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ISO...which way to go
Old 01-18-2004, 11:45 AM   #1 (permalink)
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I'm a beginner photog still on the low end of the learning and am looking for some good advice. Up until now, I've been shooting AV priority on my digital rebel and letting the camera set the shutter speed and exposure. I also have always kept the ISO at 100 on every shot. Since I've learned a bit more I've moved to manual (M) but am confused on how I should set the ISO? I've been told to move it to 400 on an overcast day but then again just keep it at 100 to play it safe. Any suggestions? [img]/ubbthreads/images/graemlins/cool.gif[/img]
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Re: ISO...which way to go
Old 01-18-2004, 12:55 PM   #2 (permalink)
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Set your ISO for each lighting situation in order to keep your shutter speed or aperature where you want them. If you have a shot in a dimly lit area and you know your shutter speed will be too slow, bump it to 400 or 800 in order to get a faster shutter speed.
 
 
Re: ISO...which way to go
Old 01-18-2004, 01:18 PM   #3 (permalink)
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To expand a little on Marc's answer...

The term ISO is a carry over from film; the numbers associated with ISO represented the film's sensitivity to light. Lower numbers meant less sensitivity BUT finer grain and better rendering of color.

Higher numbers meant more sensivity especially in low light situations BUT increased grain not as good color rendering.

here are a few links for more on this subject:

Sensitivity (ISO)

ISO discussion
 
 
Re: ISO...which way to go
Old 01-18-2004, 02:36 PM   #4 (permalink)
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Thanks for the input and the links!! I'm much clearer on the subject now!
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Exposure is an equation
Old 01-18-2004, 06:39 PM   #5 (permalink)
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The exposure equation can be summed up thus:

Exposure = reflective light x aperture x shutter speed x ASA/ISO rating (for all intents and purposes they're the same damn thing...)

In other words, a reduction in any of the above factors necessitates an increase in one or more of the others to maintain an equivalent exposure.

Given a particular light reading, the following may be a correct exposure:

1/125 sec @ f/8 @100ASA

Any of the following combinations are also equivalent:
1/250 sec @ f/8 @200ASA
1/125 sec @f/11 @ 200ASA
1/500 sec @f/8 @400ASA
1/250 sec @f/11 @400ASA

You can see therefore that having a higher sensitivity film (or digital equivalent) allows for more depth of field or a faster action or shake-freezing shutter speed given an equivalent amount of light. To make a sharp picture in lower light conditions, you might need that wiggle room. But it's not free, it comes at the cost of a "grainier" lower-definition, less saturated image.

Food for thought.
 
 
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