To get an idea of the lighting situation inside the temple complex, check out the non-HDR images in an earlier post :Golden Temple @ Bylakuppe, from a Dec 2007 visit to the same place. In that picture, the idols are too bright and the remaining portions are dark. This is indeed an ideal situation to try out the HDR technique.

This HDR is created from three pictures, taken using the AEB settings in Canon EOS 400D. I set the AEB to +/- 2EV in Av mode, f/8, ISO-100 for three images, 1/3s for the base image, 1/13s for the dark image and 1.3s for the bright image. The camera was mounted in a tripod with the Canon EF-s 18-55mm f/3.5-5.6 kit lens @ 31mm.

Dynamic Range is a measure of the camera's capability to collect details from dark as well as bright regions. When the camera's dynamic range is not sufficient enough for a scene, one may use more than one frames with different exposures and merge them using software for a higher dynamic range.

Usually three shots (mid-exposure, under-exposure and over-exposure) are taken but more than three is also possible. All shots should have exactly the same frame but different exposure values. Its advisable to use a tripod for this, since any non-aligment of frames will spoil the merge result. Also, this is possible only for frames with little or no movement. Canon cameras have a setting called Automatic Exposure Bracketing (AEB) which will help to take three different shots with different exposures.

The basic idea of merging is to use the intensity of the image itself as a mask for merging. We should use the mid-exposure image as base and add brighter portions from the under-exposed image as well as the darker portions of the over-exposed image. I have used GIMP to do this but the procedure should be similar in all softwares.

Let me outline the procedure of merging three different exposure pictures for a HDR result in GIMP:

1. Start with the normal exposure picture (lets call it the base image) and open the Layers Dialogue (Ctrl-L).
2. Create a new layer (lets name it the 'Dark Layer') and select that layer.
3. Open the under-exposed picture. Select all (Ctrl-A) and copy (Ctrl-C).
4. Go back to the base image and paste (Ctrl-V). Go to the layer's dialogue, right click on the pasted layer and 'Anchor Layer'. The pasted layer should merge with the 'Dark Layer'.
5. Right click on the 'Dark Layer' and 'Add Layer Mask' (default settings will do). Keep the selection on 'Dark Layer'.
6. Go back to under-exposed picture and convert it to gray scale (Image -> Mode -> Grayscale). Now select all (Ctrl-A) and copy (Ctrl-C).
7. Go back to the base image and paste (Ctrl-V). Go to the layer's dialogue, right click the pasted layer and 'Anchor Layer'. The pasted layer should now be the layer mask for the 'Dark Layer'.
8. Right click on the 'Dark Layer' and 'Apply Layer Mask'.
9. Right click on the 'Dark Layer' and 'Merge Down'.
10. Create another layer (lets call it 'Bright Layer') and selct that layer.
11. Open the over-exposed picture, select all (Ctrl-A) and copy (Ctrl-C).
12. Go back to the base image and paste (Ctrl-V).
13. Go to the layers dialogue, right click on the pasted layer and 'Anchor Layer'. The pasted layer should now be the 'Bright Layer'.
14. Right click on the 'Bright Layer' and 'Add Layer Mask' (default settings will do). Keep the selection on 'Bright Layer'.
15. Go back to over-exposed picture and convert it to gray scale (Image -> Mode -> Grayscale). We need to invert the image. For this open the curves dialogue (Colors -> Curves), move the bottom-left point to top-left and top-right point to botom-right. Save this, select all (Ctrl-A) and copy (Ctrl-C).
16. Go back to the base image and paste (Ctrl-V). Go to the layer's dialogue, right click the pasted layer and 'Anchor Layer'. The pasted layer should now be the layer mask for the 'Bright Layer'.
17. Right click on the 'Bright Layer' and 'Apply Layer Mask'.
18. Right click on the 'Bright Layer' and 'Merge Down'.

The HDR picture is ready. This procedure may be extended to merge more than three pictures as well. To control the layer mask, one can adjust the color curves or edit the gray scale image in step 6 and 15.