Az Bulutlu

2017.08.27

Edit Backlog #90: Notes on Infrared Photography with Darktable

Image Information

“Tree and Shadows”
2017, Datca, Turkey
Sony a6000, Samyang 21mm F1.4, Hoya B72 Infrared filter
Edited using Darktable in Ubuntu Studio

About

If you follow me in patreon or mastodon, I have been posting some work in infrared photography for the past week and half. The photo above is one of those photographs. You might notice that the colors in this final version is slightly different than the previous version. (mostly less bright reds).

I use a Hoya B72 infrared filter which as far as I am aware is one of the few if not only infrared filter that doesn’t require some sort of camera modding.

A quick research on the internet will give you the following on infrared photography:

  • Leaves look brighter, white or reddish in color.
  • red-blue channels swapped in post-processing to make the sky look natural
  • As the filter cuts most, if not all of the visible light off, you need to focus in advance
  • expect long exposures
  • white balance calibration on camera is required (don’t shoot all red photos)

 

Exposure

Infrared photography is long exposure. In almost all cases you will need a tripod, a remote shutter and bulb mode. The shutter speed needed with the kit lens that came with Sony a6000 (100ISO, F3.6) was 2-4 minutes. This was at a very bright noon at meditteranean shore.

A fast lens (a lens with lower aperture, that allows more light in), if you have one, can help reduce this to a more manageable time. With the image above, and most of the photographs I took, I used my astrophotography lens, Samyang 21mm F1.4. The image above took 33 seconds.

I liked having the ability to take multiple shots (relatively) quickly and having a bit more flexibility with less than ideal lightning, but generally most lenses are usable at wide-open (or close).

 

Focus

Under very bright sunlight, with an F1.4 lens and a very high ISO, it is possible to actually see the image through camera with the filter on. However sensor noise makes it quite hard to see the focus. In general, it is just easier to compose and focus the shot then put the filter on, even if you have the option otherwise. Since I had the white balance saved, I actually found it easier to focus and compose the image on auto white balance, and switch back afterwards.

 

White Balance

If you just shoot an image with no preparation, this is along the lines of what you will see.

Red shifted infrared raw, without white balance calibration

If your camera somehow doesn’t allow white balance adjusting, it’s alright, and it is fixable in post processing. You might have read about hacks, creating custom profiles etc (usually using Adobe Lightroom), but Darktable handles adjusting the white balance without any problem. Just ignore the temperature and tint sliders, and use the red/green/blue directly to adjust. A 1/2/10 ratio (e.g. red:0.5, green: 1.0, blue: 5.0) for R/G/B should give a good starting point, or just use the spot preset before fine tuning.

The end result should look something like this. Notice that the sky is an olive green, while most of the foreground ranges from green-grays to very faint browns.

Infrared image, unedited, with correct white balance

It is still worth calibrating the white balance in the camera through if you can, as it helps with correctly estimating exposure. You might have noticed that without the correct white balance, the photograph looked overexposed when it wasn’t.

White balance bracketing, if you have the option, can also helpful, especially if you don’t want to do too much post processing.

 

Channel swap and saturation:

Swapping red and blue channels doesn’t just make the sky blue. Most objects will end up looking somewhat close to their original colors. This is a post processing step for those infrared images with blue skies and red trees. Of course you also have the option of working grayscale, or making a different creative choice. You can switch red and blue in channel mixer obviously (and it has a preset for it). However, I would suggest making the switch using input color profile module instead. Input color profile has an option of linear infrared BGR color profile option that accomplishes this swap and has the advantage of being near the bottom of the module stack.

One thing to note that, the image is very desaturated, so the image after the channel swap will look very monochrome. I generally throw a velvia at 100% in Darktable before fine tuning white balance. Then, more saturation gets added during post processing. This particular photograph has 2 velvia modules, one at 100% another at 79%. The image below has a 100% and a 77% velvia, and some additional selective color boosts through 2 color zones modules.

Sunlight through leaves as seen in infrared filter

What to expect:

Trees actually can be a wide range of colors from reds and yellows to pale greens. I have yet to get more pure white trees, although that might be my post processing choices, the type of trees, the properties of the lighting I worked in or a combination of all three. Overall, most subjects ends up close to their original colors, depending on editing choices, infrared can actually look less surreal and more like dated film photographs. There is room to experiment and make artistic choices.

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