Lab : HDR photography explained, definition and realization.

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

HDR with Photoshop CS2

At this stage, workflow is specific to each software, while being overall the same : import the various shots, then by the different available tools, set the importance of each shot on the tonalities in the final image, the tone-mapping itself.

To illustrate this article, I had of course to take a few shots. I thus prepare my body, cable release, the whole firmly fixed on my tripod, and being time to leave the house ... impossible, due to wind ! Concrete example that this technique include its own limits ! I thus closed the shutters (house ones !), and decided to shoot a turned on spot in the dark, and get details from the lit bulb, typical cases of extreme cut in the high-lights, even our eyes are all at sea on such variations of brightness

Here is only one shot, in raw, developed with hard curve manipulation to get something usable. Anyway, it is rather dark, and bulbs are completely blown.

By measuring the values, I did get 1/1000èm and 0.3sec, but without spot metering, I started with 1/4000èm by making five shots : 1/4000, 1/1000, 1/250, 1/60, 1/15, 1/3 (thus 2IL of variation each time).
Click here to see all shots.

Inide photoshop, go to File / Automate / Merge to HDR

With “Browse”, go in the directorie containing your shots and select them, then “ok”.

Photoshop opens the files, and handle the merging. A window appears with a composite in 32bpc and the imported images, simply made “ok” without applying adjustments. You thus obtain a true HDR image in 32bpc ! Your screen being completely unable to restore such a wide range of tones, it appears blowned out, which is normal.

To visualize and set up the image such as it could be diffused or printed, it should now be converted, in 16 or 8bpc. It is preferable to convert it into 16bpc, which will make it possible to make final improvements preserving a maximum quality, but it will have then to be converted into 8bpc. In both cases, converting an HDR image into 16 or 8bpc opens a window for adjustment, to choose the manner of compressing these 32bpc into 16 or 8.

In window HDR conversion opened, you have several option. Choose “Local adaptation”, the only option allowing you to custom the curve to compress the values.

Now set the curve as you need, as well as the two adjustments “radius” and “threshold”, in order to find the best compromise, then click on “ok” to get your final image ! Done !

Here typically an image impossible to realize without such techniques !

Attention : If I had had to realize such a shot, I would not have used HDR technique. Indeed, we put ourselves voluntarily in a dirty situation, in order to create an enormeous brightness variation to illustrate this technique (on spot alone in dark environment).
In a case like that, the same spot, on, but in a clear environment like studio lighting in order to counterbalance the variations of brightness generated by the bulbs, would not have been a problem, and the result would have surely been better !

On the other hand, in landscape, especially for sunset/sunrise, it is more difficult to put the scene in a studio lighting setup ...

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Conclusion

We saw that in front of the limited dynamic range of sensors, there is a solution, relatively simple, which allows to get all the tones of a scene even with very high contrast. This solution, HDR photography, contains however constraints and so limits its application to the fixed scenes, without winds nor moving subject, and imposes to work on tripod. If results are convincing, supplying extremely rich images, this technique becomes fast very heavy if one want to generalize it on several photos. For example, if you realize 10 shots during a session, this technique will forces you to take 50 shots (or at least 30), without being mistaken (!!), and to be extremely rigorous not to mix them all !
The manufacturers of sensors must thus continue to improve dynamic range, for maybe being a day able to work in HDR with only one exposure/shot, in 32bpc raw file, and especially sensors able to catch such dynamic range. However, such files are very large in size, and not tomorrow to see a body able to shoot 10fps 32bpc raw files at 10mp !

Now, I have to admit something: although limited, the dynamic range of current sensors allows to answer the great majority of cases, and according to your artistic aspiration, HDR photography is not necessarily useful. None of the photos on this site were realized in HDR, which doesn't prevent me from fully expressing myself through my shots !
Technique is only a tool, and must meet a need, knowing such techniques don't means you should use it every day (the example of the spot illustrates it well, where a studio lighting would have been much more judicious). As an information, the photograph in top of the article does not have anything HDR, it's just a raw developed with a reversed S-curve ; -)

Nicolas Genette, on August 21, 2007