How High Can You Go?

How high can you go? How low can you go? At least when the question is ISO...the answer depends on your hardware. In the case of my Nikon D200, high ISO (shown below) means ISO 1600. Low ISO (far below) means ISO 100, so there's a 16 times difference in the amount of light being captured due to the sensitivity settings in the two photos, which were taken one after the other and post-processed in exactly the same way.

View this image larger.

It's important to understand that by increasing the ISO you are not really increasing a sensor's ability to capture light, but rather just amplifying the signal with all the negative implications for Signal to Noise Ratio you might expect.

As a practical matter, bear in mind that noise shows more in dark areas than light areas (witness the lower left of the high noise version of this photo). The implication: in long exposure time and/or high ISO situations, where you know there will be plenty of noise, try to expose so dark areas don't go really dark (at least if this doesn't mean blowing out highlights).

View this image larger.

[Both images:142.5mm in 35mm terms, tripod mounted. High ISO version: 20 seconds at f/36 and ISO 1600; Low ISO version 30 seconds at f/11 and ISO 100.]