Why are there three (!) different exposure times in the EXIF tag?

jakob.01

« on: 03 / August / 2008, 15:01:30 »

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Hi there,

I've got a Powershot A720 IS with Allbest #50. Today I took the first pictures with faster exposure times (using the Extra Photo Operations).
I understand why I can still find 1/2.000 sec in the EXIF tag and why the shutter speed is slightly different (although this is not relevant in this case anyway).
But I'm surprised what I can find in the maker notes (I'm using ExifToolGUI)....
I find an exposure time of 1/103.283, but looking further I find 1/77.398. I would have expected 1/100.000 and 1/103.283. Which exposure time was the "real one"? First I thought that ~1/80.000 is the fastest which my camera can handle, but I get the same factor 0.8 if I set 1/10.000.

There are also three different aperture values. The Canon value is F8. In the maker notes again two different values, F7.5 and F7.4.

Please find the EXIF tag attached.

Any ideas?

Thanks & best regards,
Jakob

IMG_0913.txt (5.97 kB - downloaded 93 times.)

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fudgey

1705
a570is

Re: Why are there three (!) different exposure times in the EXIF tag?

« Reply #1 on: 04 / August / 2008, 18:42:04 »

Hmm... have a look at exiftool or exif MakerNotes documentation to see if MakerNotes can actually hold two fields with the same name. I'll have to guess there's an error somewhere, naming two things the same. ExifTool default command line output isn't particularly good at keeping different categories separate in terms of naming them..

Btw remember that the Tv value you see in MakerNotes (whichever turns out to be correct) may not be "correct" either. For the cameras that have been tinkered with closely, you can set a Tv faster than the camera can actually work, but this Tv will still appear in MakerNotes. So 1/100k may actually be 1/40k and there is no way to know other than experiment to find the limits of your camera's mechanics (by testing with rotating dremel drills or similar or by photographing stable light sources).

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jakob.01

Re: Why are there three (!) different exposure times in the EXIF tag?

« Reply #2 on: 05 / August / 2008, 08:35:31 »

I think there was a misunderstanding from my side. I assumed that the "real" Tv (and Av, ISO, etc.) are written by CHDK to the maker notes. I looked at other files which I shot when I definitely did not have CHDK. There I can find also three different times like 1/30, 1/35 and 1/36.
My conclusion is that everything in the EXIF tag is written by the Canon firmware, not CHDK. The strange figures (1/77k, etc.) were written by the Canon firmware. I could only find 1/2000 because this was still the setting in the UI.

Of course I could have used the Auto CHDK to EXIF tool, but I don't want to have all the actual values in the EXIF tag. If I look at a picture, I want to know which settings I used. Thus it will make more sense for me to change the exposure time from 1/2.000 to 1/10.000 or whatever.

Finding out the fastest Tv is a different topic. I haven't got a dremel drill, maybe I find a different way. If there is someone who knows the actual limits of the A720, I would be grateful...

Best regards,
Jakob

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fudgey

1705
a570is

Re: Why are there three (!) different exposure times in the EXIF tag?

« Reply #3 on: 05 / August / 2008, 16:47:38 »

Quote from: jakob.01 on 05 / August / 2008, 08:35:31

Finding out the fastest Tv is a different topic. I haven't got a dremel drill, maybe I find a different way. If there is someone who knows the actual limits of the A720, I would be grateful...

Check the camera features page in the wiki.

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