Canon white level depends on ISO and/or aperture larger than f/4. Rawspeed can only change white level due to ISO and not aperture.
I’ve checked two Play Raw photos, one from a Canon EOS M6 Mk II, and dt matched the exif data. More investigation is needed.
It would be wonderful if camera manufacturers explained exactly what they meant by their various black and white levels.
Meanwhile, we can explore levels for ourselves with our own cameras. Point the camera at an evenly lit card, vary the exposure (assuming the aperture and shutter speeds are accurate) and examine the numbers in the raw file. Over what range are they linear with respect to the exposure? Are the ratios between R:G:B constant? If the linearity and ratios are not perfect, are they “good enough”? Or can they be fixed in post?
The desired accuracy will depend on the nature of the photography, of course. Do we need scientific precision, or aesthetic usefulness?
2 Likes
30D and optical black area. Rawspeed skips 2 lines from top and uses the 8 lines down. From left it uses all 72 columns.
When checking the optical black area at ISO 1600, There is a brighter line 2 px down from the top. At line 10 there is a grey line and can’t be used for black level.
From left, column nr 72 is grey and can’t be used for black level.
So if your camera has optical black area, take a look yourself because it may differ at different ISO.
BTW, is that a dead pixel in the corner 10 lines down and 72 columns right?
Exif or Rawspeed black levels, which one is best for 7D with these files?
CC0 license.
20230120_M7D6709.CR2 (21.6 MB)
ISO 100 Long exposure noise reduction on. Exif and Rawspeed almost same. Exif 2048 2048 2049 2048 Rawspeed 2048 2048 2048 2048
20230120_M7D6717.CR2 (33.3 MB)
ISO 12800 Long exposure noise reduction on. Exif 2053 2052 2044 2044 or Rawspeed 2040 2040 2019 2019
20230120_M7D6692.CR2 (21.6 MB)
ISO 100 Exif and Rawspeed almost same. Exif 2047 2047 2048 2048 Rawspeed 2048 2048 2048 2048
20230120_M7D6699.CR2 (32.2 MB)
ISO 12800 Exif 2034 2033 2060 2057 Rawspeed 2059 2059 2047 2047
Optical black area for the camera
optical-black-area.xcf (31.3 MB)
The largest difference you report on those files is 25 on ~2050. That is 1.2%.
Is that relevant, i.e. do you see a difference when manually forcing the one or the other value?
I see a clear difference, yes. But I don’t know which one I prefer.
There is an issue at Github talking about using black level from Exif instead of optical black. Better or not?
About white level. I checked here: Adding Support for New Raw Formats - RawPedia
I took pictures with clipped highlights and loaded them in RawTherapee. Thanks to the user guide I could use Navigator to get the raw values Editor - RawPedia
The problem is the part *
The Navigator will show the real raw photosite values after black level subtraction within the range of the original raw data.
So to reset black level that in this case was 512 I just wrote -512.
Now I could check the right white level that is 16382 at this ISO and aperture.
In darktable I loaded the same raw file. For R6, darktable (Libraw?) uses white level Canon writes in Exif (Specular White Level: 14008)
If I activate raw clipping indicator and put white point to 16384 there is no clipping.
But if I move it back a little bit to 16382 I get raw clipping.
So in this case both RawTherapee and darktable shows that the highlights are clipped at 16382.
I tried the same but with Long exposure noise reduction. This is at ISO 1600, but it looked the same at all ISO.
More about R6 from @Thanatomanic here Canon R6 CR3 support · Issue #5901 · Beep6581/RawTherapee · GitHub
Latest Hasselblad has optical black area at three sides.
@LebedevRI, three questions:
- is it possible to feed Rawspeed also with the third side, the right one?
- if yes, is it of any real use?
- the top left corner is bright and not optical black. Will it disturb the calculation of optical black with my BlackAreas settings below?
As far as i’m concerned, black area handling is completely broken
in rawspeed, and i’m not even talking about the cameras.xml part.
1 Like
That’s why you have implemented reading from Makernotes I suppose?
Not really. It’s just that it’s quite not good to hardcode stuff in cameras.xml
that is already in Makernotes. Black area handling needs to be fixed regardless.
1 Like
Just now looked at the link - wow, a lot of work went into that, by golly!
I downloaded it for info and it opens legibly in Coffee Cup (as opposed to NotePad).
Thanks!