24 Replies Latest reply on May 15, 2008 9:41 AM by Bill_Janes

    ACR 12 bit & 14 bit depth greyscale levels.

      12 bit = 4096 possible greyscale values - I seem to recall that 1 stop of underexposure robs the photographer of 2048 levels.

      14 bit=16384 possible greyscale levels. Does 1 stop of underexposure in 14 bit mode result in the loss of 8192 levels?

      Just curious.
        • 3. Re: ACR 12 bit & 14 bit depth greyscale levels.
          Panoholic Level 2
          > Does 1 stop of underexposure in 14 bit mode result in the loss of 8192 levels?

          No, it does not. Half of the levels would not be used, if the exposure was exactly one stop lower than the theoretical maximum. However, it is very difficult to achieve a perfect exposure to the right, i.e. using the entire dynamic range without clipping, even with exposure bracketing. (Let's ignore the question, how one can verify the exposure while shooting.)

          I don't know what you mean with "underexposure", but as the exposure is reduced, the range of the top 1 EV is getting smaller and smaller. Depending on the dynamic range of the scenery, "underexposure" could start for example in the third EV from the top. The numerical range for the third EV is 2048, i.e. one stop underexposure would reduce the numerical range by 2048, compared to no underexposure.

          Another issue is, that many cameras do not utilize the full bit depth. For example the Canon 30D creates only about 3260 levels instead of 4096; the Canon 40D creates about 12500 levels instead of 16384 @ ISO 100. Accordingly, the range of the top EV is much smaller than 8192, and the ranges of lower EVs are reduced in the same proportion.
          • 4. Re: ACR 12 bit & 14 bit depth greyscale levels.
            Level 1
            G Sch wrote -

            I don't know what you mean with "underexposure"

            In my case 1/10th of a stop shy of 255 for me would be a normal exposure for scenes with a full tonal range - blacks to whites- that fit into the sensors dynamic range (incident light meter re-calibrated to camera's sensor to render whites with detail at 1/10th shy of 255).

            +2/3rds over 255 (+0.66 using highlight recovery) to pump slightly more light into shadow areas, where no fill light is available, if required - depending on my aesthetic goals.

            -0.70 would be 1 stop underexposure from what I term as my normal exposure.
            • 5. Re: ACR 12 bit & 14 bit depth greyscale levels.
              Panoholic Level 2
              > In my case 1/10th of a stop shy of 255 for me would be a normal exposure

              Hold on, something is wrong. Just before you were talking about *raw* data (14bit depth, 8192 levels), and suddenly you switched to *gamma encoded RGB*.

              The original raw data and what you see displayed by the raw converter (no matter which one) are miles apart. This page http://forums.dpreview.com/forums/read.asp?forum=null&message=26905476 demonstrates the issues (never mind, that it is on a Sony thread, the underlying issues are identical for all cameras).
              • 6. Re: ACR 12 bit & 14 bit depth greyscale levels.
                Bill_Janes Level 2
                >14 bit=16384 possible greyscale levels. Does 1 stop of underexposure in 14 bit mode result in the loss of 8192 levels?

                That loss is only thoretical, since the actual number of resolved levels is limited by noise, as explained in this post by Emil Martinec, Professor of Physics at the University of Chicago:

                http://forums.dpreview.com/forums/read.asp?forum=1021&message=27650515

                Let's use the Nikon D3 as an example. The full well capacity is about 65568 electrons and the gain is 4.2 electrons per 14 bit DN (data number). At full well of 65568 e- the photon noise would be sqrt(65568) = 256 electrons, which translates to ± 61 DN of noise using the stated gain. However, the standard deviation at saturation is low since positive noise would be clipped.

                Going down 0.5 EV the electron count would be 46364 ± 215 and the DN would be 11039 ± 51. You can't resolve between individual pixel levels because of this noise. Furthermore, the eye can resolve only about 70 levels in the brightest f/stop of a digital capture (Weber-Fechner law), and the theoretical 8192 levels in the brightest f/stop are far beyond what would be required even for the most demanding application with the most extreme editing.

                The Nikon lossy compression makes use of the above facts. Their 14 bit NEF compression records only 2753 of the 16384 levels by throwing away superfluous levels in the highlights, all without any perceptible loss of image quality. I expect some to challenge this assertion of visually lossless compression, but doubt very much that they will have any data back up their challenge.

