HIGH DYNAMIC RANGE (HDR)

By Philippe Clabots

High-dynamic-range (HDR) imaging is a technique used in photography to reproduce a greater range of luminosity than what is possible with standard digital imaging or photographic techniques. Standard techniques allow differen�a�on only within a certain range of brightness. Outside this range, no features are visible because in the brighter areas everything appears pure white, and pure black in the darker areas. The ra�o between the maximum and the minimum of the tonal value in an image is known as the dynamic range. HDRI is useful for recording many real-world scenes containing very bright, direct sunlight to extreme shade. High-dynamic-range images are o�en created by capturing and then combining several different, narrower range, exposures of the same subject ma�er.

Dynamic Range (in EV)

Tone Mapping

When we create an HDR image we take a range of brightness that’s beyond what can be displayed using conven�onal display equipment. Producing an image file where the image can’t be seen is of li�le value so a process is required where we turn this into an image that can be displayed.

This is where tone mapping comes in as it will take the extended dynamic range and compress it into a range that allows it to be displayed, or printed. If you’re pushing the tone mapping too far you will end up with an unnatural (and, for me, ugly) picture. This kind of pictures has given a bad reputa�on to HDR imaging.

When is it, and when is it not, useful to use HDR ? HDR is useful when the ligh�ng condi�ons will make impossible to capture details in the highlights and in the shadows at the same �me.

The histogram on your camera is key to detect such a situa�on. If you cannot adjust the exposure to avoid clipping in the highlight and in the shadow at the same �me then HDR will probably help you to get an image with details in all the tonal range.

Keep also in mind that, If your camera is quite recent and you’re using RAW, the dynamic range of your digital image is larger than the medium used to show your image whatever it is. A typical usage is a sunset on the sea having the sun in the frame.

As HDR means taking mul�ple shots of the same scene and combining them together in post-processing it’s really not appropriate for moving subjects. The deghos�ng op�on helps but this is defini�vely not for shoo�ng sports. Mostly it will be used for landscape photography.

Crea�ng a HDR photograph

Do not change the aperture, as it will change the depth of field, but only the speed to adapt the exposure. The exposure bracke�ng mode of most cameras allows to take between three and seven pictures varying the aperture. Personally I am taking three pictures with a - 2,0,+2 exposure compensa�on.

As the pictures must be blended together you must be quite stable between the shots. A tripod is ideal but post-processing so�wares are making wonder aligning the shots. Most of the �me I’m shoo�ng HDR handheld.

Postprocessing

During the post-processing process (in camera or with your computer) you will have to merge the pictures taken at different exposures into a single picture.

Most image processing so�ware can do it and some of them are even dedicated to this process. Long �me ago I started HDR with LR/Enfuse, a free plugin for Adobe Lightroom. Now I’m relying of the standard func�on of Lightroom for doing it (Photo > Photo Merge > HDR). You can even create a panorama (mul�ple pictures) of HDR pictures (mul�ple exposure).

As you can see in this illustra�on the balanced exposure (middle) is clipped in the shadows and at the same �me is correctly exposed for the highlights. This is a typical case where the dynamic range of the subject is exceeding the capability of your camera.

The final picture combines (Lightroom HDR process) the three exposures into a single one with a balanced histogram. You have details in the highlights and in the shadows.

But in this case, as shown below, Lightroom was perfectly able to recover the shadows and highlights from the RAW file of the balanced exposure. So no need for HDR here ;-)

In some complex cases I process my HDR in Photoshop that offers a lot of control over the final result. For fun you can try some of the extreme presets …

Conclusion

HDR can be very useful in some complex ligh�ng situa�ons, when you have very bright and very dark areas in the same scene. But even if this is really quite easy to achieve, my advice is to use it sparingly as it can give an unnatural aspect to your pictures, and most of the �me you really don’t need it anymore, thanks to the dynamic range of modern sensors, to get details in the highlights and in the shadows from a single picture. As usual less is more!

Sources :

• wikiwand.com/en/High-dynamic-range_imaging

• wallpaperflare.com/landscape-photo-of-mountains-nearriver-nature-hdr-ugly-wallpaper-wallpaper-palfz

• lenscra�.co.uk/photo-edi�ng-tutorials/tutorialunderstanding-tone-mapping/

• thehdrimage.com/why-dynamic-range-is-not-tonal-range/

• researchgate.net/figure/Dynamic-range-of-various-sensors 7