Author Topic: Some questions about RAW Histogram and ETTR  (Read 3906 times)

bouncyball

  • Contributor
  • Hero Member
  • *****
  • Posts: 849
Some questions about RAW Histogram and ETTR
« on: December 05, 2013, 12:22:18 PM »
Hi,

I have some questions about the subject:
1. The histo is a little bit different for LV and Quick Review mode right after the shot. Difference is about 0.4-0.7 stop. QR mode always shows brighter. For example: LV E 0.2 before shot - QR E -0.5 after shot. Is it normal for the current implementation of the raw histogram? 0.7 stop is not a small diff btw. I can assure that the lite condition is not changing during exposure.

2. As I understand RAW histo math heavily depends on Canon's ExpoSim mode?! (I have no enough experience in coding to examine the sources for the algo). Then why on my 5D3 raw histo operates even with ExpoSim mode off (and shoes wrong value) and only shows "ExpoSim off" icon when this mode set to "when DOF button pressed".

3. In some dark but contrasty situation (quite dark wall with some quite bright highlights on it) ETTR looses control e.g. does not show messages "whoops" or "exposure limits reached" and shows ETTR hint for example E 1.5 while severe RGB clips are present like R30 G34 B28.

4. I've been experimenting a little and looking at raw histo with lens cap on (max darkness). Aperture did not make any sense only shutter and iso. I thought that raw algo in this situation mesures just a noise level but I guess I was wrong. When:
a) shutter 1/8000 and iso 100 ettr hint shows E 9.2
b) shutter 1/8000 and iso 25600 ettr hint shows E 11.9 (higher ISOs than this make no sense)
I don't understand those values while knowing that dynamic range is larger and noise lower for iso 100 (11.5 stop) than for iso 25600 (7 or something) with significantly more noise.

TIA
bb

a1ex

  • Administrator
  • Hero Member
  • *****
  • Posts: 12564
Re: Some questions about RAW Histogram and ETTR
« Reply #1 on: December 05, 2013, 12:36:08 PM »
1. the sensor is sampled differently, so the results are sometimes different; if the error is always in the same direction, it can be adjusted

2. yes (will check, but all exposure overlays should disappear when turning off expsim)

3. SNR limits

4. will check

bouncyball

  • Contributor
  • Hero Member
  • *****
  • Posts: 849
Re: Some questions about RAW Histogram and ETTR
« Reply #2 on: December 05, 2013, 06:39:28 PM »
Hi a1ex

1. the sensor is sampled differently, so the results are sometimes different; if the error is always in the same direction, it can be adjusted
2. yes (will check, but all exposure overlays should disappear when turning off expsim)
3. SNR limits
4. will check

1. Yes it always in the same direction (QR always brighter than LV). I saw differences in range from 0.1 to 0.7 but most often 0.3-0.5.
2. Histo overlay disappears ("ExpoSim is off" warning icon instead) only when "DOF button pressed" setting is set. Last checked on ML build 2013Dec03.
3. I thought so. Will check with SNR values off.
4. Will be interesting :)

regards
bb

a1ex

  • Administrator
  • Hero Member
  • *****
  • Posts: 12564
Re: Some questions about RAW Histogram and ETTR
« Reply #3 on: December 05, 2013, 06:55:53 PM »

bouncyball

  • Contributor
  • Hero Member
  • *****
  • Posts: 849
Re: Some questions about RAW Histogram and ETTR
« Reply #4 on: December 06, 2013, 10:39:44 AM »