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Old Mar 28, 2006, 4:37 PM   #1
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Digicams have a user-settable ISO value, e.g., 100, 200, 0r 400. Why is this? Seems you'd want to have the highest possible ISO all the time--or is there a downside to choosing the highest ISO value? Do CCDsdeliver grainy-er images with higher ISOs, as does film? Also, how do they alter the ISO--is there a opical filter/iris between the lens and the CCD plane, or do they bias the CCD with differing voltage values?


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Old Mar 28, 2006, 5:01 PM   #2
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The higher the ISO the more grainy the pictures are. They offer it because in some place you cant use flash so you have to raise the ISO in order to get a bright picture if you dont have a tripod. If you shot without a tripod and have the a long shutter speed you will get a burry picture because your hand isnt still enough.
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Old Mar 28, 2006, 5:29 PM   #3
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So, what's the difference between changing the ISO and changing the exposure compensation setting? If I change the camera from ISO 100 to ISO 400 will that produce the same result as changing the exposure compensation from 0 to +2?
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Old Mar 28, 2006, 6:27 PM   #4
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Exposure compensation works by telling the camera to expose either above or below its own recommended exposure level. Raising the ISO setting increases the sensitivity of the sensor but does not instruct the camera to expose differently than it's own exposure system recommends.

Example: Say that your camera is set to ISO100 and you meter a scene. The camera recommends an exposure of f/8 at 1/250 second. You decide that there is too much light behind your main subject and activate the exposure compensation feature and set it to +1. Now the camera will expose that shot at f/8 and 1/125 second or f/5.6 and 1/250 second -- anyway, one stop more exposure than the camera's recommended exposure.

Now, take the same scene and change the ISO to 200. Now, the camera will want to expose the scene at f/16 and 1/250 second or f/8 at 1/500 second. It's the same exposure that the camera recommended at ISO 100 but the camera settings to get that exposure are different. Changing ISO in a digital camera is like using a film camera with different speeds of film.

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Old Mar 29, 2006, 9:13 AM   #5
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Granthagan is right. I just want to add a couple things:

First, EC on a camera can ONLY affect aperture or shutter speed. Let's say you are shooting in shutter priority because you want a minimum shutter speed - so you set it at 1/500. Now, assume your camera is set to ISO 100 and the camera determines that for ISO 100 and shutter 1/500 the proper aperture is 2.8. Well, what if your lens is only capable of 5.6? Using EC won't keep your shutter at 1/500 because the camera can't get past 5.6. So, the only way to get a proper exposure is to adjust the ISO from 100 to 400 (2 stops worth). Similarly, if you were in aperture priority, using EC may cause your shutter speeds to fall too low and either camera shake or motion blur come into play. So, again increasing ISO may be more desirable than having the shutter speed drop.

The second point is that if you're going to shoot higher ISOs, invest in some noise reduction software from either noiseware, neatimage or noise ninja. They are some terrific products that can really make high ISO images useable.

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Old Mar 29, 2006, 10:29 AM   #6
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I'd just add for the ISO.
Increasing the ISO setting is increasing the electronic amplification of the signal from the sensor.
At the lowest setting you can think of the sensor operating at unity, no amplification cleanest signal.
The higher you go with the ISO the more the signal is amplified and the dirtier it becomes.

Yes lots of different programs like Neat Image and Noise Ninja are there to try to clean up the images taken at higher ISO. Even PS CS2 now has a cleanup filter in it.
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Old Mar 30, 2006, 5:51 PM   #7
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OK, I think I understand now. If I increase the ISO setting, I'm increasing the sensitivity (gain) of the sensor--making it more receptive to light, but adding noise. If I use the exposure compensation, I'm telling the camera aperture or shutter to throw more light on the sensor. So using the EC would produce a better quality shot if you can get away with it--however if your lens is wide open and your shutter speed is slow, the only alternative would be to increase the ISO and put up with the noise.

Thanks for the explanation everyone!

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Old Mar 31, 2006, 5:09 AM   #8
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Using the Exposure Compensation overides the camera's meter. In a low light situation changing the ISO would increase the gain and allow you to use a faster shutter speed. Using Exposure Compensation to increase the shutter speed would underexpose the shot.

The purpose of EC is for situations where the meter is getting the exposure wrong. For example if you want detail in the shadows when the meter is exposing for a lighter part of the scene. By using Exposure compensation you can force a longer exposure to expose for the shadows. On the beach or in snow the camera will tend to overexpose Exposure Compensation can be used to force a faster shutter speed and correct the exposure.
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