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how to know the true exposure when using overrides?

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Offline sphinxxx

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how to know the true exposure when using overrides?
« on: 03 / September / 2010, 11:19:45 »
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Hey I'm a little new to photography. I've been using CHDK for about a week and it's great that I can finally shoot in RAW.


When using overrides, like a shutter speed override of 1/10,000 sec, how can I quickly know the real exposure?

Since the cannon thinks its still at the original shutter speed, it won't calculate correctly and the image will obviously come out very dark.

Also, when using the flash how will I know the true exposure?

Thanks in advance!
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Offline reyalp

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Re: how to know the true exposure when using overrides?
« Reply #1 on: 03 / September / 2010, 23:44:58 »
I don't really understand what you are asking. The "real exposure" is whatever you got. Are you wanting to calculate the correct exposure for a given shutter speed ? You should be able to do this by counting stops from whatever the camera AE would have used.

You could also do this with a script. I believe there are some shutter priority (aka "Tv mode") scripts floating around.

Given the limited adjustment options available on the SD990, it's unlikely you will find many situations where you can get a correctly exposed image with that kind of shutter speed.

If you aren't clear what is meant by "stops", or what shutter speed, ISO, and aperture do, I'd suggest googling for some introductory photography materials.

For flash, keep in mind that the duration of the flash itself is shorter than any shutter speed you are likely to achieve, so the shutter speed will only change how much is picked up from other sources.
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Offline fudgey

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Re: how to know the true exposure when using overrides?
« Reply #2 on: 04 / September / 2010, 01:17:22 »
Maybe he just would like to know how much off (in Ev) the overrided shot is or will be, compared to what it would have been using without the override (in this case due to Canon's autoexposure). This is something I'd like to see as well, maybe we should add it to the override OSD.

As reyalp said, before the shot this can be done with a script: half shoot and wait for autoexposure, remember exposure settings, set override requested in script params (in this case you will not be using the CHDK extra photo operations menu), shoot. These  remembered and requested settings will give the desired information using APEX formulas.

After the shot you can't find this in PLAY mode, but you can investigate the JPEG EXIF tag on a PC later (compare makernotes to the conventional fields and calculate the difference using APEX formulas).
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Offline yvesson

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Re: how to know the true exposure when using overrides?
« Reply #3 on: 04 / September / 2010, 04:00:22 »
Hej,
Would be great to use the overrides with the canon auto exposure, especially for cams limited to 1/1250s at the max aperture.

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Offline sphinxxx

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Re: how to know the true exposure when using overrides?
« Reply #4 on: 04 / September / 2010, 15:03:32 »
Thanks all, and yes fudgey constructed my question in a more sensible way.

I have been using it and found it's easier to just do some trial and error most of the time, unless I have only one chance to get it right, and then in that case I probably don't have enough time to do all that script stuff anyway.

what I am interested in now is what you mentioend about EXIF data. Will the EXIF data (when viewed on a PC) mentioned the overridden shutted speed if I shot it useing the override in "extra photo ops menu" or only if I did it using a separate script as you mentioned?

I've gotten very used to use the extra photo menu for overrides so it would be nice if the exif data captured that info.

Last thing.. I think what you mentioned about the OSD for overrides mentioning how much +/- EV the shot is compared to the cannon exposure reading, would be a tremendous help.  If no one is working on that perhaps I could help. Not sure what kind of coding is involved but I'm pretty good with learning it usually.

Have there been any updates to the sd990 autobuild since I got it (2 weeks ago).

Thanks again.
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Offline reyalp

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Re: how to know the true exposure when using overrides?
« Reply #5 on: 04 / September / 2010, 16:35:26 »
Quote from: fudgey on 04 / September / 2010, 01:17:22
Maybe he just would like to know how much off (in Ev) the overrided shot is or will be, compared to what it would have been using without the override (in this case due to Canon's autoexposure). This is something I'd like to see as well, maybe we should add it to the override OSD.
Hmm, can you get this from misc OSD values ? I'd need to look at the code to figure out exactly what all those options show, and how what they show compares to overrides...

If not, it should be pretty easy to add one that shows how many stops off you are from the Bv.
Quote from: sphinxxx on 04 / September / 2010, 15:03:32
Have there been any updates to the sd990 autobuild since I got it (2 weeks ago).
The last change was August 21, build 923, which fixed the intermittent raw bug.
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Offline fudgey

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Re: how to know the true exposure when using overrides?
« Reply #6 on: 05 / September / 2010, 03:10:02 »
Quote from: sphinxxx on 04 / September / 2010, 15:03:32
what I am interested in now is what you mentioend about EXIF data. Will the EXIF data (when viewed on a PC) mentioned the overridden shutted speed if I shot it useing the override in "extra photo ops menu" or only if I did it using a separate script as you mentioned?

It's there in either case. CHDK actually overrides (fools) the camera the same way in either case, it's just the CHDK user interface that differs. But it seems that (at least on my a570) the camera does not update its EXIF data between shots in continuous shooting mode. That means continuous mode bracketing shots other than the first one will not have proper values in EXIF but the first shot values instead (this puzzles me as previously I thought they did).

Two examples from my camera: IMG_1509.JPG had shutter speed override enabled from the extra photo operations menu and IMG_6578.JPG didn't:

exiftool -MakerNotes:ExposureTime -ExposureTime -ShutterSpeed -ShutterSpeedValue -TargetExposureTime *.JPG
======== IMG_1509.JPG
Exposure Time                   : 1/9955
Exposure Time                   : 1/2000
Shutter Speed                   : 1/2000
Shutter Speed Value             : 1/2004
Target Exposure Time            : 1/2004
======== IMG_6578.JPG
Exposure Time                   : 1/256
Exposure Time                   : 1/250
Shutter Speed                   : 1/250
Shutter Speed Value             : 1/251
Target Exposure Time            : 1/251

The first one is what exposure time really was (unless you went past the camera's physical capabilities), the next two seem to be what the user interface showed and the last two are probably autoexposure mathematical results.
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Offline sphinxxx

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Re: how to know the true exposure when using overrides?
« Reply #7 on: 05 / September / 2010, 03:34:29 »
fudgey,

So when in playback mode, when the camera reads the files and displays the info on the screen (along with the picture), it's not actually reading the exif data? (or it is but its reading that other exposure time, as in your example, the "1/2000" exposure time).

I've also been reading that people have had the camera display the iso correctly when you set it low (lets say ISO20 override or something), but thats not working on my sd990.

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