What's in a Digital Image?

I found a good sample image on the internet at http://www.photographyblog.com/reviews_pentax_k200d_3.php:

1. Original picture downloaded from the source quoted above

(here reduced in size and converted to JPEG format)

I think that most people (myself included) will think that this is a nice, well exposed and balanced picture. However, I also find it interesting because it represents quite a range of brightness from highlight to the deepest shadow. So, nice as it is, does it reveal all information that it contains?

To check that out, let us play a little with this picture using some simple tools in PhotoImpact. One good starting point (to check out pictures - not necessarily to improve them) will always be to change gamma. Lower gamma if the picture is very bright and increase it if it is rather dark:

2. Here gamma has been increased to 1.88 and we readily see, that the bows of the barques are anything than uniformly dark/black. Their colours and textures were hidden from instant recognition  in the original picture. So, although the image has become pale and flat, we have actually gained information. This does actually resemble a bit a well exposed negative copied on soft paper. Even after saving, you may still revert to an image resembling the above using gamma and contrast manipulations only (although the resulting image will be degraded and detail irreversibly lost during such a process).

3. Trying to bring some contrast back to the picture - while preserving the detail in the bows - by simple adjustments of brightness and contrast will most likely end up something like this. This resembles the result one would expect if the image had been shout on analogue film with the exposure set by spot-metering on the shadow parts of the scene. Highlight portions are washed out beyond repair. Likewise, once you have pushed the save button, there is no way you can manipulate this digital image back to something just remotely resembling the original


4. Next, let us take a closer look at the highlight parts of the original. Aren't there some clouds in the sky. Manipulating both gamma, brightness and contrast reveals that there are indeed and that you can bring forward details in the cloud shapes barely visible in the original. Note also how the city's skyline stands out much more clearly. This resembles a somewhat underexposed slide, where exposure has been metered on the sky portion of the picture. Again, you may to some degree manipulate your way back towards the original.


Lesson learned so far: The image that is output from your camera should not be regarded as the final image. Rather, it is your precious negative from which endless variations (of which the above are just extremes) and fine-tunings may be produced for your spectators - unless you have a camera with extremely aggressive post-processing algorithms. There is nothing "unnatural" about this: The enthusiastic photographer used to do the same sorts of things with his filters, papers, chemicals in the dark ages of the old, wet dark-room.

Next question: Are there any means by which you may at the same time in one and the same image enhance detail in both highlight and shadow? (Such as combining 3. and 4. above). In fact, there are and to crudely illustrate the very basic principles let us quicly run through the following sequence:


5.a Use the selection tool in your digital imaging software to select the shdow parts (here the bows)

5.b Use gamma/contrast brightness tool(s) to enhance the selction. (Here "auto-contrast" in PhotoImpact


5.c Merge processed selection with background and select highlight portion (sky)

5.d Use gamma/contrast brightness tool(s) on your new selection (here again "auto-level" was used. Merge new selection with background....

.......and so on, if you like. Now, 5.d. is certainly not something that you would submit for a photo contest. It just illustrates that you may selectively manipulate level distributions in different brightness and colour ranges within the one and same exposure.

In practice one will only rarely work selected areas of the image but rather on selected ranges of brightness distributions in the image histogram and you may achieve (as just one example) something like this:


6. Original image enhanced using curves in PhotoImpact

To learn how this may be done, you will have to follow me through the section on High Dynamic Range Images on the next pages.


Copyright 2009 - Steen G. Bruun