.
For this reason, lenses that normally give good results may perform poorly for IR photography.
Most of the time, I shoot with the Nikkor 16-85mm f3.5-5.6:
it is an excellent quality zoom lens and performs very well in the IR spectrum.
If I want to go wider, I choose the Sigma 10-20mm f3.5 , which performs quite well, though hotspot becomes more evident as you get closer to the wider end of the zoom.
There are two major problems that generally show up: chromatic aberration - the bright halo that shows up around edges - and hotspot, a big, bright, annoying flare in the center of the frame.
Chromatic aberration is caused by different wavelengths focusing on slightly different planes: special glasses and nanocoatings do a great job to correct this issue for visible light, but not for IR, so you'll almost always have some C.A.
Hotspot is mainly due to distortions caused by the optical scheme of the lens, and if the problem persists it can be overcome simply by changing lens.
You should give your lens more than one chance though, as a hotspot may appear only at certain apertures or at certain angles.
As far as my experience goes, best results in term of sharpness are obtained between f 8 and f 16 (which is not a big surprise) but unlike for visible light, softness and diffraction will become an issue very soon out of this range.
Focus on wavelenghts
how IR affects the focal plane
When light passes through the lens it breaks down into its constituent wavelengths and each of them is deflected at a slightly different angle; this causes different "colors" to focus on slightly different planes, which is at the origin of chromatic aberration.
Lenses are engineered in order to correct this shift for visible light, but NIR is not taken into account, and CA will show up almost for certain.
As you can see in the diagram on the right red light (longer wavelengths) focuses farther away from the lens than blue; NIR focal plane is even farther back.
DSLR AF systems will in most cases give the correct read and compensate for the focal plane shift, provided that you are working with a Full Spectrum converted camera and a filtered lens.
On the contrary, compact and mirrorless cameras may give some problems after the FS conversion: I have converted an old Lumix TMZ which only focuses at the wide end of the zoom, a Canon Powershot that is sharp only at the long end, and a Lumix G3 which is completely screwed up and never gets the picture in focus.
I think that this is due to the fact that tolerances on compact cameras are smaller than on DSLRs and once you have removed the hotmirror the lens does not have enough movement range to compensate properly on the full focal length.
On the bright side, my five DSLR conversions were a full success each time.
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