High Dynamic Range images are a way of producing images that have detail in the extreme shadow areas and the near white highlight areas. The average digital camera is capable of recording about 6 or 7 stops difference between the darkest and lightest tone, HDR can extend this by at least 2 or 3 stops at each end of the range. This capability is of most use for landscape work and building interiors. Ideally the camera should be mounted on a substantial tripod and a cable/remote release used.

To produce the files requires a series of images to be taken covering a range of exposures either side of the ‘correct' exposure. The minimum range is +/- 2 stop i.e. three frames, I usually take +/- 2 and 4 stops ie five frames. It is best to do these using RAW files to maximise the data available. If you are tempted to use jpegs remember each file will have at least 25% of the available data discarded by the camera. It is also advisable to make the exposure adjustments using shutter speeds rather than apertures so the DoF is not altered between shots. How you archive this can depend upon your cameras features. Auto bracketing in AP mode is the obvious choice. The EV +/- feature can be used but this can entail touching the camera between shots which can cause movement or vibrations even with a good tripod unless a remote or cable release is used.

A more accurate method is to use a spot meter, either your incamera spot meter (for the greatest accuracy when taking these readings use the longest focal length on your lens) or for preference a hand held 1 degree acceptance angle spot meter. Take and note a reading from the highlight and from the shadow areas. For a scene such as the old foundry the highlight reading was EV14 (1/125 @ f8 ISO100) the shadow reading was EV6 (1 @ f8) so intermediate exposures of 1/60, 1/15, and 1/4 @ f8 will give us five frames at a spacing of EV2 to cover the whole dynamic range of the subject.

When processing the RAW files you need to modify the methods detailed in the RAW processing worksheet. Do not use the eyedroppers and make sure all Auto boxes are left unchecked as shown below. Change the W/B to the correct setting and save the file without any cother corrections or processing.

Save these files as 16 bit Tiffs.

You should get a series of images looking similar to those below.

-1stops

as metered

+ 1 stop

If you are using PS CS2 you can merge these files by using File>Automate>Merge to HDR

I prefer to use a programme called Photomatix Pro, details can be found here . This can be used as a stand alone or as a PS plug-in. I prefer the former and think the extra 10€ is well spent. The files are merged using the 'Standard Response Curve'. When the files have been merged you need to 'Tone Map' the result so it can be viewed on a monitor or printed. Tone mapping is pretty straight forward after a bit of trial and error. The Photomatix site can explain the fine details far better than I can, the FAQ's make very interesting reading. The above files produced this result.

The images appearance after merging may seem disappointing

After tone mapping it is spectacular, in fact tone Mapped HDR images can take some getting used to.

You can also use HDR to improve the result from a single RAW file by making three images in your RAW conversion programme at the exposed value and +/- 1 stop; merging the results and tone mapping. As above enter the correct WB and leave all the auto boxes unchecked. This can recover nearly blown skys but as you are only using the total dynamic range from one file the result will never equal the multiple exposure method, however the results can be far better than the original image. The shot below of the Tower of the Winds at Shugborough shows this, the centre frame is as shot the others two are +/- 1.5EV. The larger shot is the tone mapped HDR image. This shot also shows why multiple exposures are preferable to making three 'exposures' from one frame, the cloud in the top right hand corner is still 'blown'.

A slide show produced using this technique can be viewed here

 

You can also get some startling results by tone mapping a 16 bit Tiff file on it's own without any HDR blending.

The original on the left, on the right a 16 bit version that has been tone mapped three times using different values.

 

The Photomatix Pro also has a feature to blend various exposures to maximise highlight and shadow detail. See here.