Aperture Value, thats a lie

[blackest]

I use old lenses on Pentax, Canon and Panasonic Cameras
For canon I use a chipped adapter that says 50mm f1.4
For Pentax I have a focal length of what I set the shake reduction to and no F value
For my lumix I have no lens.

ExposureTime                    : 1/100
FNumber                         : 1.4
ExposureProgram                 : Aperture-priority AE
ISO                             : 400

ShutterSpeedValue               : 1/100
ApertureValue                   : 1.4
MaxApertureValue                : 1.4
Flash                           : Off, Did not fire
FocalLength                     : 50.0 mm

MeteringMode                    : Evaluative
FocusRange                      : Not Known
CanonExposureMode               : Aperture-priority AE
MaxFocalLength                  : 65535 mm
MinFocalLength                  : 1 mm
FocalUnits                      : 1/mm
MaxAperture                     : 1.4
MinAperture                     : 1.4
FlashActivity                   : 0

FocalType                       : Zoom
AutoISO                         : 100
BaseISO                         : 400
MeasuredEV                      : 5.75
TargetAperture                  : 1.4
TargetExposureTime              : 1/102
ExposureCompensation            : 0
WhiteBalance                    : Cloudy
SlowShutter                     : None

MeasuredEV2                     : 5.75

LightValue                      : 5.6

Ok so the useful bits are iso 400 , shutter speed 1/102  aperture f1.4  focal length 50mm (really 135mm) measured ev=5.75 exposure compensation 0
----Experiment -----
taking a series of 6 shots canon m42 lens f2.8 max aperture
f2.8  12.5
f4.0  11.50
f5.6  10.25
f8     9.12
f11   8.12
f16   6.75   

Ev 14.5 would be a pretty good guess at the real light level I think

Switching to my pentax
first a da 18-55 set to 28mm(28.1) its largest aperture is f4 this time its a different LV value in the exif
-EffectiveLV
f4.0 15.1
f5.6 14.9
f8.0 15.0
f11 14.6
f16 15.1

That seems pretty consistent the bodys saying its around 15EV here this afternoon, the canon maybe 1/2 a stop darker
Next a KM 28mm -EffectiveLV
F2.8 3.5
F4.0 3
F5.6 1.5
F8.0 -0.5
F11 -1.5
F16 -3.0
f22 -4.0

Ok now i'm really confused but then this is a km in manual mode, same lens in AV mode shoots wide open. Is it possible to figure the actual light level and the aperture used?

[Phil Harvey]

It is physically not possible to figure out the actual light level if the camera doesn't know the lens aperture and the light sensor is behind the aperture.

- Phil

[blackest]

It is physically not possible to figure out the actual light level if the camera doesn't know the lens aperture and the light sensor is behind the aperture.

- Phil

What if you know more than the camera e.g with the canon the camera thinks it is behind f1.4 and I know it's behind f2.8 so the Light is 2 stops higher than it thinks it is.

I just don't know what to make of the Pentax values + and - what does it think it's seeing?


Thank you for replying, no one can know this tool better than you
Its very useful
exiftool -AFPointsInFocus . > focus.txt
and then a search for none finds me all the photo's my camera thinks are out of focus.
kinda wish i could figure a way to call exiftool with an option in lightroom.
It's kind of handy pentax has so few focus points because they line up quite well on the rule of thirds grid.