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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.

Note: Not all cameras have an ND filter. I know that the IXUS 70 (SD1000), IXUS 80 IS (SD1100), IXUS 800 (SD700), IXUS 860 (SD870) and many other similar models have them in place of a real aperture.

Resources

A disassembled IXUS800 (SD700) with photos of lens and ND Filter - http://chdk.setepontos.com/index.php/topic,548.0.html

External Resources

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