Technical photography question using ISO to control exposure?

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zanthus

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Ok, lets say I am using a little point and shoot digital camera...it has mainly automatic settings... but for some reason, even in landscape mode, where I would assume it would pic a low shutter speed and higher fstop number, very frequently it will pick something like 1/500 f2.8 instead of 1/60 f8...(as an example)

Here is my question...everything else being equal, if i take a picture
and the setting says 1/500 f2.8 and I am using iso 100, if i increase
the iso, say to 200, will i accomplish my goal of forcing the camera
to pick maybe something like a 1/250 f 3.5, and if I further increase the ISO, will the camera be forced to select something like 1/125 f5.6???? IE, force the camera to select a lower shutter, higherfstop number by increasing the ISO...

If I am correct, WHY would increasing the ISO cause this to happen...I am missing a basic fundamental of the relationship of how ISO can control exposure....
 
It's really up to the camera. You have the right understanding in the relationship, but if your camera is already doing 1/500 f2.8 in landscape mode, I can't tell that it won't just go to 1/1000 f2.8 if you bump the iso up.

Honestly, your best bet is to just try it. I'd think that once it maxes out its shutter speed it would start closing up the aperture, but it's already a little puzzling that it chose 2.8 for landscape mode.

It sounds like you get it, but since you asked for a "why" even if you were correct: ISO controls how sensitive your film is. The higher the number, the more sensitive. So if you make your film more sensitive, the only way to compensate for it is to either have a faster shutter speed or a smaller aperture, so that less total light hits the sensor. The hope is, as you guessed, that the camera would finally pick a smaller aperture instead of a faster shutter speed and give you a better depth of field, which is typically the priority for landscape photography.
 
As Ramon said, you have the math right, however the camera will do what ever the spot meter inside tells it to. Best bet is check the manual for the location in the viewfinder where the spot meter "looks" for the light setting. Some point and shoots have setting for many points and some have only one a general spot meter. On one of mine I can set up the meter to look in the corners or the center.

This is why I still perfer film, I find it easier to force whatever setting I want.
 
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