3 Replies Latest reply on Nov 12, 2008 6:37 AM by Bill_Janes

    ETTR and ACR

      I've just started to use ACR with CS3 (Windows platform) and wondered if anyone can direct me to on-line resources that help with processing images with regards to the newer controls, Highlight recovery, vibrance, clarity etc.??

      Also, I read somewhere that if highlights are "clipped" when exposing a scene (LCD is "flashing in those clipped areas) that ACR can recover one stop of exposure in these areas? I assume that this means that in theory, you could actually add further exposure to an image once the LCD flashes to warn you that highlights are being clipped.

      Is the info completely gone if the LCD is flashing?

      Any thoughts on this would be appreciated?

      Best,

      JH
        • 2. Re: ETTR and ACR
          Panoholic Level 2
          > I read somewhere that if highlights are "clipped" when exposing a scene (LCD is "flashing in those clipped areas) that ACR can recover one stop of exposure in these areas?

          Careful with this "recovery".

          1. The flashing indicator on the camera's LCD reflects the result of the in-camera JPEG conversion (even if recording only a raw file, a JPEG preview is embedded).

          2. The highlight clipping indicator (the non-flashing red light) shown by ACR reflects the result of the conversion with the currently effective parameters.

          Neither of these stays in direct relationship to "clipping" (saturation) of the raw pixels. Factual clipping may have occured without being indicated, or the indication may show clipping even if there was no raw clipping, even if the shot was actually underexposed.

          Particularly, highly saturated colors may cause clipping in sRGB or Adobe RGB; changing the color space to ProPhoto RGB eliminates most of such clipping.

          Such "clipping" can be created or eliminated by several adjustments (exposure, recovery, brightness, contrast, saturation, but particularly by WB). "Recovery" does not recover anything here.

          Another issue is, when actual pixel clipping occured. *Some* of that may be recovered *under circumstances*.

          If you reduce the intensity (by the "exposure" slider) *a lot* and see a sharp line at the right edge of the histogram, then you can be sure that there was quite much clipping. Small clipping is not shown this way, but you can always check if a certain spot was clipped: try to pick WB on it. If you can, then there was no clipping. If the WB does not change, click once more and you get an error message, explaining that that particular spot is not suitable. The red clipping indication can be eliminated for example by reducing the "exposure", but even then you can not make ACR using that spot for picking WB.

          > I assume that this means that in theory, you could actually add further exposure to an image once the LCD flashes to warn you that highlights are being clipped

          Again, you must not rely on the in-camera indication. If you want to maximize the utilization of the dynamic range of your camera, you have to coax it into making a such a JPEG conversion, that the resulting R, G and B channels resemble the raw channels. This is not a simple subject.

          There has been much written about ETTR in general, but not so much about how to attain it.

          See
          http://forums.dpreview.com/forums/read.asp?forum=1037&message=26905476
          http://forums.dpreview.com/forums/read.asp?forum=1019&message=26534648
          http://forums.dpreview.com/forums/read.asp?forum=1019&message=27554522

          Keep in eyes, that whatever you are doing, the displayed histogram will not be linear; it will reflect the power function applied in the sRGB conversion. The consequence is, that

          1. you can rely on the flashing clipping indication, but
          2. you have to practice it to get a feeling, how much you can increase the exposure (or how much you have to decrease it), because changing the histogram when it is close to the right edge requires much higher adjustment in the exposure than achieving the "same amount of change" while the shot is underexposed.
          • 3. Re: ETTR and ACR
            Bill_Janes Level 2
            >1. The flashing indicator on the camera's LCD reflects the result of the in-camera JPEG conversion (even if recording only a raw file, a JPEG preview is embedded).

            >2. The highlight clipping indicator (the non-flashing red light) shown by ACR reflects the result of the conversion with the currently effective parameters.

            >Neither of these stays in direct relationship to "clipping" (saturation) of the raw pixels. Factual clipping may have occured without being indicated, or the indication may show clipping even if there was no raw clipping, even if the shot was actually underexposed.

            As Gabor indicates, you have to do some experimentation with your own camera to determine how the blinking highlights and camera histogram relate to the raw data. Gabor's program Raynalize is the easiest way to look at the raw data.

            I took some shots of a Stouffer wedge with a Nikon D3 with the camera set to aRGB with the standard picture control settings and compared the camera displays with the raw data and the ACR previews. For the D3, ACR applies a baseline exposure of +0.5 EV, so you have to use an exposure compensation of -0.5 EV. The light source was a 5000K light box.

            Here is the camera histogram indicating exposure just short of clipping:

            http://bjanes.smugmug.com/photos/415568824_PHcTE-O.png

            the corresponding raw histogram:

            http://bjanes.smugmug.com/photos/415568835_GUxuh-O.png

            and the ACR preview with the required -0.5 EV adjustment:

            http://bjanes.smugmug.com/photos/415587225_kFgtv-O.png

            Here is the camera histogram indicating exposure with minimal clipping:

            http://bjanes.smugmug.com/photos/415568791_bAmZJ-O.png

            The raw histogram:

            http://bjanes.smugmug.com/photos/415568796_KW2oZ-O.png

            And the ACR preview with -0.5 EV:

            http://bjanes.smugmug.com/photos/415568785_PB3Qm-O.png

            From the above, it is apparent that the camera histogram and blinking highlights are slightly conservative.

            Now for exposure recovery, I took shots of a Macbeth color checker illuminated by a Solux 4700K bulb.

            Here is the image with proper exposure:

            http://bjanes.smugmug.com/photos/415580816_DbKCi-O.png

            The raw histogram confirming the exposure just short of clipping:

            http://bjanes.smugmug.com/photos/415580822_VBYS5-O.png

            Here is overexposure:

            http://bjanes.smugmug.com/photos/415578667_ekd4n-O.png

            And with highlight recovery in ACR:

            http://bjanes.smugmug.com/photos/415578691_DHoEf-O.png

            Since the brightest (white patch) is clipped, it is not possible to bring the value of that patch below 255 in ACR, but the second from right patch can be brought down and the colors appear reasonable.