Recent Travels, August 2020

ISO - Exposure Triangle Gone

Hey Mom’s and Dad’s – is your child’s height almost ISO with yours?  What word would you fill in for ISO?  Even? The same? Equal to?  Guess what?  That’s what ISO means.  When we speak of ISO200 film, we mean the film is equal to a standard (of the International Organization for Standardization) that specifies how fast it must respond to light. Except for color rendering, all film that has the same ISO “number” reacts at the same speed within certain tolerances.  So, since when we opened the shutter, film, shutter and aperture affected exposure at the same instant, we had the famous exposure triangle. And, ISO was one leg.

Enter digital where film has been replaced by a sensor – a device that captures light.  Since we all came over from film, it was easy to say that ISO was a change in the sensor’s sensitivity to light (and some are still saying that – pros too). Arghh!  Our camera sensors do not have venetian blinds that open and close depending on ISO setting.  ISO refers to an amount of amplification of the signal from the sensor and it happens after the shutter closes. Since this amplification takes place after the shutter closes, exposure is now affected by shutter speed and aperture only – goodbye exposure triangle. 

So, let’s follow the path of an image: Open the shutter. Light comes streaming in. Our camera sensor gathers that light into cells containing filters for either red, green and blue (RGB) creating pixels of color. Shutter Closes. That information is converted to electrical signals right off the back of the sensor. Now those little signals are amplified (how much depending on our chosen ISO) just before and during an analog to digital conversion (which happens in the camera’s Analog to Digital Converter – duh?) where the electrical signals are converted to numbers which are “printed” onto our memory cards.

Finishing our trip from capture to image, we plug our memory card into a computer or computer-like device. What do computers do? They crunch numbers! Crunch, crunch, crunch and those numbers are converted back to pixels. What a journey.