The Random-Showing-Up Distortion
When shooting IR you will almost certainly face two major problems that will need a fix in postproduction:
Hotspots and Chromatic Aberration.
A Hotspot is a typical spot-or-doughnut-shaped flare in the center of the frame, caused by IR light reflecting inside the lens.
Thus prime and old manual lenses with a simpler scheme generally tend to suffer less from this issue.
It is not a matter of flares, as Hotspots are independent of the direction of light; it is most likely due to the fact that every lens is engineered for optimal performance in the visible spectrum and not for the NIR.
Hotspots show up every now and then, depending on the lens, the focal length and aperture, the time of the day, and so many other factors that it is almost impossible to know if it will show up until you have taken the shot.
As far as my experience goes, Hotspots are more common at wider focal lengths (between 10mm and 20mm on a DX sensor) and become less and less invasive the more you zoom in.
On LCD previews Hotspots are generally less evident compared to how they appear on your PC screen; postproduction fixes range from tedious local correction to laborious total substitution of the sky.
Hotspots are independent of light direction.
I've experienced that several times when shooting panoramas:
the Hotspot shows up always in the center of the frame
and with the same intensity, regardless of the direction that
I am pointing the camera.
Doughnut-shaped hotspot on Nikon 16-85mm at 16mm
Spot-shaped hotspot on Fujinon 18-55mm at 18mm
IR Chromatic Aberration
Annoyingly popping out
Chromatic Aberration shows up as a bright halo around sharp edges: it's caused by the different wavelengths of light being bent at slightly different angles when they pass through the lens of the camera.
All lenses are engineered to correct and minimize C.A. for visible light, but not for IR, so it will eventually show up in your pictures, more often with mid-telephoto lenses and beyond.
Most of the times C.A. won't be a big issue and you'll only have to fix some critical points, but on some occasions, you'll have to go through a patient and time-consuming work of retouch, especially if you're going to print your picture.
Software C.A. correction filters can do only a partial work and most of the times you will have to go for a manual fix.
Hoover mouse to see detail
This picture from the series Milano>720nm suffered from serious C.A. all around the edges of the horse and the windows and rails of the building.
IR digital noise
Pros and Cons of high sensitivity
IR Digital Noise is usually less invasive up to ISO 400 - 800, depending on the sensor and the image processor, but at higher ISO settings it tends to increase more dramatically than in the visible spectrum.
This is possibly due to two main reasons: as the digital sensor is more sensitive to IR wavelengths, the same ISO settings requires a higher shutter speed than visible light, thus generating lower noise.
That same higher sensitivity of the sensor will exponentially amplify thermal noise as you increase ISO settings.
My experience is actually limited to Nikon cameras, so please take all this with a grain of salt.
Postproducing my work, I had to heavily reduce noise on portraits, mostly shot at ISO 1000, while landscapes shot between ISO 200 and ISO 400 required almost no correction of digital noise.
Some issues may come up in the sky midtones if you tweak the contrast too much, and you'll possibly have to locally reduce noise not to affect the contrast in the rest of the picture.
In critical situations at higher ISO settings, quite often the standard noise correction tools won't do the work and you'll have to come up with some creative postproduction technique.
© all rights reserved