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need help with ND filter

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need help with ND filter
« on: 20 / February / 2009, 14:44:34 »
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Hello, if I set the ND (neutral density) filter out to force high shutter speeds I get overexposure in bright sunlight.

camera: SD800

I have read the FAQ which states:

The final shutter speed will be the product of "Value factor" times the "Override shutter speed value".

http://chdk.wikia.com/wiki/CHDK_firmware_usage/AllBest#Override_shutter_speed_value


Is there a page that puts this into "layman's" terms?

Re: need help with ND filter
« Reply #1 on: 22 / February / 2009, 20:52:40 »
Can't answer your particular Q, but to can mention one caveat - make sure you're actually using the setting you think you are. In particular, check you're not also overriding ISO or shutter speed and/or using a camera mode the "does it for you".

And how I know this... <G>


Cheers, Rufus

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Offline fe50

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Re: need help with ND filter
« Reply #2 on: 23 / February / 2009, 03:22:38 »
Hello, if I set the ND (neutral density) filter out to force high shutter speeds I get overexposure in bright sunlight.
camera: SD800
I have read the FAQ which states:
The final shutter speed will be the product of "Value factor" times the "Override shutter speed value".
http://chdk.wikia.com/wiki/CHDK_firmware_usage/AllBest#Override_shutter_speed_value
Is there a page that puts this into "layman's" terms?

Hi sf_photo,
the Canon firmware swings in the ND filter in brigth light conditions, e.g. in direct sunlight (that's the equivalence to the highest F-stop number, e.g. F/8.0). Then it calculates the exposure time with this self-choosen ND filter state - if you set the ND filter to OUT, the original Canon firmware don't know this, the calculated exposure setting is wrong & your shots will be overexposed.

You should also override the exposure with CHDK or you can compensate the exposure time manually from the Canon menu.

2nd Question:


"Value factor" OFF disables the CHDK shutter speed override, to enable it set it to 1 (maybe on your camera there are only 2 possible settings, OFF and 1).

The "Override shutter speed" is a value in seconds (64s in the screenshot), multiplicate it with the "Value factor" (set it to 1) and you'll get your CHDK shutter speed...

[64    ] * [1 ] --> 64s shutter speed
[1     ] * [1 ] --> 1s shutter speed
[1/4   ] * [1 ] --> 1/4s shutter speed
[1/100k] * [1 ] --> 1/100.000s shutter speed

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Offline whoever

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Re: need help with ND filter
« Reply #3 on: 23 / February / 2009, 03:49:37 »
Hello, if I set the ND (neutral density) filter out to force high shutter speeds I get overexposure in bright sunlight.
See here: http://chdk.setepontos.com/index.php/topic,2654.msg26940.html#msg26940
for an example of a simpe script that removes ND and compensates exposure with Tv (you cannot do such sort of things using internal CHDK overrides)


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Offline waldo

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Re: need help with ND filter
« Reply #4 on: 23 / February / 2009, 08:00:21 »
 
Quote
See here: http://chdk.setepontos.com/index.php/topic,2654.msg26940.html#msg26940
for an example of a simpe script that removes ND and compensates exposure with Tv (you cannot do such sort of things using internal CHDK overrides)

Actually, SDM has this feature.  Hopefully someday it will be added to the main CHDK build.

 

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