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More about image Dynamic Range

Dynamic Range (DR) is most probably the single most discussed aspect of digital photography in forums, magazines and blogs. I believe it has become one of the most powerful marketing tools manufacturers use to influence buyers, buying decisions. How often do we read or hear, the smaller sized sensor has less DR, or this camera had poor DR in camera reviews, or full frame is better than APC sized sensors. I decided to add this short article on dynamic range and hope to remove some of the "street science" from dynamic range. My previous articles on dynamic range looked more at the technical aspects of what dynamic range is and how it effects the image. For this article I decided to take a very practical approach to explain what it is we see, in terms of the possible effects on image quality and how to best work around the limitations of the average digital image sensor. Also see the Blog article I wrote on Dynamic Range……

Dynamic Range (DR) definition:-
Dynamic range is the ability of the digital camera sensor to record details equally good in the shadows, the midrange and the bright areas of the image. To see for yourself how difficult it is for the digital camera to record details in dark areas plus bright areas in the same image you can do the following easy test. Let your partner or a friend stand with his or hers back facing a window in a room. Take a digital camera and point it to the window, preferably with bright daylight on the outside and take a picture. Study the picture and you will see one or more of the following interesting aspects in the image:-

  • Possibly the window area is very bright, the rest of the room dark and the person facing the camera not well exposed.

  • Possibly the darker areas below any furniture in the room has no detail and is only black.

  • No matter how you try, you could not manage balanced exposure.

  • Moving closer or further away from the window just worsened the described effects.


Another good example is when you try to take a picture outside in the city streets with a bright blue skyline. If you expose for the area between the buildings and down on street level then the skyline seem to over expose. Vise versa if you expose for the sky line then the street level detail will be poor because it will be under exposed. With these two examples I think you will better understand what it is we refer to when talking about dynamic range.
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Digital images are saved on the camera memory stick in 8 bit JPEG format. This 8 bit file format does not have enough information “holders” to record/hold all the image information coming of the sensor on a bright sunny day. One way of expressing this problem is to say that a bright sunny day could have as much as 16 stops of information and we try to compact that into a 8 bit file format. I do not want to go into too much detail in this article realizing that much more theory needs to be discussed in terms of DR. Important to know is that by studying the histogram one can see if an image was under or over exposed plus one can see if the exposure exceeded the dynamic range of the 8 bit file format. For example if the conditions was too bright then the bright areas will go white. In a similar way under exposed areas will just turn into black. Once tuned White or black, these areas does not have any unique shade or brightness information.

When you learn more about working with histograms you will see that it is possible to see exactly when the exposure conditions exceeds the capacity of the 8 bit file format. For example the histogram can tell you if it was a clear or a hazy day. When reading this article one could say that dynamic range are only related to the 8 bit file format. That is also not 100% correct because the actual digital image sensor also has a limited dynamic range. The typical dynamic range of camera sensors range between 8,5 and 13 or 14 stops.

As you learn more about exposure and dynamic range, you will see it is not correct to only consider the image sensor or only the file format. There are several ways to deal with high dynamic range situations plus most important high dynamic range scenes can also assist us to create unique artistic looking images. There are wonderful examples of how photographers used a high dynamic range to create wonderful images or works of art. Please do not allow people to make you doubt you equipment or the camera you bought. Digital cameras in general are good and capable of doing much more than what we expect.

There are several ways to master dynamic range. One can basically divide these methods into exposure techniques or one could do it using software. If you like to know more about exposure techniques then please ask in the forum area. I the next paragraphs I will discuss a few software techniques one can use to increase the dynamic range. Interesting is that the results will again be saved in JPEG format. The difference is that this time we as the artists tuned the final image.
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Adjust DR in Photoshop
The image below is a visual illustration of an image exceeding the dynamic range the file format or the sensor can manage. I prepared the same JPEG image to be under exposed in one case and over exposed in the next. Straight from the camera the bottom of the image is completely under exposed when the sky is correctly exposed or the sky is over exposed when the bottom part is well exposed.

Having said that it is just was not possible to correctly expose all parts of the scene at the same time. No matter what I tried, in one instance it would over expose like on the right and in the next instance it will under expose as in the image on the left.

One way to fix this limitation is to work with RAW images. RAW files are 16 bit files and hold much more information than the 8 bit JPEG file format. That means I can recover details in the over exposed or under exposed parts using the RAW file format.

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Another method is to work with layers in a image editing package like Photoshop or Pixelmator. One can use blending modes and layers masks to step by step develop separate parts of the images and to then join the corrected image parts into one final image. I used this method on the above two pictures. On the left picture I used the mountain plus skyline and on the picture on the right I used the bottom "green" part. The picture below shows the result using this method.

Another method is the so called High Dynamic Range (HDR) method. This option requires a series of differently exposed images plus dedicated HDR software. This is a fun option and very creative depending on the style of the photographer. HDR images can be very natural looking or they can be creative, the choice is that of the photographer.

The images I used for this article was taken with the Olympus E420. With digital SLR cameras its possible to save images in both JPEG and RAW. At the time of taking this image I did not use the JPEG plus RAW option. I therefor only had the JPEG version of this image when writing the article. This is frustrating because one can do so much more with the RAW version. My advice therefor is to save files in RAW plus JPEG whenever you can. As your image editing skills improve you will find it is great fun to work with RAW images and to go back and re-edit previously keeper images

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Conclusion

During the analog film days only aperture and shutter speed were considered to adjust exposure. Film speed also influenced exposures but different to what ISO values does in digital photography. The film speed (ASA) stayed the same for the duration of the type of film used. Instead of having only the 2 variables used in the film days we now have a active third variable, namely ISO. In addition we are faced with limitations as discussed in this article.

In the film days we had only limited access to image editing opportunities. Image editing was a hands on process and it was also expensive to set up a home lab. Digital photography has changed that and today there are a number of free editing packages that are very powerful. In the old days the photographer had to master exposure plus it was critical to do things right the first time. Digital photography has opened the the image taking experience and to combined that with editing software to reduce the limitations of the format we use.

What is the right way and what should be allowed? In the old days photographers had to master their skills based on the given variables. I think its important not to limit digital photography and to allow the modern photographer to work within the new set of new variables we see with digital photography as creatively as possible. In addition digital photography has become so competitive that it is only those who master all aspects of digital photography that will make it to the top.

I hope you enjoyed this article, it surely is a interesting subject.

Siegfried