Many small form factor cameras such as the IXUS/SD series only have one real aperture setting, and that is f/2.8 at wide-angle.
The lens in all point and shoot cameras (and some D/SLR zoom lenses) have a variable maximum aperture (lowest f number), which means if a camera manufacturer lists the camera with the following zoom specs 6.2 - 18.6mm and aperture f/2.8 - 4.9. In this case the maximum aperture will be f/2.8 when the camera is at wide-angle (6.2mm) and increase as you zoom until you reach the telephoto setting (18.6mm) to an f-stop maximum value of f/4.9.
Which is why you may get different aperture values when zoomed in or not on these cameras despite having a single fixed aperture.
How the ND filter in these cameras work
When your camera detects a really bright scene, it'll swing the ND filter in. This in effect makes the scene look darker and will effect the exposure (but not depth of field like a real aperture would).
With the ND filter in, the camera can select a slower shutter speed without over-exposing the entire scene. Also, with the ND filter swung in, the camera will capture an image with an aperture of f/8 - This is a fake aperture value caused by the ND filter swinging in.
With CHDK, you can keep the ND filter out, and just select a faster shutter speed to compensate. In CHDK, you can perform this operation by going to:
Extra Photo Operations -> ND filter state
Just select one of the options from there: Off/In/Out
- 'Off' means the camera is in control of the ND filter state.
- 'In' means the ND filter is swung in.
- 'Out' means the ND filter is not in.
- Not all cameras have an ND filter; all cameras of the Ixus series have a ND Filter instead of a real aperture, also some other ones, e.g. the A450, A460, A530, A550, A560, A1000, TX1.
- Some higher end cameras have both an ND filter and a real aperture, e.g the cameras of the G series.
ND Filter Scripting Functions
get_nd_present() - return status indicates whether camera has an ND filter and/or a diaphragm
set_nd_filter() - 0=auto, 1=in, 2=out
A disassembled IXUS800 (SD700) with photos of lens and ND Filter - http://chdk.setepontos.com/index.php/topic,548.0.html
- Learn more about Neutral density filters - http://en.wikipedia.org/wiki/ND_filter
- Learn more about Minimum/Maximum Apertures - http://en.wikipedia.org/wiki/Aperture#Maximum_and_minimum_apertures
- Understanding Camera Lenses - http://www.cambridgeincolour.com/tutorials/camera-lenses.htm