So now we're looking at the third piece of the puzzle, of what is traditionally referred to as the Exposure Triangle.

When you combine the three elements of the Exposure Triangle, they interact with each other in what's often referred to as the law of reciprocity. 

This reciprocating relationship between aperture, shutter speed and ISO allows us to take control of the light coming into the camera - and we chose which of the three elements will take priority, which side effects will dominate and which do not.

What actually is ISO?
I meet very few photographers who fully understand what ISO is.  I meet even fewer who know what the initials ISO stand for.   

ISO is an independent, non-governmental international organisation that looks after thousands of standards worldwide.  Based in Geneva, Switzerland, the International Organization  for Standardization controls over 24,000 standards (at the time of writing) from health and the environment, to electrical engineering and telecommunications, and of course... photography. 

Its range of influence on world standards is huge and from our perspective, any camera manufacturer, wishing to produce a new camera sensor, has to follow ISO's guidelines to comply with international standards.

Photography has a number of standards controlled by ISO, but perhaps ISO12232:2019 is the one we're most interested in today. 

It specifies:

'the method for assigning and reporting ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras. It is applicable to both monochrome and colour digital still cameras'


Back to our cameras
ISO replaced the old film-speed rating ASA (American Standards Association) around 1987.  Back in the day, you would buy a roll of film which had a fixed speed rating (eg. ASA100, ASA200, or ASA400) and you were locked into that rating. 

Come the arrival of digital cameras, we found that we were able to change that speed rating (or ISO) to virtually whatever we wanted, and a whole new era of flexible exposure control began. 

This week's Tog-Talk is quite a short one, but I think you will find it interesting. 

If you have any questions, write to me at: [email protected]



Some links from this week's Tog-Talk

• Introduction To Photography (1-day Workshop)

Run four times a year, this one-day photography workshop will equip you with the knowledge you need to move from simply being a camera-owner… to becoming a real photographer !

Whether you're photographing your kids, snapping your holidays or you're hoping to progress your photography to a more creative level, you'll still need a solid grasp of the basics.

PLEASE NOTE: All course fees will be going up on 1st April (2022).  And no, this is not an April fool !  If you register for any course before that date, you will get it at current prices. 

• Photography Assessment Tool:

Use this online tool to assess your photography knowledge and get a snapshot of your strengths and weaknesses.

• Hampshire School

Kevin Ahronson is founder of Hampshire School of Photography: www.hampshirephotoschool.com
More teaching and video tutorials are available at the Photography Teacher

Email: [email protected]