Full Dynamic Range Tools
Preface HDR photography
Copyright © 2002-2011 Andreas Schömann

### HDR photography - why and what for

Digital photography is fun. Digital photography would be even more fun without those reiterative exposure problems. Annoyingly they occur especially in situations, where light is most beautiful - for instance on days with crystal clear air, cloudless sky and bright sunshine. Digital cameras simply can't cope with the very intensive interplay of light and shadow. Consequences are overexposure in light image areas and/or strong noise in dark areas. "Correct" exposure is often impossible.

Cause for this nuisance is the technical imperfection of todays digital cameras. A thing the human eye manages effortlessly - the clean visualization of even the greatest intensity differences without "overexposure" or "noise" - is utterly impossible for a digital camera.

Exposure series and the result of HDR processing with FDRTools |

The following consideration shows a way out of this unsatisfying situation: what is impossible with one image, can be accomplished with several, differently exposed images. This way all areas of a scene are present optimally exposed, though in different images. The "only" thing one needs to do now is to combine these photos to a single image in a way that lets overexposure and noise disappear.

Such an image is named HDRI (High Dynamic Range Image). An HDRI comprises all information contained in the images of an exposure series. Normal output devices like monitor or printer can not reproduce the information content of an HDR image in a natural looking way. An other step - named Tone Mapping - is needed to make this possible. Tone Mapping converts an HDRI to an LDRI (Low Dynamic Range Image) preserving all relevant details of the scene. The LDRI can then be displayed using monitors and printers.

The figure to the left shows an example for such an exposure series and the tone mapped result obtained with FDRTools.

### Natural and creative HDR image processing

Result of a dramatising image processing with FDRTools |

The tone mapped image resulting from combining three images as shown above looks quite natural. However, achieving dramatising effects with FDRTools is also possible. The premium tone mapping method named Compressor with ease converts boring looking scenes into exciting images.

But there is more to merging images than just scene HDRI and TM. FDRTools offers a method named Creative. Creative allows for mixing images with completely differing content thus broadening the possibilities of HDR imaging and supporting creative photography.

### Preparing exposure series

One of the essential functions of FDRTools is to merge the images of an exposure series into a so called HDR image. When doing such an exposure series you should observe the following:

1. The camera must not jiggle while taking the exposure series. Therefore the following is recommended:
   - Mount your camera to a tripod and look out for save stand of the tripod.
   - If you have a cable release you should use it.
   - If your camera has a so called mirror lock-up function you should use it.
2. Pay attention that your exposure series covers the whole dynamic range of the scene. In the brightest picture all detail of the darkest areas should be clearly visible. Contrary in the darkest picture all detail of the brightest areas should be clearly visible.
3. The images of an exposure series should have exposure differences between two and four EV or f-stops. Using the Automatic Exposure Bracketing functionality that most digital cameras possess can be beneficial.
4. To alter the exposure value vary the exposure time. Keep the aperture value constant.
5. All images of an exposure series should be taken with identical white balance setting.
6. We recommend to use RAW format whenever your camera supports this.