Author Topic: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?  (Read 2833 times)

aexiff

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hi everybody,
a picture taken with a Canon PowerShot G12, shows this EXIF results:

Code: [Select]
Scale Factor To 35 mm Equivalent: 10.6
Focal Length                    : 30.5 mm [b](35 mm equivalent: 324.2 mm)[/b]


however the Canon G12 spec says:
5x-Zoom: 6.1 (W) – 30.5 (T) mm
(equivalent 35 mm : 28 (W) – 140 (T) mm)
which suggests a scale factor of about  4,59  ( 1/1.7"  sensor)

ExifTool Version Number : 10.36, OSX 10.10, cmd: exiftool file.jpg

cheers, aexiff

Hayo Baan

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #1 on: December 14, 2016, 02:06:31 AM »
Can you post a small sample file that exhibits the phenomenon (a picture taken with the lens cap on will suffice)?
Hayo Baan – Photography
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aexiff

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #2 on: December 14, 2016, 05:24:02 AM »
thanks Hayo Baan, for having a closer look!
attached, you'll find my example pic.

however, I failed in re-producing such results.
new pictures (and many old ones I checked) show "correct" results, varying acceptably just a little bit:
Code: [Select]
Scale Factor To 35 mm Equivalent: 4.7
Lens                            : 6.1 - 30.5 mm (35 mm equivalent: 28.5 - 142.3 mm)
or:
Code: [Select]
Scale Factor To 35 mm Equivalent: 4.6
Lens                            : 6.1 - 30.5 mm (35 mm equivalent: 27.8 - 139.2 mm)

so, if it's just this single example, that fails, I have no problem in closing this/my case.

thank y'oll for this wounderful exiftool !




Hayo Baan

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #3 on: December 14, 2016, 12:37:14 PM »
Right, as I suspected, this is a cropped image. The scale factor to 35mm equivalent is calculated from the image size (amongst others) so if the picture is resized (which this sample is), the scale factor is going to be off. So basically the scale factor is unreliable :-\ If you really rely on its value, you could e.g. do what I have done and that is create a small table of scale factors for the different cameras in use.
Hayo Baan – Photography
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aexiff

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #4 on: December 15, 2016, 09:46:02 AM »
thanks, good to know!
how'd you find out the cropping?
is there some exif information telling you?
or do you know the different picture sizes of the G12 ?

Hayo Baan

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #5 on: December 15, 2016, 09:59:25 AM »
The image wasn't at full pixel size so that was easy ;)
But now you mention this, I think that if you take an image in camera at non full size (e.g. small), the scale factor will be off too. Never tried this though.
Hayo Baan – Photography
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aexiff

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #6 on: December 15, 2016, 11:52:02 AM »
actually, exiftool calculates for my G12 also for small pixel-size pics the somehow correct Scale Factor To 35 mm Equivalent: 4,6 - 4,7.
to my knowledge, the Scale Factor is calculated as:  (35mm-diagonale) / (sensor-diagonale) , and from there you can calculate the 35 mm Equivalent zoom range as e.g. done in the camera product descriptions & specs - independent of the pixel size you chose for your pictures.
for cropped pics, it may become a bit more tricky to calculate correctly in the exiftool, dependent on manufacturers data provided.

Hayo Baan

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #7 on: December 15, 2016, 12:04:31 PM »
Ah right, that's possible. The scale factor calculation in exiftool is quite sophisticated. For Canon, for instance, exiftool uses the sensor resolution info from the camera to calculate the diagonal. So I guess with e.g. small, that resolution is adapted. Still, if the image is cropped, the info is going to be off.
Hayo Baan – Photography
Web: www.hayobaan.nl

Phil Harvey

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Re: Scale Factor To 35 mm Equivalent for Canon PowerShot G12 correct ?
« Reply #8 on: December 19, 2016, 07:13:00 AM »
If it helps, the Composite tags documentation lists the tags used to derive ScaleFactor35efl.

- Phil
...where DIR is the name of a directory/folder containing the images.  On Mac/Linux, use single quotes (') instead of double quotes (") around arguments containing a dollar sign ($).