                Exposing to the right is important for achieving the best signal to noise ratio. The number of levels in the highlights is not a proper justification, but the number of levels in the shadows could be important if noise were not a limiting factor.
                • 7. Re: ACR 12 bit & 14 bit depth greyscale levels.
                  Level 1
                  Can we keep this to a less technical level. If two RAW files were exposed of the same scene under the same lighting (contrast range that fits the sensor), would the image underexposed by one stop less than 255; as judged in a RAW converter (gamma corrected), have substantial less greyscale levels in comparison to the image exposed to 255?

                  Whether our eyes can observe the differences further down the workflow chain, after Photoshop adjustments, is another topic.
                  • 8. Re: ACR 12 bit & 14 bit depth greyscale levels.
                    Bill_Janes Level 2
                    >Can we keep this to a less technical level. If two RAW files were exposed of the same scene under the same lighting (contrast range that fits the sensor), would the image underexposed by one stop less than 255; as judged in a RAW converter (gamma corrected), have substantial less greyscale levels in comparison to the image exposed to 255?

                    I think that everyone would agree that with linear integer encoding one would lose half the possible levels with one stop of underexposure. If you encoded in log or floating point, the loss would be less. If you reduce the bit depth and introduce gamma encoding, levels can be lost as shown by Bruce Lindbloom's levels calculator.

                    If you ignore the effects of noise and the limitations of human perception, then I think the analysis is meaningless.
                    • 9. Re: ACR 12 bit & 14 bit depth greyscale levels.
                      Level 1
                      Bill wrote,

                      "I think that everyone would agree that with linear integer encoding one would lose half the possible levels with one stop of underexposure."

                      Bill,

                      This reply takes me back to my original question, answers it, is what I suspected, and Jeff Schewe has also confirmed. Gabor, IMHO, although in good faith, has over-complicated matters.

                      I seem to recall Bruce on this forum, approximately 3 years ago, also stating with eloquent reasoning, that half of all linear levels were lost by 1 stop of underexposure.

                      I am aware that a digital camera perceives light in a linear manner and that it is necessary for a RAW converter to redistribute the linear tonal information to correspond closer to the way we perceive brightness (gamma correction).
                      • 10. Re: ACR 12 bit & 14 bit depth greyscale levels.
                        Bill_Janes Level 2
                        >I seem to recall Bruce on this forum, approximately 3 years ago, also stating with eloquent reasoning, that half of all linear levels were lost by 1 stop of underexposure.

                        Yes, and you can add Thomas Knoll to the list of luminaries making this statement. However, when you take into account noise and the limitations of human vision, the number of useful levels lost in the highlights is much less. While the highlights in raw files have more levels than is actually needed in most cases, the effect of one stop underexposure also halves the number of levels in the darker f/stops and this can have practical significance. With underexposure, you should worry more about noise and posterization in the shadows.
                        • 11. Re: ACR 12 bit & 14 bit depth greyscale levels.
                          Level 1
                          >would the image underexposed by one stop less than 255; as judged in a RAW converter (gamma corrected), have substantial less greyscale levels in comparison to the image exposed to 255?

                          Yep...more or less.
                          • 12. Re: ACR 12 bit & 14 bit depth greyscale levels.
                            Panoholic Level 2
                            > Can we keep this to a less technical level

                            Your question is of technical nature. You are already confused; you should either drop the issue, or try understanding it. "Dumbing down" the answer does not help.

                            You are talking about *raw levels*. The top stop from the maximum possible exposure (pixel saturation) occupies half of the levels the camera can create in that given situation. However, the RGB value 255 does *not* correspond to the maximum pixel value.

                            Furthermore, the red, green and blue pixels are usually not equally exposed. For example it is typical in daylight, that the red raw channel is at least one stop lower than the green, and the blue is in-between. That means that starting out with the maximum exposure without clipping, the upper half of the range is not used by the red channel. When you reduce the exposure by one EV, then all pixel values will be halved. Thus the green pixels "release" or "lose" one half of the entire numerical range, but the red pixels "release" or "lose" only one quarter of the range.

                            I put a demo together in form of a layered TIFF, with explanation inside:

                            http://www.panopeeper.com/Demo/ExposureAndRawLevels_Demo.tif
                            • 13. Re: ACR 12 bit & 14 bit depth greyscale levels.
                              Level 1
                              > "Dumbing down" the answer does not help.

