Studying Canon Tags

[AttilaTolga]

Hi to all ... I've decided to study all the Canon tags available in .CR2 files and I started this topic for getting help from other forum members about the tags which are not so easily clear for me. Thanks a lot in advance.

My first question: When does the "MacroMode" tag have value of "Macro"?

[Phil Harvey]

This is set for some PowerShot models when the focus range is set to Macro.

- Phil

[AttilaTolga]

Thanks Phil.

What is MeasuredEV2?
Is there a tag for the measured photometric exposure in lux?

For a sample shot, I have AutoISO 100, BaseISO 308 and EXIF:ISO 320.
Applied ISO on the camera is 320.
What are the others?
If you have time, can you please explain them briefly?
Or, can you please advise me a link explaining them, if any on the Internet?

[Phil Harvey]

MeasuredEV and MeasuredEV2 should be a measure of the light level in EV.  These are all just guesses because there is no documentation for the proprietary makernote tags.

- Phil

[krzysiu]

I'd give a shot that AutoISO is always 100 - it could be the preferable ISO for auto, usually the lowest. The camera will try to reach that if the lighting conditions are fine. That "always 100" it's true at least for my camera (details on the bottom). Some cameras have advanced settings for auto ISO (i.e. range) and maybe that's when it's really used.

BaseISO is the ISO which is the best for camera. If you have base ISO, let's say 100, and the current ISO was shown as float, i.e. ISO 0.8 was 0.8*100=ISO 80. And as now Canon doesn't use this tag anymore, it always equals ISO (that's my guess). I hope I won't be accused of spam: if you want to read more about base ISO, there's a lot of great posts there: http://www.dpreview.com/forums/thread/2881374

But why it's 308 in your case if ISO is 320? In my theory it's all about float point conversion. The camera conversion is not ideally precise (as it's not always possible with floats) and it shows the close value. Look at not converted values (-n switch) in my case (format: baseiso[tab]iso; autoiso is ommited, as it's always 100):
100     100
200     200
308.442165081588        320
400     400
616.884330163176        640
800     800
1037.47164372081        1000
1233.76866032635        1250
These values may mean that camera have some constant internal ISO, and it uses not very precise multipler to count base ISO - that'd explain why some numbers are fine and some aren't.

The test was made using Canon 50D (using newest firmware) CR2 files.

So briefly (according to my theory, which could be wrong):
ISO - good one, always use that one
BaseISO - close to good one, but with conversion arifacts, so in case of our cameras useless
AutoISO - more like technical data about camera, not about pic. As you aren't using auto, you shouldn't care about that and if you do, you'd still prefer to use ISO field.

You can make some picture with autoiso with good lightning conditions and bad and check if AutoISO changes.