As you have probably noticed, some of my photographs are taken in infrared. I think that an explanation of this shooting technique can be interesting, whether it's by the purely physical aspect of this technique, but also by its aesthetic aspect.
This article will therefore be divided into two parts, starting first with the physical part and its impact on the image itself.
Let's start with the very definition of the term "infrared", here we will rather speak of photography in the "near infrared" to be correct.
The eye is able to perceive light waves corresponding to values ranging from 350nm to 780nm approximately. Here 350nm is deep blue, 555nm is green, and values around 700nm are red, then dark red.
The sensitivity of the human eye to wavelengths
The "near infrared" corresponds to values ranging from 800nm to 1100nm approximately. Personally, I use a filter letting the values pass around 800nm and another filter letting the values pass around 950 / 1000nm.
I like to divide the notion of near infrared photography in two, in fact we can have quite different types of images.
1 ° Infrared in "color"
"Color" infrared photography corresponds to infrared images taken in the spectrum ranging from 700nm to 810nm approximately. We thus notice that these filters let pass at the same time wavelengths visible to the eye (from 700 to 780nm) and at the same time an invisible part (780 to 810nm). This has the effect of giving a "mix" between colors that the eye perceives and astonishing shades on certain other elements, such as foliage for example.
where infrared begins, the eye's sensitivity curve drops
We can note that the more we advance in the spectrum, the more the saturation of the colors will drop (because outside the sensitive spectrum of the eye). Therefore, the filters allowing wavelengths greater than 830nm to pass through will give a monochrome image . (Refer to the diagram above)
2 ° Infrared in "black & white"
Thus we pass to the second category of photography taken in the near-infrared: monochrome images.
You will find it more often than infrared-color images, on my page, because I particularly like black & white.
In addition to this, you need to know more about it.
In black and white infrared, there are images that can be very contrasted if the sun is present, with very white foliage and black shadow areas .
We have a rather different result than if we simply ran a black & white color image on post-processing software.
The aesthetic aspect
You may have noticed that my color infrared images do not have these red tints, but tend to be rather blue and yellow.
It is simply decided in post-production on the computer. I prefer to modify the colors to find a blue sky, more natural for the eye, therefore, the foliage, rather grayish, tends to go towards the yellow ... which does not displease me!
In the end we are in the world of infrared, where everything is only interpretation ... because the eye does not see these colors.
Comparison between a "raw" file, and a file processed to appear more natural and pleasing to the eye
Infrared and visibility
You've probably already noticed that some of my photographs present remarkably sharp and contrasting landscapes, despite their distances. It gives the impression of having peaks several hundred kilometers much closer.
This is explained by " Rayleigh scattering ", to put it simply with a lot of shortcuts: the filter only captures wavelengths greater than 1000nm (for my monochrome infrared filter) as well, particles of sizes less than 1 / 10th of these wavelengths do not block the passage of the electromagnetic wave (light). There is therefore less "dust" leading the distant summits to be invisible.
The particle ("a) is shorter than the wavelength, it does not block the arrival of light to the lens.
Here we can see a comparison between an image taken with an ordinary camera, and an infrared camera, with a 1000nm filter. We note that the image is in black & white, and it allows to distinguish Corsica much more easily than the ordinary camera.
Infrared photography is fun to practice but is far from easy to master. It is indeed necessary to have a good experience in post-production because the raw files are not necessarily publishable without retouching (adjustment of contrasts / colors ...).
In addition, it is necessary to have a de-filtered camera to practice this technique, the IR-cut filter must be removed to allow the infrared to pass through, so the camera must be dismantled.
The infrared filter blocks a large part of the light, does not necessarily have excellent transmittance and the CMOS sensor in photo cameras is not very sensitive to near infrared wavelengths.
Diagram of the approximate wavelengths "unlocked" in dots after de-filtering of a camera.
We therefore end up with exposure times of around 1 / 50th of a second to 1 / 15th of a second in broad daylight for photos taken with a 1000nm filter, so you have to use a tripod in most cases. For infrared filters located around 750 / 800nm, no worries, exposure times remain around 1 / 1000th.
There is also a "hotspot" concern on certain optics, due to the optical treatments of manufacturers which is not suitable for near infrared wavelengths.
Likewise, the number of reflections explodes as soon as the sun shines on the lens, they are not treated against these wavelengths. We must therefore be very careful where the Sun is located.
I hope that these explanations can enlighten you a little bit about infrared photography ! Do not hesitate to contact me by email for any further questions.