Wednesday, 31 January 2007

HDR.... fad or the future...?


HDR or High Dynamic Range is method of merging images taken at different exposure settings, such that both the highlights and shadows are maintained.

How many times have you tried to take a picture from inside a room and try to get the view out the window as well..? What normally happens is that you can get the view out of the window correctly exposed, but then the room is dark, or the room properly exposed and the view completely blown out. Thats because the dynamic range of your film, transparencies or CCD is limited, so it can't record the whole range of light values present. So you have to choose whether you want detail in the shadows, or in the highlights because you can't have both... until now... 'you mean I can have it all !!!'... well sort of....

You can now take three or more exposures covering the mids, highlights and shadows, stuff these images into a piece of software like Photomatix which then combines the images, performs tone mapping and gives you back an image with everything..... Sounds good doesn't it... so where's the catch ?? Well.... all your images have to be the same, pixel for pixel, so you need a tripod, then you have to adjust your camera up and down at least 2 stops. You now have to process 3 files instead of one and well... the tone mapping can give you some pretty funky results, very artistic but not exactly true to life... (I imagine sometime in the future all this will be done in-camera in the blink of an eye, but until then...)

The web is awash with interesting examples, like here and here. If you want to read a good tutorial, try this one... and have a go.. its lots of fun.

4 comments:

Mauro said...

Quite interesting. My camera has an feature to make 3/5 photos with automatic different exposure and you can select the exposure distance of each one. It is great to capture a mixed outside/insider photo (such as a cave entrance). But I agree with you: it is mandatory to use a tripod to get something pretty.

Neil... said...

Normally, 3 exposures of +/- 2 stops is enough. What you will get with 5 exposures is slightly smoother transitions in the shadows.

There is a way to do HDR photography with 1 image, but your digital camera will have to be able to output in raw, then using your raw converter you create two more files moving the exposure up and down 2 stops..... advantages.. you don't need a tripod, you can make HDR images of things that move... disadvantages... the tonal seperation won't be quite so good, but unless you are a perfectionist you not going to notice that much...

Mauro said...

Thanks for the explanation. I supposed that raw exposure control was somehow worse (my camera has a shorter range: -1..+1). Next time I will try to use it and test the differences :)

Neil... said...

If your auto exposure bracketing only has a range of +/- 1 stop, you can 'trick'it a bit by using the exposure compensation as well. Such that you set the bracketing add +1 of exposure compensation take a photo, then change the exposure compensation to -1 take the second of the sequence, and return it to 0 for the last. Try it out to see if it works on your camera...