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F.-Stops and Shutter Speeds Explained, Really

I'm going to see in how few words I can explain camera exposure controls, the dreaded f.-stops and shutter speeds.

Before starting the clock (word counter?) let me suggest your current confusion might be caused by the habit of some camera makers to provide clever workarounds which only create new complications of their own. So, for the moment, forget that you know anything.

Here goes:

There are two, and only two, tools by which a camera controls how much light reaches the sensor (or film).

First, the shutter (which really ought to be called the “opener”) regulates the amount of time that light coming through the lens splashes onto the sensor. The time used is called the shutter speed.

From slow to fast... 2 , 4 , 8 , 15 , 30 , 60 , 125 , 250 , 500... Yes, bigger number, less light--stay tuned.

Second, between two of its many glass elements every camera lens contains an opaque disk with an adjustable hole or “aperture” in it. The size of that hole is described by its “f.-stop.”

From big to small... 1.4 , 2 , 2.8 , 4 , 5.6 , 8 , 11 , 16... Yes again, bigger number, smaller hole, less light.

The important thing so far is this: shutter speed controls the amount of time light will be allowed to hit the sensor. F.-stop controls the light's intensity during that time.

To create the appropriate blast of light for a given situation and sensor sensitivity (ISO), any one of several f.-stop/shutter speed combinations can work. The choice will depend on picture priorities like action-stopping, depth of field, and other needs to be described elsewhere.

Chances are that if you're with me in principle, you're still confounded by those crazy numerical values. First, think of them as the undersides of fractions (remember “denominators?”). "250" is slower and passes twice as much light as "500" because it really means 1/250 of a second, and that's twice as long as 1/500 of a second. Every value on the list above is twice as bright as its successor. So far so good.

F.-stops are less intuitive and require a little faith on your part. F.-2.8 passes more light than its neighboring f.-4.0. (1/2.8 is more than 1/4.) Not just more light, though, but exactly twice as much. Why is that? Shhh.*

Modern electronic cameras add confusion by offering one or two settings between the full-stop settings in the sequences above. That's useful, but for now try to keep your eye on those greatest-hit values I showed you.

Last word, this about ISO settings. They refer to the sensitvity of film or sensor, and they're mercifully easy. 400 is twice as sensitive as 200 is twice as sensitive as 100. See? Easy.

Okay. Stop the clock.

*If you must know, oh, faithless one:

The technical definition of f.-stop is “focal length of lens divided by the diameter of the aperture.” Now that you know that fact, you can forget it, except for that word “diameter.”

Remember high school, when you learned about how if you double the diameter of a circle you quadruple the area? Well, it's still true. Also true: if you double the diameter of the hole in your lens, you'll let in four times as much light. Now, shave those values down a little: increase the diameter by about 1.4 (the square root of 2, if you must know) and you'll double the light.

Look at those f. numbers again: notice how each one is the value of its predecessor multiplied by 1.4.

There you go. Clear as mud. If this is the first time you've encountered this bit of information, then I suspect your head is in some pain right now. As the surgeon will  tell you about your post-bypass agony, "This is normal and healthy." The same applies here. Come back in a day or two and read this again. The mist will clear, believe me.

©Michael Bailey