@Lesnums), Samuel Boivin
Posted on 01/12/15 at 09h53 Share:
La HDR photography is a set of processes to surpass the technical capabilities of cameras, in order to reveal a greater dynamic range and get a little closer to human vision. We will see in this article how to create a photorealistic image, from the Shooting to the post production.
In this type of photography, without HDR processing, the buildings in the foreground would be backlit.
The principle
Before getting down to business, it's important to understand how HDR photography works in theory. We talk about HDR photography for High Dynamic Range, or large dynamic range in Spanish, but what is the dynamic range of a photograph?
This is the difference between the darkest shadows and the strongest lights in your image. Since the human eye has a greater dynamic range than the technical capacity of your camera, it is often difficult for you to faithfully transcribe in your image what you see. The HDR technique allows you to overcome this limit and obtain more realistic images, where the shadows shadow areas are uncovered.
Technically, our boxes take photos in 8 bits (256 color shades) or in 16 bits (65 shades) for the most efficient, which is still too little to come close to human vision. To create an HDR image, you will have to take the same photo at least three times, but at different exposures: once correctly exposed, a second time underexposing it and a final time intentionally overexposing it.
Once your shot is complete, using software, you must assemble your three photos into a 32-bit file (4 shades) to expand your dynamic range. Our computer screens and digital printers cannot display as many shades, so we will have to convert this file into a new 294-bit file usable by the tone mapping technique. Using an HDR fusion algorithm, we will choose the most suitable pixel among the three photos in order to compose an image correctly exposed in all areas.
If by this technique one can obtain photorealistic images, one can also create images with a surrealistic rendering closer to drawing or painting. Everything will be a question of setting the tone mapping, and tastes.
Illustrations of different more or less realistic treatments of the same photograph in HDR.
The shooting
As we saw earlier, the basis of creating an HDR photo is to photograph the exact same scene at least three times at different exposures. To do this, there are three ways to proceed.
The first and easiest is to use your camera's bracketing function. Today, the vast majority of cameras are equipped with it, whether reflexes, bridges, compacts and even some smartphones. With this function, you will be able to ask your camera to make three releases (or more) by choosing the difference in exposure between shots. Once configured, all you have to do is make the first trigger and your box takes care of the rest.
If this method is accessible to all, it does not allow you to have control over your settings. Thus, it is not impossible that to vary the exposure, the camera modifies the opening of the diaphragm from one release to another, with the consequence of varying the depth of field, which will be harmful for your final rendering.
If you are faced with this kind of problem, or if you prefer to be in control of your settings, then activate the manual mode of your camera and vary only the shutter speed. Underexpose and overexpose by 1-2 EV if shooting RAW, or 0,3-0,6 EV if shooting JPEG. Either way, overexpose as much as you underexpose.
The last solution is to use a remote control specialized in HDR photography. It will allow you to adjust the bracketing as you wish (number of shots, exposure difference between each, etc.) and will trigger all the photos in a row, thus limiting the movement of elements, even if this can be corrected afterwards.
After you've finished shooting, if you want to check that your photos are consistent for HDR processing, you can use your camera's histogram. The underexposed photo should contain no highlight information (right) and the overexposed photo should contain little shadow information (left). If so, you have successfully taken your shot.
Three differently exposed photos before their processing in HDR and their final rendering.
The box settings
It doesn't matter how you shoot: either way, there are a few common settings to follow to get the best HDR image possible.
- – Use a tripod, or failing that, put your case on a stable place, in order to avoid a change of framing, however small. Realigning images is always possible in post-production, but will never be as effective as a steady shot.
- – In the same way, use a remote control and/or the self-timer to avoid moving the camera by pressing the shutter button.
- – Set the sensitivity as low as possible: as tone mapping generates noise, you might as well avoid having any when shooting.
- – Set your white balance manually to prevent it from varying from one shot to another.
- – If your camera allows it, shoot in RAW. To produce the best possible HDR image, you will need to modify a number of settings in post-production. A file free of any compression will be of the greatest help to you to express your creativity.
Night photography techniques combined with HDR can sometimes reveal lights that were previously impossible to capture.
The creation of the HDR image
Now that you have your photos exposed in a different way, you will be able to generate your HDR image using specialized software. If the algorithm varies a little according to the program used, the way will always be more or less identical. Either way, you'll need to create a 32-bit container with your differently exposed shots, convert it to a 16-bit file by tone mapping, and then handle the post-processing in your usual software.
We will detail two different methods to do this, but there are a plethora of programs available depending on your operating system, all of which more or less take up these common bases.
With Photomatix Pro
Certainly one of the most used software for creating HDR photos, Photomatix Pro has the advantage of being affordable and very intuitive, while producing good quality images for beginners.
