Why doesn't the Nikon D90 use ISO lower than 200 in very bright areas? [duplicate]

Question

I've got a D-90 Nikon and a f1.4 lens and I've found in very bright areas like beaches in the Caribbean, when I have the camera doing auto ISO and speed and fixed f-stop, sometimes I get overexposed pictures.

These are shot at 1/4000 of a second and ISO 200, which appear to be the limits. Other than turning down my f-stop, is there anyway to configure this camera to avoid this situation? I presume 1/4000 is the limit on shutter exposure, but what about ISO? I would think a setting below 200 should be possible, but I don't seem to be getting it. There are HI and LO settings when you set ISO manually, but I do not know what these mean nor why they are not used by the auto-ISO setting.

Answer 1

Since the aperture directly affects image composition, it might not be desirable to stop your lens down when you hit the minimum possible shutter speed and ISO for your camera. The LO ISO option is pointless as well, since this is just an image pulled one stop in software to ISO 100 (in your case), which can be done (and probably better) in your postprocessor of choice.

I'm afraid the only choices in this type of situation are to either use an ND filter of appropriate density to cut the amount of light going through the lens or to use a body with a higher minimum shutter speed.

You might be able to use a circular polariser instead of an ND filter, as these are cheaper and also cut light by 1-2 stops, but can also change the character of the image quite substantially.