Lab : HDR photography explained, definition and realization.

HDR, definition
Application to photography
HDR photography, realization (shooting)
HDR with Photoshop CS2
Conclusion

Application to photography

In the figures, our HDR file is thus exceptional and makes it possible indeed to quantify any scene with high contrast. Now remain to inform these informations in the field so we can use them ! There are two ways of doing this, only one relly being “HDR”:

Multiple RAW developpment.

A raw file contains more information than a jpg file, being quantified on 12, 14 or 16bpc instead of 8. A simple way to extract all the raw informations is thus to develop different versions from a single raw, one under exposed, one neutral, one over exposed, and merge them to an HDR file. However, as said above, even if the dynamic range of a raw is wider than that of a jpg, it doesn't have anything extraordinary since it corresponds in fact to the real dynamic range of the sensor, which is limited. We could have 32bpc file, if the sensor "fill" only 10bpc informations, this is non sense.
Moreover, if this technique allowed a few years ago to recover some delicate shots, today derawtisers are very sophisticated and allow to directly achieve suchs work, thanks to precise adjustments on the curves and to certain special functions (in particular the recovery highlights).

Multiple exposures.

The real HDR shooting method consist in taking multiple exposures of the same scene, at different durations (by making “bracketing”), for successively collecting different part of the scene (in term of brightness/chrominance), and then merging these exposures in only wide one. We can, for example, shoot once under expose by 2 stops, another one at neutral metering, and a third one over exposed by 2 stops. The under exposed shot will give us high-lights informations, over exposed one shadows informations, and neutral one will enable us to join the whole around average information. Here, each shot containing entire sensor dynamic range, with three shots, we get three times the sensor dynamic range (not exactly as each exposure intersect others) !
You can shoot in jpg or raw, but in raw, takes cares to develop them at neutral exposure (0 EV compensation during development).

On the other hand, we see that such technique is doable only in certain conditions. That's why manufacturers works a lot to improve their sensors dynamic range. Indeed, requiring multiple exposures of the same scene, it can be considered only on tripod, with fixed subjects. Even for landscape, you will have to avoid any wind that would ruins sharpness of foliages since each exposure would be different not only in exposure ! Good-byes thus sport shooting, street shooting, portraits, etc…

Rmq : This technique also work with films, by scanning the various exposures then by following the same workflow.

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HDR photography, realization

Now that we understand what is an HDR file and how realizing such file with multiples exposures, we have the theory and tools, so go for it !

Shooting

Use a tripod, perfectly stable, with if possible a cable release. And avoid windy days …

Set your body on manual, select  ISO (usually 100 ISO on tripod), and set your desire aperture.

Measure high-lights and shadows of the scene. For that, in spot metering or Av, point on these zones and note exposure time indicated by the body metering (or use a spotmeter). Without spot metering you can zoom on each zone to get more reliable measures.
Once you have these two values, you have the variation (range) of the scene. The other values should be determined for averages values. Generally, we use variations of 1.5 or 2IL (1IL measuring itself by 1 stop) between each exposure, for 2 stops, it is thus necessary to multiply or divide the exposure time by 4.

Example: Metering get 1/1600èm in high-lights, and 1/8èm in shadows.
While proceeding every IL, we have : 1/1600, 1/800, 1/400, 1/200, 1/100, 1/50, 1/25, 1/12, 1/6. What gives us 8IL variation.
By taking a shot every 2IL, we have five exposures : 1/1600èm, 1/400èm, 1/100è, 1/25èm, 1/6èm.

Note that the majority of the sensors take around 6IL without problems, 8IL should be ok (in raw) but will have less tones in shadows, whereas in HDR, much more tones will be captured in shadows, since over-exposed. Indeed, the response of a sensor is linear, which means that the more light (or photons), the more captured tones, therefore even with only 6IL, an HDR file will be richer in shadows. However, with less than 6IL variation, HDR is not really necessary, unless seeking some special effects.

Manually adjust the exposure time for each shot. Some bodies have bracketing functions for that.

Software tools for HDR.

Once realized your shots, you will have to merge them to form only one full image. This stage is done with some specific softwares able to handle such large dynamic range :

 -Photomatix pro
- Photoshop which has this function since version CS2.
- easyHDR pro, which also has a free version.
- Photogenics HDR

The goal will be to merge the 8bpc images in one 32bpc image, then to reduce this image in 8bpc again, by compressing HDR informations through curves. Indeed, screens and printers, except some rares ones, are able to restore only 8bpc, you won't be able to see your HDR file on a screen such as it is really, you will only see part of it ! This technique of compression through curves from multiple exposures is called “Tone-mapping”.