Setting Your ISO

Hotel Del Coronado
Coronado, CA, United States

Canon 5D Mark II

Focal Length 47

Aperture 18

Shutter 20 sec.

ISO 50

This is a picture that I shot of the Hotel Del Coronado, in Coronado, CA.  I used a very low ISO (Exposure) so keep the image crisp.  By keeping a lower ISO you will have less "grain" or "noise" in your image.   Grain is rather of hard to describe so I will provide an example.  The above image was shot with a digital camera - an EOS Canon 5D Mark II.  

The below image is simply a variation of the first image that I retouched in Adobe Photoshop.  In photoshop i selected the Filter > Noise>Add Noise (Noise was within the filter drop-down).  This was done to mimic the effect that using a higher numbered (Faster ISO) would have.  This is an extreme example, and you would probably have to shoot in upwards of 6400 (depending on the model of your camera) to obtain an image with this much noise.  This would only be necessitated by very low light levels.  

By using a faster Exposure (ISO) you in essence using faster film (it is capable of being developed faster, although with digital cameras we aren't actually using film - just a digital file), which enables you to shoot at a higher shutter with the same aperture, thus not sacrificing depth of field.  

This can be useful when you are not shooting with a tripod.  If you are "hand holding" your camera you don't want to shoot at anything slower than 1/60th (1/50, 1/40, 1/30, etc.) of a second.  If you do this there is a chance that there will be motion blur in your shot.

For a more detailed discussion on how Aperture works in conjunction with Shutter Speed please refer to this post.

Unless you like a grainy look, which actually looks appealing for certain subjects, especially if you want a rougher or older feel to them, keep the ISO as low as possible.  

(Above) This is a good example of "Noise" that would be encountered with a (very) high ISO.  Even if  the Mark II's ISO limit was reached (25,600) you would likely not see such visible grain.