Understanding Stops in Photography

by Mike Schumacher

The stop is a basic unit of light in photography. Stops can be controlled by shutter speed, f-stop and iso setting. Shutter speeds double or half the amount of light.

Standard shutter speeds are:

1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 sec

A step faster halves the amount of light and a step slower doubles the amount of light. Going from 1/60 to 1/30 sec doubles the amount of light. Going from 1/30 to 1/60 halves the amount of light.

Iso is the sensitivity of film or the digital sensor to light. The higher the number the more sensitive the film or digital sensor is to light.

Standard iso settings are:

50 100 200 400 800 1600

Increasing the iso one step doubles the amount of light (one stop), decreasing iso one step halves the amount of light (one stop). Caution should be used when adjusting iso to a high value. Higher iso settings can introduce noise especially in the shadows.

Aperture settings are described in f-stops. F-stops specify the light gathering ability of the lens. A specific f-stop on one lens, say f8, will be the same on another lens.

Standard f-stops are:

1 1.4 2 2.8 4 5.6 8 11 16 22

The higher the f stop number the less light and the smaller the aperture of the lens. The lower the f stop number the more light and the larger the aperture of the lens. If we use f4 as an example going to f5.6 halves the amount of light (one stop) and going to f2.8 doubles the amount of light (one stop). In our f4 example going to f5.6 would be referred to as closing down the aperture and going to f2.8 would be referred to as opening up the aperture. Aperture also controls depth of field. F16 will have more depth of field than f5.6. Depth of field can be used very creatively to direct the viewer’s eye.

If you’ve gotten this far you might be wondering, why do I care about all this? It is very useful to understand these inter-relationships. Controlling aperture for depth of field and shutter speed for freezing or blurring action are basic tenants of creative photography.

More importantly we’ve all had fantastic photo op’s and tried to capture what we see. The human eye can see between 12 to 14 stops of light. Your digital or film camera can see about 5, maybe 6 or 7 if you play a bit in the darkroom. So when we are in the forest with the sun shining through the treetops we have at least 5 stops of light or more between the shadows of the forest and the sunlight at the top of the trees. Matrix metering combines the highlights and the shadows to create an average. So if you have bright sunlight and deep shadow it creates a compromise that has either plugged up shadows or blown highlights. This is where knowledge of stops in photography comes in handy.

When we meter a subject we are measuring the light reflected off the subject, this is called reflectivity. Snow has a reflectivity of about 90%. A black bird or building has about 20% reflectivity. Your camera sees midtones or middle reflectivity commonly called middle gray or 18% reflectivity. This is why with snow we add a stop or so of light. The camera will try to render it a mid tone.

If we use our forest example, metering for the sunlight will render the forest floor as underexposed. Metering for the forest floor will blow out the treetops. In extremes like this you have to make a decision. Your camera will try to render the whole scene and more than likely not do a very good job. Using the spot meter on your camera, meter the forest floor and subtract one stop of light. Check your histogram, it should be a nice bell curve in the middle. It shouldn’t be bunched up on the left or right side. If it isn’t adjust exposure compensation in 1/3 or 1/2 stop increments until you get a nice histogram. If there is too much dynamic range (contrast) in the scene you will have to make a decision as to which part of the scene to render. When using your spot meter you are telling the camera which element in the scene to render as a midtone. You can meter the treetops or the sky and let the tree trunks and forest floor fall into shadow. Waterfalls in bright sunlight will often fool a meter. Subtract one stop of light and check your histogram!