Where do stops come from?

Photographers tend to talk about stops rather than precise changes in aperture so changing from f/16 to f/8 increases exposure by 2 stops.

DAVID BRYSON FRPS

The first of Robert Galloway’s photographs of cameras on page 8 shows a lens which uses Waterhouse stops as well as a collection of stops. Stops were invented by John Waterhouse of Halifax, England in 1858; previously lenses had to be unscrewed to change aperture, whereas Waterhouse stops with suitable lenses or attachments could be slotted in and out of lenses as needed.

Thankfully nothing we have to use these days as photographers with apertures changed at the touch of a dial; but we still talk about stops when discussing changes needed for better exposures.

Working with stops is a very useful way to explain changes in exposure, for example when it comes to High Dynamic Range Imaging (HDRI). We can use multiple exposures to capture highlights and shadow detail in different images, then combine (or ‘stack’) those images into one. Rather than specifying apertures for the individual exposures, we can say “plus or minus 5 stops or 8 stops”, whichever is suitable for the subject.