overexposure compensation with SX240 - General Help and Assistance on using CHDK stable releases - CHDK Forum
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overexposure compensation with SX240

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Offline shinnen

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overexposure compensation with SX240
« on: 08 / November / 2021, 13:27:59 »
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 Hi,

I'm trying to use 'custom auto iso'. to create a more even exposure, shooting (white) swans in a nearby pond; and am wondering if anyone can suggest starting settings for 'overexposure threshold', and 'overexposed pixel percentage'. I'm also wondering if the the 'overexposure compensation' value is applied to the entire image or only the overexposed area.
Any help is appreciated.

........ john
 
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Offline Caefix

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Re: overexposure compensation with SX240
« Reply #1 on: 08 / November / 2021, 13:53:18 »
Could be interesting to experiment with http://chdk.wikia.com/wiki/SUIx  8)
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Offline reyalp

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Re: overexposure compensation with SX240
« Reply #2 on: 08 / November / 2021, 16:18:20 »
Quote from: Caefix on 08 / November / 2021, 13:53:18
Could be interesting to experiment with http://chdk.wikia.com/wiki/SUIx  8)
I believe shinnen is asking about the "custom auto ISO" feature for shooting manually. How would a timelapse script for long timelapses be helpful?


Quote from: shinnen on 08 / November / 2021, 13:27:59
I'm trying to use 'custom auto iso'. to create a more even exposure, shooting (white) swans in a nearby pond; and am wondering if anyone can suggest starting settings for 'overexposure threshold', and 'overexposed pixel percentage'.
@shinnen:
I'm afraid I've never used this feature much, and have found the code confusing, so I can't provide a definitive answer. However, https://chdk.fandom.com/wiki/CHDK_1.5_User_Manual#Custom_Auto_ISO says

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Overexp Compensation

A fixed value exposure adjustment applied when the overexposure threshold is exceeded for more than a defined percentage of pixels (see below). Set to Off to disable this functionality.
I read that as saying *if* the percent of overexposed pixels exceeds "OverExposure Threshold", *then* the exposure is reduced by the value set in "Overexp Compensation"

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I'm also wondering if the the 'overexposure compensation' value is applied to the entire image or only the overexposed area.
It can only be the whole image, there is no capability in CHDK to change exposure only for parts of the image.
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Offline shinnen

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Re: overexposure compensation with SX240
« Reply #3 on: 09 / November / 2021, 09:32:34 »
Hi reyalp,
    "I read that as saying *if* the percent of overexposed pixels exceeds "OverExposure Threshold", *then* the exposure is reduced by the value set in "Overexp Compensation"
       Yes, that would seem to be the case.
"It can only be the whole image, there is no capability in CHDK to change exposure only for parts of the image."
    Right OK. I was thinking that maybe in the 'evaluative' setting CHDK was somehow able to affect changes in the over-exposed areas of the image.
   I've tried the extremes of the 'overexposure threshold', and 'overexposed pixel percentage' settings and do not see any difference?
    If what you say is true, it would seem that finding the best Zebra settings, in conjunction with manipulating exposure compensation, would achieve the same results as using the custom auto iso over exposure?
...... john
 
« Last Edit: 09 / November / 2021, 10:05:30 by shinnen »
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Offline Mlapse

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Re: overexposure compensation with SX240
« Reply #4 on: 09 / November / 2021, 10:11:52 »
SUIX can be triggered with a usb remote, so that's covered.
and although SUIx has settings that prevent overexposure, it would only do so below 1 second exposure and i do not think you are looking for that.
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Offline reyalp

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Re: overexposure compensation with SX240
« Reply #5 on: 09 / November / 2021, 13:06:31 »
Quote from: shinnen on 09 / November / 2021, 09:32:34
   I've tried the extremes of the 'overexposure threshold', and 'overexposed pixel percentage' settings and do not see any difference?
Perhaps some other settings are preventing it from having an effect. What shooting mode are you using? According to the documentation, it's disabled in M, Tv and Stitch

It could also fail if the required exposure values are out of the usable range. For example, if you set Min ISO to a lower value than the camera can actually achieve, then the calculation would be incorrect. Or if the shutter speed needed to get the required exposure was shorter than the camera can actually do.
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    If what you say is true, it would seem that finding the best Zebra settings, in conjunction with manipulating exposure compensation, would achieve the same results as using the custom auto iso over exposure?
Yes. It just manipulates Tv and ISO, so in principle you can do pretty much the same using zebra and setting those yourself. 
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Offline shinnen

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Re: overexposure compensation with SX240
« Reply #6 on: 10 / November / 2021, 14:36:25 »
Hi reyalp,
         Yes, I could be asking the camera to do what is not within it's capability. I'll try some other settings. I've been using P mode.
Thanks again for your help.  I appreciate it.
....... john
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Offline Caefix

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Re: overexposure compensation with SX240
« Reply #7 on: 10 / November / 2021, 14:51:10 »
More or less interesting,  ??? there´s a i-contrast setting in record menu...
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Offline shinnen

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Re: overexposure compensation with SX240
« Reply #8 on: 11 / November / 2021, 14:59:15 »
Hi Caefix,
          I don't think i-contrast will effect the kind of change I want. Thanks for the suggestion.
........ john
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