HDR Photography

High Dynamic Range (HDR) photography is a relatively new genre in photography. The idea behind HDR is to capture much more of the information in a picture that what one can achieve with a single frame. Therefore more comparable with what you can see with your eyes. In order to create a HDR image one typically captures 3,5 or 7 images with different exposures in order to have details in most parts of the photograph. Once the images are captured they are blended by using software. Software such as Adobe Photoshop and Photomatix are examples of such programs. A variety of Photoshop plug ins also exist that can be used

If you look at the "correct" exposure photo on the right then you will

Correct exposure

Over Exposed (+1)

notice that although there is detail in most parts, the engine has little detail, there could be more detail under neath and the background is too bright. So in order to rectify this we take two more photos , one that is 1 fstop over exposed and one that is 1 fstop under exposed. That is the next two images. Please bear in mind that the data in each photograph can be represented by four histograms, namely: Highlight and Shadow, Blue, Red and Green (seeing we are working in an RGB or sRGB color space). Therefore if we are able to capture a wider range of information in the highlights mid tones and shadows, we will have more details in these areas.

 And if we are able to capture more shades of Red and more shades of Green and Mores shades of blue we will end up with a photograph that is vivid and punchy in color and it has details in areas where there would normally just be shadows or over exposure. 

Under Exposed (-1)

In order to take these photographs one can use AV mode, TV Mode or Manual mode. In manual mode one would look at the exposure graph in the view finder and literally take a correctly exposed image, adjust the shutter speed to get an exposure that is fstop under exposed and then adjust the shutter speed again to have a 1 fstop over exposed image. One can also use AV/TV mode and adjust the exposure by using the ev (exposure value) setting of the camera. Or the best way is to use bracketing, where the camera adjusts according to the way you have setup the bracketing.

One last point to consider is whether to use AV mode or TV mode. In AV mode you keep the depth of field constant but you vary the shutter speed. However in TV you keep your shutter speed constant but you vary the depth of field. Be careful of this as you may now have a photo with f stop of f4 and another with f16. This might create problems between areas in your photo. I prefer to use AV and vary my shutter speed. Especially when I photograph static objects. So when you photograph something with movement like the sea, what then? Well one option would be to try TV. The other is to use one correctly exposed RAW file and create three JPGs from it that has different exposures. And then to do the HDR blend with these JPGs.

HDR Blended image of the above three exposures

The last comment to make on this post is a warning. Remember that you are blending images, and subsequently my find that images show artefacts of these actions. Look out for anomalies in the final picture as well as halo's. If these exist it is necessary to carefully clean then up afterwards in software such as Adobe Photoshop. Unless it it is the intention be careful that the HDR effect is not pushed to far. As this can spoil the image a take away the realism of the photo.

Recommended reading: The HDR Book - by Rafel "RC" Conception.

by Francois Venter APSSA
venterfa@ibi.co.za