Once started, simply load your batch of bracketed images; a settings window will open, offering you some presets such as image alignment, reduction of chromatic aberrations or white balance adjustment. If you have followed the previous few instructions, you should not have much to do at this stage. Always check the reduction of ghosting, they are produced by elements that would have moved from one shot to another, such as trees caught in the wind, clouds, traffic, etc.
Your 32-bit HDR container is now created but is not yet viewable on your computer screen. Now you need to activate the tone mapping to convert it to 16 bits and make it readable in order to start the job. Photomatix Pro then offers you two ways to process your image.
You can use the list of available presets, these can be fine-tuned manually: they represent a good way to become familiar with HDR processing and understand the problem, but will frustrate the most demanding.
You can also develop your photo by setting everything manually. Photomatix Pro being rather well developed, even a novice will manage to obtain a satisfactory rendering in a short time.
With Photoshop and Merge HDR Pro
Since the CS4 version, Photoshop natively offers the Fusion HDR Pro module. Creating an HDR photo with Photoshop is a bit more complex, but has the advantage of producing higher quality images. As the algorithm generates much less noise, this method is more suitable for those who would like to make large-format prints and experienced users in photographic retouching.
You will find the Merge HDR Pro module in the "File > Automation > Merge HDR Pro" menu. Once you have entered the location of your files, you have two options.
You can directly generate a 16-bit file that you will "tone-map" directly from the module. This technique has the advantage of being faster, but its rendering is much less advanced.
Alternatively, you can generate a 32-bit container that will load in a new Photoshop window. For tone-mapping, you can then use adjustment layers and blending masks to choose the best-exposed pixels yourself for each area of your image, or use Photoshop's tone-mapping tool: Virage HDR. You will find it in the "Image > Adjustments > HDR Toning" menu.
Using this tool, you will create a 16-bit file that you will develop using the classic tone mapping method. If the settings are essentially the same as on Photomatix Pro, you will need a greater mastery and understanding of HDR photography before obtaining a result similar to what other specialized software could offer you.
Similarly, if you can't manage with Photoshop's tool, you can simply save your 32-bit container in Radiance format to import it into another software like Oloneo PhotoEngine, or in TIFF format to use it in Lightroom.
Even if the HDR photo helps to balance the tones, sometimes, like in this photo, certain lights remain overexposed.
Understanding Tone Mapping
In order to achieve a satisfactory result during your tone mapping, especially if you do not want to go through presets, it is necessary to understand certain parameters specific to this technique or having another impact compared to conventional photography.
- Intensity: this is the setting that will give the HDR effect to your image by balancing the highlights and darks. It is measured from 0 to 100; the closer you get to the maximum, the more optimal local exposure you will have. However, if you want to keep a photo-realistic rendering, be careful not to force too much and stay around 70-80. As you can see, this parameter generates noise. To counteract this issue, use micro-smoothing as needed.
- Saturation: when it comes to HDR photography, this setting is even more important than usual. Compressing tones has an effect on color rendering and can have a drastic impact on the final look of your image. Preferably, keep a light hand on the general parameter and prefer a tone-by-tone adjustment of the saturation.
- The contrast of details: unlike conventional photography, in HDR, the contrast adapts to the exposure of the different areas concerned. As a result, it will have a direct impact on the rendering of your photo by giving it more or less liveliness. For once, it is even advisable to force this setting, while making sure to keep a realistic rendering.
- Lighting adjustment: due to the compression of the different shots, you will have noticed that the light intensity of your composition is no longer faithful to reality. Adjusting the lighting will allow you to restore good brightness throughout your image. Play with the slider until you get a satisfactory result, while taking care not to overexpose part of your photograph. The interest of the HDR technique lies in the fact of having neither underexposure nor overexposure.
- The range: it allows you to adjust the brightness of the midtones in your scene. So far you've balanced the shadows and highlights with the intensity; the gamma will affect all other tones. For optimal adjustment, guide yourself using the histogram: it will be your ally to know where you are.
You have now set the main tone mapping factors that are almost systematically found on all software. Additional settings, such as adjusting the white point, compressing tones or even retouching the white balance, are offered depending on the software, but the main and essential problem in creating an HDR photo lies in these previous points. .
Your HDR photo can now be saved, either directly in JPEG format, or in 16-bit TIFF format for further editing in your usual post-processing software.
If the HDR technique makes it possible to reveal details hitherto impossible to capture, it does not adapt perfectly to any scene.
Conclusion
You should now be able to start creating HDR photos by mastering the basics of its design. Many points can and should be explored according to your own experience and your desire for rendering, but you are now equipped to embark on this new photographic universe.