                              FWIW - I've gotten benefit from the more technical descriptions from
                              yourself, and Bill. The OP does have a point, however that there are ways
                              to explain these concepts that are clearly understandable to a reasonably
                              educated person who lacks the deeper technical foundation. It is no mean
                              feat to accomplish that without dumbing down the answer.
                              • 14. Re: ACR 12 bit & 14 bit depth greyscale levels.
                                Level 1
                                G Sch wrote -

                                "Furthermore, the red, green and blue pixels are usually not equally exposed. For example it is typical in daylight, that the red raw channel is at least one stop lower than the green, and the blue is in-between."

                                I am fully aware that the green channel will be ahead of the red and blue channels due to the nature of the Bayer filter. I have several of my own images of a GM colorchecker that have have been put through Dave Coffins DCRAW. It's straightforward to select a grey patch and witness the RGB imbalance - I was aware of this some 3 years ago. People may know a little more than you think so it's not wise to assume.

                                Had you been more courteous in your reply instead of suggesting that I am confused (I am not in relation to the question that I posed) and to either drop matters, or understand the issue, I would have responded in a different manner - my earlier post referring to you mentioned in good faith (courteous) - this time I have decided not to display any people skills.

                                I suggest that you try and read peoples posts carefully, before you reply, to gauge an appropriate response, the hint in my original post ended - just curious! I appreciate that this is a skill that cannot be taught.

                                Jeff Schewe answered both of my questions in an appropriate manner, you failed miserably.
                                • 15. Re: ACR 12 bit & 14 bit depth greyscale levels.
                                  Bill_Janes Level 2
                                  >Jeff Schewe answered both of my questions in an appropriate manner

                                  If you consider Jeff Schewe's answers to be appropriate, you are not going into the matter with sufficient depth to understand the issues properly. These issues are more than theoretical, i.e. with current cameras does one gain anything by going from 12 bit to 14 bit? In most cases, no, because of noise. Can you discard redundant highlight details for visually lossless compression? Yes, you can.

                                  Look at the following chart, which uses Roger Clark's data for the Canon 1D Mark II, a large pixel camera with excellent noise characteristics. Data values and noise are shown for highlights, midtones, and shadows according to a Kodak Q-14 target.

                                  The highlight tones (Step A) have a data number (DN) of 3650 ± 13.2. Five levels down at DN 3555, the range is 3633 to 3659. The two values (3650 and 3555) are not significantly different. You have not resolved individual levels.

                                  For the shadows (Step "B"), the DN is 92 ± 2.3. Five levels down at DN 87, the range is 85 to 89, which is significantly different from 90-94. You have resolved at least some of the levels.


                                  http://bjanes.smugmug.com/photos/295632078_ZTR87-O.gif
                                  • 16. Re: ACR 12 bit & 14 bit depth greyscale levels.
                                    Hudechrome-sd9sPI Level 2
                                    Bill, would you please define Data Number? I googled it and came up mildly perplexed, as it has meanings all over the map!
                                    • 17. Re: ACR 12 bit & 14 bit depth greyscale levels.
                                      Bill_Janes Level 2
                                      >Bill, would you please define Data Number? I googled it and came up mildly perplexed, as it has meanings all over the map!

                                      Lawrence,

                                      Data Number in this context refers to the raw pixel value. I don't know if it is formally defined, but DN is commonly used by people who evaluate raw files, e.g. Roger Clark:

                                      http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html
                                      • 19. Re: ACR 12 bit & 14 bit depth greyscale levels.
                                        Level 1
                                        Bill wrote

                                        If you consider Jeff Schewe's answers to be appropriate, you are not going into the matter with sufficient depth to understand the issues properly

                                        Bill,

                                        I actually learnt a great deal from you several years ago, about highlight recovery, it was you that showed me a GM Colorchecker with the RGB histogram imbalance from a linear file. That is all I needed to know to have some understanding of how highlight recovery worked. I am grateful for this help.

                                        I fully understand that a linear file, after demosaicing and gamma correction is a different beast. However I examine RAW images, with the software (ACR, Lightroom, Phase One or RAW Developer) that I use in the field at sporting events, and not Rawnalyze, or other geeky (term of endearment) software for detailed analysis of RAW data.

                                        I go to great lengths to tie in my incident/spot meter to my sensor so that I can expose an image and know where the channels will rest in the RAW converter without relying on inaccurate camera, jpeg derived, histograms - No I don't use Julia Borg's UniWB in my Nikon's to improve the accuracy of the camera's histogram as I have a very accurate RAW workf