So, for those of you who are venturing into the big bad world of CHDK and all the wonders it reveals, you will have seen the option in the Extra Photo Operations Menu for ND Filter State.
For those who don't know, an ND filter is a Neutral Density filter and is essentially darkened glass that is placed over the front of your camera lens. This glass allows you to take a photograph in a brightly lit environment without over exposing your image.
The most common example is with Waterfalls and rivers. Have you ever seen those pictures where the water looks so smooth and silky? This is done with a long exposure. Taking an image for approimately 1.5 seconds or more, allows the photographer to capture the motion of the water which leads to this very silky looking water.
However, if you have ever tried to take a long exposure of even half a second, in daylight, that image will be incredibly over exposed/too bright. So we place this dark glass over our lens. The dark glass means that light slowly creeps into the image, without flooding in too fast. This allows our picture to capture the movement of the water over time, e.g for 2 seconds, without too much light hitting the sensor too quickly.
The result? A nice balanced exposure, but that great smooth looking water from our long exposure.
Some cameras have a Neutral Density filter built in and those cameras apply the filter to images in this sort of situation. The camera is quite often doing this for the user without the user actually being aware of it.
When the camera has a built in neutral density filter, it will swing the filter down between the lens and the camera sensor to act the same way a filter in front of the lens would work - stopping too much light hitting the sensor too fast. The SX40 doesn't have a built in ND filter but the override in CHDK allows us to have a bash at it!
The options for the ND filter state can sound confusing but now we know how a built in ND filter works, we can understand the odd phrasing. The options for the Neutral Density Filter are Off (self explanatory), In (The filter is swung down in front of the sensor) or Out (The filter is swung out and not in use).
Assuming we had a camera with a built in ND filter we would have the following options with CHDK:
Off - Let the camera choose whether or not to use the ND Filter
In - Force the camera to swing the ND filter down in front of the sensor
Out - Force the camera to stop using the ND filter and swing it out of the way
As the SX40 doesn't have one built in, all we really need to see is Off and In options as Out will essentially be the same as switching it off for us.
When CHDK activates the ND filter state, it overrides our camera to allow a long shutter speed but maintains a darker image. As we don't actually have a built in filter, it can't work miracles and if you were trying a 15 second shutter in daylight, it can't actually override our settings in a way to stop that being over exposed but it certainly works to a point as I demonstrate in the new video.
I hope that's pretty clear and have fun playing around with it :)
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