Canon "MeasuredEV" and "MeasuredEV2"

[JohnMoyer]

Thanks for exiftool.

I have been confused when trying to understand these two tags. MeasuredEV and MeasuredEV2.

https://exiftool.org/TagNames/Canon.html says

3   MeasuredEV   int16s   (this is the Canon name for what could better be called MeasuredLV, and should be close to the calculated LightValue for a proper exposure with most models)

This is from an EOS R5. For my EOS Rebel T1i and EOS 80D, measuredEV and measuredEV2 are usually closer together. MeasuredEV does not seem to me to correlate with LightValue, but I might guess that it is similar to the EV reading from a light meter before cameras had auto exposure.

Code Select Expand

exiftoo-s -G0:2 -exposuretime -fnumber -ISO -autoISO -baseISO -"measuredEV*" -colortempasshot -WB_RGGBLevelsAsShot -lightvaluee /Volumes/jr_p_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

I based those 4 shots upon a table I found in https://www.canon-europe.com/pro/infobank/low-light-eos-r-system/ and pointed my camera at the sky. I selected EV 13 from the table, but maybe the table is wrong or maybe the EOS R5 does not treat ISO in the same manner as older cameras. The table seems to me to be in agreement with https://en.wikipedia.org/wiki/Exposure_value#EV_as_an_indicator_of_camera_settings
 

My wild guess is that the difference between measuredEV and measuredEV2 might have something to do with the dynamic range of the scene.

I put 4 png files with raw histogram from rawtherapee, 4 CR3 files, and 4 out of camera JPG files at https://www.rsok.com/~jrm/raw/measuredEV/

https://en.wikipedia.org/wiki/Exposure_value#Meter_indication_in_EV

Meter indication in EV
Some light meters (e.g., Pentax spot meters) indicate directly in EV at ISO 100. Some other meters, especially digital models, can indicate EV for the selected ISO speed. In most cases, this difference is irrelevant; with the Pentax meters, camera settings usually are determined using the exposure calculator, and most digital meters directly display shutter speeds and f-numbers.

Recently, articles on many web sites have used light value (LV) to denote EV at ISO 100. However, this term does not derive from a standards body, and has had several conflicting definitions.

Please let me know if more examples would be useful.

Thanks again.

[Phil Harvey]

My guess is that MeasuredEV2 doesn't apply to your model.  From the tag names, I would assume that this had the same value as MeasuredEV at the time it was decoded.  But without more information I can't say for sure.  How do these values change for scenes of varying light value?

- Phil

[JohnMoyer]

For the images I provided when trying to figure out pitch and roll, it seems to me that the relationship between measuredEV and measuredEV2 might have something to do with dynamic range in the scene. https://www.rsok.com/~jrm/raw/tilt_test2.zip

