Croping a picture - how to update crop factor ?

[7tonin]

Hi,
a picture file is croped from ImageSize: 5152x2904 to ImageSize: 640x480
All metadata are copied, but how to update crop factor? field of view?

[7tonin]

I'm happy with this formula, which is the inverse ratio of the diagonals: ird = SQRT( ( 5152 ² + 2904 ² ) / ( 640 ² + 480 ² ) ) = 6.978...
Then read equivalent focal length

Code Select Expand

exiftool -FocalLengthIn35mmFormat filename.jpg
Focal Length In 35mm Format     : 150 mm

calculate 150 * ird = 150 * 6.978... = 1046.80...

Apply

Code Select Expand

exiftool -FocalLengthIn35mmFormat=1046.80 filename.jpg

Then -FOV (field of view) is updated, and maybe some others.

[7tonin]

The follogwing fields are updated (and maybe some others):

Code Select Expand

-FocalLengthIn35mmFormat
-ScaleFactor35efl
-CircleOfConfusion
-FOV
-FocalLength35efl
-HyperfocalDistance

field	from	to
-FocalLengthIn35mmFormat	150 mm	1047 mm
-ScaleFactor35efl	4.4	30.8
-CircleOfConfusion	0.007 mm	0.001 mm
-FOV	13.7 deg	2.0 deg
-FocalLength35efl	34.0 mm (35 mm equivalent: 150.0 mm)	34.0 mm (35 mm equivalent: 1047.0 mm)
-HyperfocalDistance	60.62 m	423.13 m

[StarGeek]

You can check the Composite tags page to see what these tags are derived from.

[7tonin]

Thank you very much !
By the way, I made a mistake in my ird value calculation. But my explaination is still ok.