Friday, May 18, 2012

Stops in Photography




Lots of beginners to photography hear the word "stop" or f-stop" and become quite confused. So, I thought I would try and explain the theory behind the mysterious "stop".

In photography, "stop" is the word used to mean double the previous amount of a value applied to a camera setting - if the value is increasing - or half the amount if the value is decreasing. It applies to ISO, shutter speed and aperture settings.

The term f-stop refers specifically to an aperture setting - because on old film cameras the ring around the lens that was used to change the aperture values had a notch it in when the next "stop" for the aperture setting was reached.

But, for shutter speed and ISO changes, the term "stop" is also used (without the "f") to signify a change in the value. 

With ISO (which start from 50) a one stop increase is 50 to 100 or 100 to 200 or 400 to 800. Two stops is 100 to 400 or 400 to 1600. So, from 100 to 200 ISO you are making the sensor twice as sensitive to light, and if you go from 200 to 100 ISO, you are making the sensor half as sensitive to light. 

With shutter speeds, a one stop change is 1/30th second to 1/15th second, or 1 second to 2 seconds, for example. Two stops is 2 seconds to 8 seconds (all examples - there are others). A one stop shutter speed change lets in half as much light if the shutter speed is doubled (slower speed) or twice as much light if the shutter speed is halved (faster shutter speed). With most cameras,  shutter speeds start at 30 seconds and go to 1/4000th or 1/8000th second.

For ISO and shutter speed settings, the values are arithmetical - so you can easily work out the change needed for any number of stops - just double or half the previous amount. If you want to increase an ISO by 3 stops and you are starting at ISO 100, you go to ISO 800 (100 x 2 x 2 x2). If you want to increase the shutter speed by 4 stops and you're starting at 2 seconds, you go to 1/8th of a second (2 seconds halved, and halved again, and halved again and halved again).

Just be aware that there are a couple of changes in the shutter speeds that are not exactly arithmetical.Shutter speeds change from 1/8th second to 1/5th second (not 1/16th) and 1/60th second to 1/125th (not 1/120th), for example.

With Apertures:

Depending on where you start, 3 clicks of the dial (on most cameras) is one full stop up or down. If you start at f4 it goes: F4, f/4.5  f/5, f/5.6 (f5.6 is one stop smaller than f4). At f5.6 the aperture is now half as big as it was at f4 and only half the amount of light that was coming through the lens when set at f4 will reach the sensor.

One stop from 5.6 is f8 (f5.6, f6.3, f7.1, f8). Now half the light that was coming in at f5.6 is coming in at f8 (the aperture is only half as big again).

You can also see that there are now 2 stops between f4 and f8 (6 clicks). 4 times less light is coming in at f8 than there was at f4.  If the exposure was correct at f4, it's now 2 stop UNDER-exposed (4 times less light).

The list of f-stops below starts at f4 (and ends at f32) with the full stops in bold.
f4, f/4.5 , f/5, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16, f/18, f/20, f/22, f/25, f/29, f32

Each stop is letting in half as much light as the previous one (or twice as much more - if you work backwards and open the aperture up).

I am grateful to Scott D Coulter for clarifying the mathematical relationship between f-stops. Basically, to find the next f-stop you multiply the one before it by the square root of 2 (1.4 plus some decimal points) and round it up or down. So, to find the next f-stop from f8, you multiply it by 1.4 = 11.2, rounded down it becomes f11. 

The smallest aperture numbers signify a wide aperture. The larger aperture numbers signify smaller apertures. The wider apertures give the effect of what is called "shallow depth of field" (lots of blurring in the foreground and background of the photograph) and smaller apertures give the effect of "wide depth of field" (lots of sharpness throughout the entire photograph).

See the two photos above for examples of a wide and small aperture used on the same scene. 


You can read more about "Depth of Field" in another of my blog posts here.