Tag Archives: Photoshop

High Dynamic Range (HDR)

Scenes with very high dynamic range (containing very bright and very dark sections) are difficult to capture in a single shot without either blowing out the highlights or underexposing the shadows. Although common sensors in current DSLRs have 7 to 8 stops of dynamic range, many natural scenes would normally have a much greater dynamic range, too large to be captured accurately, even though they would look perfectly normal to our eyes. This is because each photoreceptor in the retina can adjust its gain independently (or almost independently) of each other, while an ISO setting in the camera applies equally to all pixels in the sensor. HDR imaging is a way to extend the dynamic range of a capture by taking sequential exposures of the same scene with bracketed settings. High-end DSLR cameras can do this automatically after dialing in the desired bracketing sequence and interval. Typically one shot is taken at the “correct” exposure indicated by the metring system, while additional shots are taken above (say +1, +2 or +3 EV) and below (say -1, -2 or -3 EV). The trick is to get all highlights and shadows correctly exposed in at least one of the exposures in the series. The images in the series are then combined to generate an HDR image using specialized software. Later editions of Adobe Photoshop can do this. Dedicated HDR software, such as Photomatix, is however easier to handle and yields very nice results.

The three photographs below -taken during a recent visit to Pompei- were shot at 1/125, 1/500 and 1/2000 sec, respectively, on the EOS 40D with EF-S 10-22mm lens at 15mm, ISO400, f/11.0 (always apperture priority for HDR). The corresponding histograms are shown under each image:


Detail in the shadows is totally lost in the first image, but all highlights are inside the histogram and the sky exposed correctly. The opposite aplies to the far right image: shadows are now resolved but highlights are completely blown out. The middle image is the “normal” exposure, what one would have been left with from this scene had one not decided to take bracketed images.

Below is the HDR image produced by Photomatix from the three photographs:


A well exposed, nice image, with detail in the highlights and the shadows, just like my eyes saw it! Incidentally, the marked diagonals add to enhance the composition here. Very pleased with this one!

Another Raw conversion: Pompei

Here is one classic Pompei shot with Vesuvius in the back. Midday sun, crowds all over the place, too bright sky, poor contrast:

Cropping (in Camera Raw) allows focusing on the center temple and eliminates the worst part of the crowd:

Here is the Raw conversion: mainly tone curve for added contrast, graduated filter to pull back the sky and adjustment brush to bring back top of the building and columns that were too dark after the grad filter:

Here are the histograms before (left) and after (right):

One final clone tool in Photoshop removes the unwanted visitors. Voilá:

Posted in POTN here.

Capturing RAW

I am often asked why I shoot my photographs in RAW format. RAW capture refers to the direct transfer of the information acquired by the sensor of a digital camera to the memory card without any in-camera processing. In the 18-megapixel Canon EOS 7D, this translates into files of 25MB, compared to the 6MB of a high-resolution JPG-compressed file. Why would one like to shoot RAW files? The RAW format contains all the information captured by the sensor and is therefore most amenable to corrections of exposure, saturation, chromatic aberrations and noise during post-processing. Compressed JPG files contain a reduced amount of information and so are much more limited to adjust during post-processing. Why would someone want to adjust a photograph? Shown below are three examples taken during a recent trip to the Otavalo valley in Northern Ecuador. In all cases, the top image is straight-out-of-camera, while the lower image is after conversion in Adobe CS4 Camera Raw (no Photoshop in any of these examples!).


With the early afternoon sun blazing over these beautiful mountains at the feet of the Imbabura and a very bright sky, the top image straight-out-of-camera is flat and lacks clarity and contrast. The sky is overexposed, detail in the clouds is lost, the different tones of greens in the foregorund do not come out as they looked in reality. There is also uneven exposure across the frame. In the second image, all this is corrected. This picture has vibrant colors, even exposure in foreground and background and detail in the clouds. Comparison of the histograms before (left) and after (right) also reveals these differences:

The highlights are clipped (outside of range) in the first histogram but tamed in the second, which also has a more even distribution of tones.

This awesome wide-angle view of the flooded crater of Cuicocha was taken at the wide end of a 10-22mm EF-S lens aided with a circular polarizer B+W filter. The filter did a good job at taming reflections on the lake, giving a deep beautiful blue color to the water. But the top image lacks pop, the clouds are overexposed, and the greens in the foreground are wahsed out. The lower image shows the lake in all its glory, even under such an intense midday sun. A comparison of the histograms before (left) and after (right) tells the story:

Highlights are now better resolved (although some clipping persists) and the tones show a much nicer dynamic range.

This last image was taken at the Cascada del Peguche, a small but very beautiful waterfall near Otavalo. (But… you ought to watch out for those tiny mosquitoes; they are truly deadly!) Here, some small rapids downstream the river called for a long exposure to show off the flowing water. The photograph was taken handheld with a Canon EOS 7D and the100mm 2.8 IS L macro lens at 1/5 second and f/25 (barely open!). At 1/5 sec exposure, image stabilization came really in handy here! The shadows on the left image have no detail, it can barely be made up what’s below those stones on the upper left and bottom right. Neither the grass or the water shine as they should (and did!). On the right, the water is pure silk and the shadows have detail and pop. Much better contrast as well. OBS! no polarizer was used here. (I still need to get a 67mm polarizer for this new lens.) The compressed shadows of the original show very clearly in the left histogram:

All those clipped dark pixels (left) are now back and resolved very nicely (right).

Why capturing RAW? The conversions shown here would have been much more difficult to obtain from in-camera JPG files. On the other hand, the disadvantages are larger files, hence the requirement of bigger memory cards and disk space, and post-processing time at the computer. As the examples above illustrate, the results are well worth the extra effort.