Tutorial #5

HDR – What’s that all about?
A beginners guide to High Dynamic Range

Updated: November 2009

HDR or ‘High-Dynamic-Range’ is all the rage at present. These images allow a far greater range between the light and dark areas of a photograph than normal. The purpose is to attempt to represent the human eyes perception of the wide range of intensity levels from the deepest shadows to the brightest sunlight. Studies have shown that the human eye scans a scene taking multiple 'snapshots' - each one with its own individual exposure level. The brain then assembles them together to provide an instant view, and the subsequent memory of it. A camera can only take an 'average' exposure of the whole scene so high-dynamic-range and tone-mapping software go some way towards replicating the 'eyes and brain' imagery.

The basic principle is to take multiple shots of the same scene but with different exposure levels. These images are then combined and 'tone-mapped' with computer software to produce one. Look at the three images below of a small, disused, railway bridge that I took in Shropshire, England.

To get the field beyond exposed correctly leaves the bridge arch in darkness. But increasing the exposure to lighten the arch results in total over-exposure of the field. An image exposed mid-way fails on both points. My eye could see both the arch and the field perfectly– but my camera could not. However good modern technology may be, a cameras sensor is still nowhere near as ‘flexible’ at capturing an image as our eyes (and brain) are.

Help is at hand, though, with HDR. Basically what we do is blend three differently exposed versions of the same image. The details from the dark image combine with the details from the light image to produce one image perfectly exposed – that also seems to have a slight three-dimensional effect, possibly because the effect is so much more like the one we see from our own eyes.

Unfortunately, the initial result from blending these three images has so much dynamic range that a monitor or printer cannot process the information correctly. This is where tone-mapping really does come in useful – by reducing some of the range so that the image can be viewed acceptably.

So how do we do it?

There is simply no point in making an HDR image just for the sake of it. If your camera can take an acceptably exposed image with the given light levels - then take it! HDR comes into its own when it is impossible to take an acceptably exposed image – as with my three bridge pictures. Therefore, the HDR process begins at the site that the photograph is being taken. It is at that point that you will know that your camera will not be flexible enough to handle the contrast in light levels.

So what do we do?

1) Use a tripod. It is imperative that all three images are as near to being identical as possible. A cable release
would also be desirable.
2) Set up your camera so that the aperture remains constant and white balance remains constant.
3) Take three successive photographs with the minimum of time between them, adjusting the exposure
compensation by between 2 and 4 stops each time. (Note: many cameras have an automatic bracketing
mode that can be set to do just this – check your manual).

When you get back in front of your computer, you will need some special software to process the three images. As ever with my tutorials, I want to show you how it can be done for free – and there are numerous free programs available (just search for ‘free HDR program’) - but I have chosen the one that seems to get good user reviews:

FDRTools Basic can be downloaded here

Note:
There is an excellent users manual for the software on the same site so I have no intention of providing a step-by-step lesson on how to use it. Below is a summary of how to process my three bridge images so that you can see the effect of HDR. You will need to experiment yourself with your own images.

Start the program and press the ‘One’ button (this is because you are opening one project, not one picture).

Navigate to your three image files, highlight all three and press OK. Because of the size of the files it may take a little while (especially if you shoot RAW) but eventually three thumbnails of the images will appear in the lower window. Rename the project to something meaningful.

Press the ‘Edit’ button and the three images will be merged together and the result will show in the Navigator screen. You may be disappointed to see that there is probably a blurred effect – this is because, even with a tripod, some movement may have crept in.

Press ‘Alignment, then ‘Align’ and the three images will be adjusted to match each other.

We are now ready to produce the HDR image, so press the ‘HDRI Creation’ button and then the ‘Tone Mapping’
button.

You can now choose between the ‘Simplex’ view and the ‘Receptor’ view – the latter giving slightly more control
over the final image, although I always seem to get better results using the Simplex view.

Adjust the slider bars and curves until you get an acceptable image (the preview window will only show the effect
when you let go of the slider).

Save your image to file and, if required, fine tune in Photoshop or similar.

Roll mouse over image to see original (middle-exposure) version

So here we have it – the HDR image of my original three files. Of course, I could have spent ages masking areas and making adjustment layers in Photoshop– but this was so much quicker and gives a seamless result. In fact, it looks just like I saw with my eyes!

Updated: November 2009: It's been over a year since I first wrote this tutorial and HDR has now become a common 'tool' for the digital photographer. Some have embraced the artistic creations that can be achieved by 'pushing' the processing to the limits and whilst it can give results somewhat like a watercolour painting or pastel drawing it is likely that this will be a short-term fad (rather like starburst filters were in the 1980's). More importantly is the ability to combine an HDR version with the original image in layers and then using blending modes and layer masks to create one with much improved exposure levels throughout the scene but without the dramatic tonal ranges that instantly identified an HDR image in the early days. I aim to add a tutorial on this technique at a later stage.

 

If you want to try HDR but haven’t, as yet, taken your three exposures you can download the three that I have used to make this tutorial from the link below and experiment with them. If you get a better result than mine please let me know.

My three images for you to experiment with are in a zip file here (click on the link then save to disc when prompted)

In the next tutorial we will look at my learning curve with strobism.

If this tutorial was useful to you, please let me know by signing the guestbook. Thanks.

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