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Image-ExifTool
Posted on 2006-05-25 21:32:47-07 by geve
Minolta makernote Fnumber formula
I was comparing the minolta F-values for
- Tag 0x9202 ApertureValue
and
- Tag 0x927c MakerNote / Sub Tag 0x0001 / sub Tag 0x000a FNumber

See my table

#1. First 2 columns is how the camera should calculate the Exif value
F5.6 and F5.7 resulting in the same Exif value 48

#2. The formula for calculating from using the ExifTool formula resulted in green fields when ApertureValue and Fnumber match. This is mainly for the Dimage 5 and 7
I tried to change the formula (try-1 and -2), but was only more succesful for th A1

Do you have any thoughts about this?

Ger
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Posted on 2006-05-26 01:52:18-07 by exiftool in response to 2355
Re: Minolta makernote Fnumber formula
Very interesting. I am convinced that the constants I have used to decode the aperture (and exposure time, and ISO) are correct. You won't be able to solve anything by changing these. (You can see that everyone is in agreement for whole values such as f/4.0 and f/8.0.) The problem is how odd values are rounded off.

I am sure that the inconsistencies arise because cameras use internal lookup tables to convert these values, and that individual elements of the table are tweaked by hand to give "nice" aperture values. I don't think there is a single mathematical formula that can give the same values that a camera displays.

But to me the interesting thing you have demonstrated is that Minolta is not self-consistent within their own model line-up, and they appear to be using different conversion tables for each model. This is truly weird. I bet not even their own software can convert these numbers properly because it is unreasonable to expect a piece of software to maintain a different set of lookup tables for each camera model.

By the way, this is FAQ number 4. But the FAQ doesn't go into as much detail as we have in this thread.

- Phil
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