You may be saying to yourself if I can shoot at higher ISO speeds then why not do so? This may be fine for you or it may only be fine in certain situations. Its all really up to you, but here is a trade off. It is called digital noise, or image noise, or just noise. Digital noise is random color information on your photo that varies in brightness. It is kind of like grain on film except grain is from the emulsion and noise is from the electronic circuitry of the camera. As the technology gets better I expect the noise will become less and less noticeable.
I have a few photos below to illustrate noise. All are with the Canon EOS 5D Mark II, 24-105 f/4 lens, no filter, f/8 aperture, 50mm focal length using the Evaluative metering mode. The camera is on a tripod triggered by a shutter release. Lighting is via a Verilux lamp with a 27 watt natural spectrum fluorescent light. ISO is changed from photo to photo as shown. The digital file was opened in RAW, not altered, then exported to Photoshop. In Photoshop the file was changed to a bit depth of 8, the same area was selected for each enlargement then saved as a JPEG file. Image size was not otherwise changed. If bit depth was left at 16 then the file could not be exported as a JPEG.
First we have the overall scene, a simple still life, not artsy, but colorful, and definitely underexposed. All I did was follow the meter, but that will be for another discussion. When I have time to shoot this again I will, but for now these show the noise and that is what this is all about.
In the still life we have an Economist magazine, the UV filter that normally resides on the lens used for this shot, a pack of gum, some cheap U.S. postage stamps, a roll of ribbon, the manual to my 5D and an Orange County Council BSA shoulder patch. In case you don’t get the joke look at the koala bear holding a cup of tea then note we were a Quality District (koala tea).
I chose an area to enlarge away from the center of the lens. The example area is the same in each photo. I start to see the differences between ISO 640 & 800. Remember, that is my eye, not yours. Others may see noise at different ISOs. The higher ISO photos show the noise best in the green of the camera manual, but noise is still seen elsewhere in the enlargement.
Noise kind of reminds me of reciprocity failure in film. You use a long exposure on film or a high ISO on digital and you get something you may not want. In either case stretching the technology to the extreme renders a photo, but may render some issues with it. I doubt this is a valid comparison, but I can’t help thinking of it anyway.