Code Select Expand

exiftool -s -G0:2 -"measuredEV*" -lightvalue *.CR3                                                                                   
======== IMG_0281.CR3
[MakerNotes:Image] MeasuredEV                   : 6.25
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0282.CR3
[MakerNotes:Image] MeasuredEV                   : 6.25
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0283.CR3
[MakerNotes:Image] MeasuredEV                   : 6.25
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0284.CR3
[MakerNotes:Image] MeasuredEV                   : 6.12
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0285.CR3
[MakerNotes:Image] MeasuredEV                   : 6.12
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0286.CR3
[MakerNotes:Image] MeasuredEV                   : 6.12
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0287.CR3
[MakerNotes:Image] MeasuredEV                   : 6.12
[MakerNotes:Image] MeasuredEV2                  : 1
[Composite:Image] LightValue                    : 7.9
======== IMG_0288.CR3
[MakerNotes:Image] MeasuredEV                   : 6.12
[MakerNotes:Image] MeasuredEV2                  : 1.5
[Composite:Image] LightValue                    : 7.9
======== IMG_0289.CR3
[MakerNotes:Image] MeasuredEV                   : 7.00
[MakerNotes:Image] MeasuredEV2                  : 3
[Composite:Image] LightValue                    : 7.9
======== IMG_0290.CR3
[MakerNotes:Image] MeasuredEV                   : 7.12
[MakerNotes:Image] MeasuredEV2                  : 3
[Composite:Image] LightValue                    : 7.9
======== IMG_0291.CR3
[MakerNotes:Image] MeasuredEV                   : 7.50
[MakerNotes:Image] MeasuredEV2                  : 5
[Composite:Image] LightValue                    : 7.9
======== IMG_0292.CR3
[MakerNotes:Image] MeasuredEV                   : 6.88
[MakerNotes:Image] MeasuredEV2                  : 3.5
[Composite:Image] LightValue                    : 7.9
======== IMG_0293.CR3
[MakerNotes:Image] MeasuredEV                   : 6.62
[MakerNotes:Image] MeasuredEV2                  : 2.5
[Composite:Image] LightValue                    : 7.9
======== IMG_0294.CR3
[MakerNotes:Image] MeasuredEV                   : 6.88
[MakerNotes:Image] MeasuredEV2                  : 3.5
[Composite:Image] LightValue                    : 7.9
======== IMG_0295.CR3
[MakerNotes:Image] MeasuredEV                   : 7.00
[MakerNotes:Image] MeasuredEV2                  : 4
[Composite:Image] LightValue                    : 7.9
======== IMG_0296.CR3
[MakerNotes:Image] MeasuredEV                   : 7.00
[MakerNotes:Image] MeasuredEV2                  : 4.5
[Composite:Image] LightValue                    : 7.9
======== IMG_0297.CR3
[MakerNotes:Image] MeasuredEV                   : 6.75
[MakerNotes:Image] MeasuredEV2                  : 3
[Composite:Image] LightValue                    : 7.9
======== IMG_0298.CR3
[MakerNotes:Image] MeasuredEV                   : 6.62
[MakerNotes:Image] MeasuredEV2                  : 3
[Composite:Image] LightValue                    : 7.9
======== IMG_0314.CR3
[MakerNotes:Image] MeasuredEV                   : -0.50
[MakerNotes:Image] MeasuredEV2                  : -29.5
[Composite:Image] LightValue                    : -0.8
======== IMG_0315.CR3
[MakerNotes:Image] MeasuredEV                   : -1.00
[MakerNotes:Image] MeasuredEV2                  : -32.5
[Composite:Image] LightValue                    : -1.0
======== IMG_0316.CR3
[MakerNotes:Image] MeasuredEV                   : 0.12
[MakerNotes:Image] MeasuredEV2                  : -27
[Composite:Image] LightValue                    : 0.0
======== IMG_0317.CR3
[MakerNotes:Image] MeasuredEV                   : 4.12
[MakerNotes:Image] MeasuredEV2                  : -9.5
[Composite:Image] LightValue                    : 4.0
======== IMG_0318.CR3
[MakerNotes:Image] MeasuredEV                   : 5.75
[MakerNotes:Image] MeasuredEV2                  : -3.5
[Composite:Image] LightValue                    : 5.7
======== IMG_0319.CR3
[MakerNotes:Image] MeasuredEV                   : 5.75
[MakerNotes:Image] MeasuredEV2                  : -3
[Composite:Image] LightValue                    : 5.7
======== IMG_0320.CR3
[MakerNotes:Image] MeasuredEV                   : 5.38
[MakerNotes:Image] MeasuredEV2                  : -5
[Composite:Image] LightValue                    : 5.3
======== IMG_0321.CR3
[MakerNotes:Image] MeasuredEV                   : 5.12
[Composite:Image] LightValue                    : 5.0
======== IMG_0322.CR3
[MakerNotes:Image] MeasuredEV                   : 5.12
[Composite:Image] LightValue                    : 5.0
======== IMG_0323.CR3
[MakerNotes:Image] MeasuredEV                   : 5.12
[MakerNotes:Image] MeasuredEV2                  : -5.5
[Composite:Image] LightValue                    : 5.0
======== IMG_0324.CR3
[MakerNotes:Image] MeasuredEV                   : 5.38
[MakerNotes:Image] MeasuredEV2                  : -4.5
[Composite:Image] LightValue                    : 5.3
======== IMG_0325.CR3
[MakerNotes:Image] MeasuredEV                   : 5.50
[MakerNotes:Image] MeasuredEV2                  : -4
[Composite:Image] LightValue                    : 5.3
======== IMG_0326.CR3
[MakerNotes:Image] MeasuredEV                   : 5.38
[MakerNotes:Image] MeasuredEV2                  : -5
[Composite:Image] LightValue                    : 5.3
======== IMG_0327.CR3
[MakerNotes:Image] MeasuredEV                   : 2.12
[MakerNotes:Image] MeasuredEV2                  : -17.5
[Composite:Image] LightValue                    : 2.0
======== IMG_0328.CR3
[MakerNotes:Image] MeasuredEV                   : 0.50
[MakerNotes:Image] MeasuredEV2                  : -25.5
[Composite:Image] LightValue                    : 0.3
======== IMG_0329.CR3
[MakerNotes:Image] MeasuredEV                   : 5.50
[MakerNotes:Image] MeasuredEV2                  : -4.5
[Composite:Image] LightValue                    : 5.3
======== IMG_0330.CR3
[MakerNotes:Image] MeasuredEV                   : 6.25
[MakerNotes:Image] MeasuredEV2                  : -1
[Composite:Image] LightValue                    : 6.3
   35 image files read


[JohnMoyer]

The table on the Canon web page I looked at appears to be incorrect or I was reading it wrong.

instead of EV==13 values in table, they should be

1/1000s, f/2.8, ISO 100

1/1000s, f/4, ISO 200

1/1000s, f/5.6, ISO 400

1/1000s, f/8, ISO 800

What is labeled "measuredEV" in exiftool appears to me to be the illumination of the scene as measured by the camera.

What is labled "lightvalue" in exiftool appears to me to be the exposure and is correctly calculated.

I might also be confused about how things are named.

Thanks again