EOS R5 EV does not behave as I expected

[johnrmoyer]

I read the web page at: https://www.canon-europe.com/pro/infobank/low-light-eos-r-system/ where there is a table for EV as a result of setting shutter speed, F-stop, and ISO.

Is the EOS R5 expected to behave differently than the description at the Canon web page?

I made 4 images of the sky with the settings from the table for EV 13. My expectation was that all 4 images at EV 13 would have the same brightness.

The tag named "measuredEV" in exiftool stays the same for all 4 images, as does the tag named "measuredEV2". It seems to me like "measuredEV" might be the scene illumination and be like EV reported by a light meter from before there was automatic exposure.

I made screen shots of the raw histogram as displayed by rawtherapee. The brightness of the sky did not change, but the brightness of the images did change. Maybe ISO in the EOS R5 does not match how ISO is used in the tables?

The Wikipedia page for EV https://en.wikipedia.org/wiki/Exposure_value seems to have a table that is consistent with the table in the Canon web page. 

exiftool -s -G0:2 -exposuretime -fnumber -ISO -autoISO -baseISO -"measuredEV*" -colortempasshot -WB_RGGBLevelsAsShot -lightvalue /Volumes/jrm_photos/2024Jul02_measuredEV/*.CR3
======== /Volumes/jrm_photos/2024Jul02_measuredEV/IMG_0589.CR3
[EXIF:Image]    ExposureTime                    : 1/1000
[EXIF:Image]    FNumber                         : 2.8
[EXIF:Image]    ISO                             : 100
[MakerNotes:Image] AutoISO                      : 100
[MakerNotes:Image] BaseISO                      : 100
[MakerNotes:Image] MeasuredEV                   : 14.75
[MakerNotes:Image] MeasuredEV2                  : 34
[MakerNotes:Camera] ColorTempAsShot             : 5807
[MakerNotes:Camera] WB_RGGBLevelsAsShot         : 2044 1024 1024 1745
[Composite:Image] LightValue                    : 12.9
======== /Volumes/jrm_photos/2024Jul02_measuredEV/IMG_0590.CR3
[EXIF:Image]    ExposureTime                    : 1/1000
[EXIF:Image]    FNumber                         : 5.6
[EXIF:Image]    ISO                             : 200
[MakerNotes:Image] AutoISO                      : 100
[MakerNotes:Image] BaseISO                      : 200
[MakerNotes:Image] MeasuredEV                   : 14.75
[MakerNotes:Image] MeasuredEV2                  : 34
[MakerNotes:Camera] ColorTempAsShot             : 5807
[MakerNotes:Camera] WB_RGGBLevelsAsShot         : 2044 1024 1024 1745
[Composite:Image] LightValue                    : 13.9
======== /Volumes/jrm_photos/2024Jul02_measuredEV/IMG_0591.CR3
[EXIF:Image]    ExposureTime                    : 1/1000
[EXIF:Image]    FNumber                         : 11.0
[EXIF:Image]    ISO                             : 400
[MakerNotes:Image] AutoISO                      : 100
[MakerNotes:Image] BaseISO                      : 400
[MakerNotes:Image] MeasuredEV                   : 14.75
[MakerNotes:Image] MeasuredEV2                  : 34
[MakerNotes:Camera] ColorTempAsShot             : 5807
[MakerNotes:Camera] WB_RGGBLevelsAsShot         : 2044 1024 1024 1745
[Composite:Image] LightValue                    : 14.9
======== /Volumes/jrm_photos/2024Jul02_measuredEV/IMG_0592.CR3
[EXIF:Image]    ExposureTime                    : 1/1000
[EXIF:Image]    FNumber                         : 22.0
[EXIF:Image]    ISO                             : 800
[MakerNotes:Image] AutoISO                      : 100
[MakerNotes:Image] BaseISO                      : 800
[MakerNotes:Image] MeasuredEV                   : 14.75
[MakerNotes:Image] MeasuredEV2                  : 34.5
[MakerNotes:Camera] ColorTempAsShot             : 5807
[MakerNotes:Camera] WB_RGGBLevelsAsShot         : 2044 1024 1024 1745
[Composite:Image] LightValue                    : 15.9
    4 image files read

[p4pictures]

I think there's something odd here and the table is rather confusing.

1st shot - 1/1000s, f/2.8, ISO 100 - EV13

2nd shot - 1/1000s, f/5.6, ISO 200 - you have increased the ISO by 1-stop, but closed the aperture 2-stops

3rd shot - 1/1000s, f/11, ISO 400 - you have increased the ISO by 1-stop, but closed the aperture 2-stops

4th shot - 1/1000s, f/22, ISO 800 - you have increased the ISO by 1-stop, but closed the aperture 2-stops 

So for each shot after the first they are 1-stop underexposed compared the previous shot.

The sequence should be…

1st shot - 1/1000s, f/2.8, ISO 100 - EV13

2nd shot - 1/1000s, f/4, ISO 200 

3rd shot - 1/1000s, f/5.6, ISO 400 

4th shot - 1/1000s, f/8, ISO 800