Digital,Camera,Dynamic,Range,I technology Digital Camera Dynamic Range - an Investigation

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Introduction Many digitalcamera sensors have a depth of 12 bits per colour. The bit depthdefines the dynamic range of the camera. However, it is difficult tofind any recommendation by any of the camera manufacturers that weshould be using all 12 bits by working with 16 bit rather than 8 bitfiles. In addition, many of the features of image processing software(such as Photoshop) do not support the 16 bit mode as they may only beused in 8 bit mode. Potentially, the result of using the 8 bit mode,which causes 4 bits of information to be thrown away, is that thedynamic range of the camera is reduced and information is lost. Toovercome this problem, many photographers process their work in 12 bitRaw format, converting to 16 bit tif when the final result is to beprinted. This method of working is detrimental to workflow as itprecludes the use of many of Photoshop's most useful features. Assupport for the 16 bit mode by the manufacturers is half hearted to saythe least, there is a suspicion that the 4 bits, that are lost whenworking in 8 bit mode, do not contain any coherent data; they containonly noise. It is also possible that the 4 bits are partially swampedby noise i.e. the higher order bits of the four contain coherent datawhilst the lower order bits contain noise. Method Thefollowing method was used to investigate whether the 4 'lost' bitscontain any coherent data. The method involves placing a marker in theimage. This provides an easier method of locating the correspondingdata addresses in the 8 bit and 16 bit versions of the files, than byhand calculating the data addresses. As the 4 bits ofinterest are most likely to contain coherent data at the camera'slowest sensitivity, the sensitivity was initially set to ISO 100. Theeffects of raising the sensitivity to ISO 400, ISO 800 and ISO 3200 wastried in later tests. 1. An extremely out of focus imagewas taken and saved as a Raw file. A badly focused image was used toavoid the possibility of any texture in the subject interfering withthe results. All camera parameters, including noise reduction, wereturned off. Sensitivity was set to the camera's minimum of ISO 100. 2 The Raw file was saved as a 16 bit tif using Nikon Capture. 3. A small random section (about 5%) of the image was selected usingPhotoshop's Rectangle Marquee Tool. The image was cropped thusproducing an image of manageable size. 4. A section of the cropped image was filled, using Photoshop's ColourPicker and fill tool, with a distinctive bit pattern (R=hexAA, G=hexAAand B=hexAA) to act as a marker. 5. The image was saved in 16 bit and 8 bit modes to separate files. 7. HexEdit (an excellent freeware editor) was used to view the binarycontents of the two files. The data immediately after the markersection of hexAA entries was located and compared. Results and Conclusions Thetable below provides a small representative sample of results from animage taken using a Nikon D2X at ISO 100. All values are given inhexadecimal. Row 1 indicates the RGB colour channel. Row 2 containsdata obtained from the tif file saved in 8 bit mode. Row 3 containscorresponding data to that in row 2, obtained from the tif file savedin 16 bit mode. Row 4 contains the low order bytes of the 16 bit data. Colour Channel (red, green, blue) R1 G1 B1 R2 G2 B2 R3 G3 B3 R4 8 bit B8 BE 90 B8 BF 90 B9 C0 8F B9 16 bit, ms byte B8 BE 91 B9 BF 90 B9 C0 90 B9 16 bit, ls byte DD C1 B9 2B 81 B9 A1 0F 0B 15 Theresults for the low order bytes in the fourth row of the table indicatethat there is no correlation for each colour channel, in either valuesor bit patterns. Taking the red channel results, for example, R1 = DD,R2 = 2B, R3 = A1 and R4 =15. This would appear to be random data thataffects all of the four bits. Similar results have been obtained fromall the other locations that have been analysed. Comparing the second and third rows, it can be seen that there is avery close correlation as the difference between the data values neverexceeds unity. This difference in some of the values indicates thatnoise is affecting the least significant bit of the higher order byte. Further results have been obtained for the D2X at ISOs of 400, 800 and3200. They show the higher order byte of the 16 bit word formatbecoming progressively swamped by noise as the ISO is increased. Similar results have been obtained for the Nikon D100. Implication Theimplication of these findings is that, for the Nikon D2X and D100,working with the 16 bit mode or the Raw format to maximize dynamicrange is ineffective as no coherent data is lost when using the 8 bitmode.