This series of articles is excerpted from Rocky Nook’s Enthusiast’s Guide to Exposure by John Greengo covers the controls — shutter, aperture and ISO. The week’s topic tell how the shutter effects the photograph.

Shutter by numbers

The shutter speed will impact a photograph in two ways: how much light falls on the sensor and how sharp or blurry an object that moves will be rendered. The amount of light is pretty easy to understand. Let’s start by moving from one full second to two full seconds. Twice the time equals twice the light. A shutter speed of 1/2 second means half the light as a one-second exposure. On traditional cameras, the shutter speeds are approximately doubled or halved in sequence. For instance, the available shutter speeds are listed as 1s, 2s, 4s, 8s, 15s, 30s as the exposure times get longer, and 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, etc., as the exposure gets shorter.

In theory, we can go as far as we want in either direction. In reality, most cameras will top out at 1/4000s or 1/8000s. This is due to mechanical limitations on the shutter unit. On the long end, most cameras will bottom out at 30 to 60 seconds. Camera displays, in the viewfinder or on the back or top of a camera, will often display one second as 1” (with the quotation mark indicating that the number is equal to or greater than a full second). All other numbers are fractions. Thus, 60 means 1/60 of a second and 4 means 1/4 of a second.

Photo by CJ Dayrit on Unsplash


With most modern cameras, you can use a setting called “Bulb” that will allow you to leave the shutter open as long as you like. To use this feature, press the shutter release the first time to open the shutter. When you want to end the exposure you’ll press the shutter a second time to close the shutter. This might be handy for exposures that take place at night or in a very dark environment. Most photographers will limit this feature to exposures between 1 and 10 minutes since anything longer will exhibit excessive noise that detracts significantly from the photo.

A tripod and remote cable release are important tools in this situation because they allow you to keep the camera steady, and you can start and stop the exposure without touching the camera.

One Stop

Going from 1” to 2” is often said as exposing for one more stop of light. “One stop of light” simply means we have doubled the amount of light being received by the sensor. We can work this in reverse as well: 1/2s is one stop less light than 1”. To change by one stop of light is to either double or cut in half the amount of light.

Shutter speeds in 1/3 stop steps

In order to be as exact as possible, shutter speeds are available in third stops. Going from 1 to 2 seconds looks like this: 1”, 1.3”, 1.7”, 2”. Set the camera to manual and turn the shutter speed dial to see for yourself. For example, working our way down from 1/1000s we have 1/800, 1/640, and 1/500. Some cameras may show slightly different numbers; for instance, some cameras display 2, while others display 0”5 to indicate 1/2 of a second. The reason we have third stops and not fourth or tenth stops is that it’s about the smallest difference a person can see when comparing two photos side by side. Tenth stops would allow us to be very precise, but would also require us to rotate our shutter speed dial a whole lot more! So if you wanted to make a photo that is just the slightest bit brighter or darker, you should adjust the exposure by 1/3 of a stop.

Opening photo: ©Kevin Ames

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