An Introduction to HDR Imaging for Adobe Photoshop CC

“The sensitivity range of the human eye is enormous. From full sunlight to starlight represents an illuminance range of more than 10 million to 1.” Graham Saxby, The Science of Imaging, IoP Publishing, 2002, p30.


In effect, what this is saying is that our eyes see The whole dynamic range …but our camera only is one or the other

This is because the dynamic range of even modern digital cameras is around 12 stops, whereas our eyes and our brains can cope with exponentially more than this. Most amateurs or hobbyists however assume that an HDR image looks like what you typically see on amateur urban or urban exploration sites .  That is not an HDR image. It’s a tone-mapped image from an HDR source. It’s also a badly tone-mapped image, and is fast becoming a cliché. It also destroys the quality of the image by adding artefacts such as halos around contrast boundaries, and high levels of noise. The purpose of a HDR image is to map the wide range of tones into something that is believable as a normal image. In order to do this, a series of RAW files are produced over a wide range of exposure values, from very under exposed to reveal fine highlight detail, to very under exposed to reveal fine shadow detail. The images are then loaded into software to create a very high bit-depth, wide dynamic range file. The dynamic range of this file cannot be seen on the screen, nor can it be printed, as no print process or screen exists that can display such a high contrast range. The purpose of the file is merely contain all the detail, which is then remapped into a visible dynamic range to be seen or printed. There are many pieces of software that can do this. Photomatix is popular, but is mainly used to create HDR special effects as seen above. One of the best tools for creating realistic HDR images is already part of Photoshop: “Merge to HDR Pro.” In order to use it, you must have a series of suitable RAW files ranging from at least 2 stops under, to 2 stops over exposed, and preferably in 1 stop increments, so at least 5 images is normal. Less can be used, but less than 4 or 5 begin to degrade the process. The more files, and the wider the range of exposures captured the better the results. As several files of differing exposures are needed, the subject matter dictates what’s possible or not. Scenes with lots of movement do not work well, as there will be significant differences between frames. Landscapes, interiors and still life are usual subjects. Tripod use is essential.

Step 1: Creating the 32bit HDR file. Place your range of RAW files in a folder, then in Photoshop, go to File/Automate/Merge to HDR Pro. A dialogue box will appear. You can navigate to a folder, or to files. If Folder is selected, then all files within that folder will be loaded into Merge to HDR Pro. If Files is selected, then you open the folder and manually select the files to load. Once selected the files will be displayed.

Ticking “Attempt to Automatically Align Source Files” will align files in case there was any movement between frames. Click OK. The RAW files will load into the software, and the Merge HDR Pro window will open and display your image. Step 2: Creating the Tone-mapped image With Photoshop CC, we now have the option of converting the images into a single Smart Object  and actually use the raw files directly within Adobe Camera Raw. This means that it’s not only a 32bit image, but can be re-edited at any time, and no permanent changes are being made. 32bit images are not directly useable, as their dynamic range is too wide for monitors, printers etc, but first creating a 32bit Smart Object file and saving it gives you a “digital negative” that contains the exposure width of ALL combined raw files. In order for the smart object to continue working, your files can not be moved from their original location, so this is another reason to have a sensible file and folder strategy . Once Merge to HDR has loaded all your files into layers, you have a new option in Photoshop CC for 32bit working, and that is “complete toning in Adobe Camera Raw” Ensure this is ticked, and it may be worth also ticking “remove ghosts” as this helps remove items from individual raw frames that are not present in all, such as a bird flying past in one frame only.

Then click “Tone in ACR” The raw files will be ALL opened in ACR as a Smart Object.  At this point, Just click OK without adjusting anything. You’ll see that once the image is now open in Photoshop, it’s layer palette icon has changed, and there are new adjustment layers. These are indicating that the image has been converted into a Smart Object, and there are Smart Filters applied. If you save the image now using File/Save As, as a PSD (photoshop file) you will have a 32bit digital negative that will allow a huge range of adjustment with much less loss of quality than any other method. Once you have saved this smart object 32bit file, the raw files it links to cannot be moved or it will break the link. Make sure you put your raw files in a place you are happy to leave them (see session 1). The beauty of smart objects and smart filters is that you can go back in and make adjustments at any time, and no permanent changes are being made to either the linked raw files, or the actual smart object itself. Double clicking on the “Camera Raw Filter” layer will re-open ACR and let you carry on editing. Initially, lowering “highlights” and increasing “shadows” in ACR will demonstrate how much more dynamic range has increased by combining all 5 raw files into a smart filter. this may look a little flat however, so you can make adjustments to suit your needs, but to take advantage of the HDR file, high shadow settings, and low highlight settings are generally desirable. If the image looks flat, you can make adjustments to ‘blacks’, and also make curve adjustments. You have to remember at this point, that we are still working at 32bit level, and this file is not useable at this point. It can’t be accurately printed, nor even viewed, as printers and monitors cannot handle 32bits of colour depth and dynamic range. Once we have the image roughly adjusted as we want, we can now convert it to 16bit, and this is where CC’s Smart Filters come into their own, as even after converting to 16bit to be viewed and edited, it still links to the original raw files, and is still fully reversible. Select ‘Image/Mode/16bit’ (next page) You will initially be greeted with this warning You may ignore this – it’s a general warning, and not specific to this exercise. However… the NECT warning should NOT be ignored. If you merge layers, it will convert the smart object back to a normal layer, and your ability to adjust the image by using ACR will be lost. SELECT “Don’t Merge”. Your image will now be 16bit, and the HDR 32bit tones are being remapped and compressed into a 16bit space. You can always check what bit-depth you are working on by looking at the file name in the image tab. This is now a 1bit file. As the tonal range has been compressed, the image will look flatter at this stage, but don’t worry, as double clicking the ‘Camera Raw Filter’ smart filter layer will re-open ACR and allow you to continue editing the 32bit HDR set of raw files, but will now accurately convert them to 16bit. As we are essentially working on raw files (via the smart filter) we can also make some quality adjustments other HDR methods do not allow, such as removing chromatic aberration or applying sharpening/noise reduction. The biggest advantage of this method of HDR management is that this is not only reversible, but does not add the artefacts other methods to, such as hallows, fringing, noise etc. HDR images created by this method just look like normal, full tonal range images, nut with great shadow and highlight detail. Just to demonstrate the power and advantages of using CC’s new ACR Smart Filter method of mapping HDR images, I’ll try to recreate this using only the 1 “correct” exposure raw file. (see next page) Proper HDR conversion Recovered shadow detail from a single raw file. At any time, you can save this file using ‘File/Save As’ as a PSD file, and carry on re-editing it using the Smart Filter in ACR so long as the raw files have not been moved.

Once you are completely finished and happy, you can if you want flatten the image to archive as a TIFF file.

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