IR Light sources
the light you cannot see
The Sun is a major source of Infrared light, and that has been the case with my whole production, except for portraits, which I'll talk about later.
Infrared photography is generally shot in full daylight and those very midday hours which tend to be avoided by traditional photography turn out to be an optimal condition for IR photography.
IR converted cameras are between 30% and 50% more sensitive to IR frequencies than standard cameras are to visible light (take a look at this graph), which translates into a 1 to 2 stops gain.
A rule of thumb for IR photography is to keep your light source behind you, as incident light sources may cause unwanted flares and reflections much more frequently than in traditional photography, as almost every lens is optimized to reduce flares and distortions for the visible spectrum, but not for IR light.
Besides the technical issues, if you are shooting landscape and are looking for deep black skies with contrasty clouds, then just point away from the sun.
The more you point your camera towards the sun, the greater is the amount of IR radiation reflected by the sky, which will consequently lighten up and loose contrast in your pictures.
IR radiation, as well as longer light wavelengths
(i.e. red and yellow light), are efficiently absorbed by the surrounding atmosphere; high-frequency blue light is scattered around much more and for this reason the farther you look away from the sun, the bluer the sky appears to your eyes as longer waves become less and less invasive.
Atmospherical conditions and season of the year also have a deep impact on the final result.
Comparing summer and winter photo sessions I can see how the latter generally produced better results in terms of detail crispness and sky contrast.
This is likely due to air humidity, which is coherent with other situations that I've experimented in Oman, Sri Lanka and Peru: cold and dry air contributes to image sharpness just as they do with visible light, but IR Photography is more sensitive to this issue.
You also have to take into account the fact that almost no lens is optimized for the NIR spectrum.
Artificial light and IR
we don't want to be energy saving
Energy saving lightbulbs deserve their name because they waste only a tiny portion of the energy that they absorb by emitting light outside the range of the visible spectrum, i.e. the Infrared.
On the contrary, old-fashioned incandescent and halogen lamps do emit a wide range of IR light, which makes them an ideal light source for IR Photography.
LED lights are useless as they have very narrow bands of emission limited to one color of the visible spectrum.
You can easily find LED IR flashlights at 850nm or 940nm: they have a narrower emission band compared to incandescent lamps, but they dive deeper into
the NIR spectrum and they are extremely versatile
and leave room to a wide range of experimentation.
I actually own a pair myself, though I haven't really made any significative use of them yet.
LED noramlized emission spectra
Candles, as well as light matches and cigarette lighters, are a great source of Infrared radiation; I've been using these sources in my IR portrait sessions with remarkable results.
Candles and lighters have a similarly weak emission in the visible spectrum, but they both have a stronger emission in the NIR spectrum than incandescent light bulbs.
Typical settings for a 760nm portrait lit by only one candle
(as per what you can see in my Under the Skin gallery) would be something around f 5, 1/60 sec, ISO 1600.
This is quite a critical setup, as you will probably face some issues with noise reduction in postproduction, but it is a really promising starting point.
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