Respecting RawExposureBias tag for Fujifilm RAF files
Fujifilm RAF (RAW) files might contain proprietary tag RawExposureBias (exiftool lists it with tag id - 0x9650). This tag tells raw processor how much exposure in RAF file is shifted away from correctly exposed image as it is intended by camera and expected by user.
For Fujifilm X cameras value of this tag is dependent on ISO. I.e, for Fujiflm X-E2:
ISO 200-1600: -0.72 EV
ISO 3200: -1.23 EV
ISO 6400: -2.23 EV
I am not sure why it is done like that. Maybe Fujifilm is exploiting invariant ISO and saving chip cycles or whatever...
Currently darktable ignores this tag and shows initial image underexposed. This is especially evident at XE2's ISO 6400 where image is over 2 EV too dark.
I have uploaded RAFs and camera JPEGs here: https://drive.google.com/open?id=0B0PhIMgeBzO8TUxPTDlvWmhYbTQ
It is the same scene, the same lightning, just different ISO settings and shutter speeds to keep the same EV. Brightness of all images should be the same, and it is with camera JPEGs, but not with RAFs developed in darktable.
With this feature request I am asking to respect this Fujifilm's tag and automatically apply EV correction depending on value in this tag, so that user is freed from doing that manually.
According to discussion on IRC, one possible way of implementing this: turning on exposure module by default for newly imported RAF images with this tag, and automatically entering EV correction depending on value in tag.
Personal opinion: if architecture of darktable allows it, maybe the code of exposure module can be reused and new module "Raw exposure correction" can be created. This module would be executed just before exposure module, would essentially do what exposure module does, but would lack any user's inputs (except ability to turn module on/ff). Exposure correction value would be internally taken depending on mentioned tag, and shown to the user in this module ("Exposure correction applied: X.XX EV"). I think it would be more clear to user in this way than preapplying correction to the slider in exposure module.
#4 Updated by William Brown about 1 month ago
I'm seeing this issue also.
I have dumped the data from my camera with a series of test images (one at each ISO) and EV set to 0 (shutter on A). This yields the following exiftool data:
Raw Exposure Bias : -0.7 ISO : 200 -- Raw Exposure Bias : -0.7 ISO : 250 -- Raw Exposure Bias : -0.7 ISO : 320 -- Raw Exposure Bias : -0.7 ISO : 400 -- Raw Exposure Bias : -0.7 ISO : 500 -- Raw Exposure Bias : -0.7 ISO : 640 -- Raw Exposure Bias : -0.7 ISO : 800 -- Raw Exposure Bias : -0.7 ISO : 1000 -- Raw Exposure Bias : -0.7 ISO : 1250 -- Raw Exposure Bias : -0.7 ISO : 1600 -- Raw Exposure Bias : -0.7 ISO : 2000 -- Raw Exposure Bias : -0.8 ISO : 2500 -- Raw Exposure Bias : -1.2 ISO : 3200 -- Raw Exposure Bias : -1.5 ISO : 4000 -- Raw Exposure Bias : -1.8 ISO : 5000 -- Raw Exposure Bias : -2.2 ISO : 6400 --
Examining this, I took a "test JPG" image for optical comparison. It appears the test JPG is the same as the view -0.7 image. As a result, the correct exposure to apply is not "+2.2" for ISO 6400, but actually +1.5 (2.2 - 0.7).
As a result, you only need 5 presets to exposure to correct this.
+0.1 at ISO 2500
+0.5 at ISO 3200
+0.8 at ISO 4000
+1.1 at ISO 5000
+1.5 at ISO 6400
Hope this helps. I can provide all test images if required.