BetterPhoto Q&A
Category: Traditional Film Photography

APERTURES

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March 18, 2007

You are confusing f/numbers with shutter speeds. Maybe I can help.

When we change the camera aperture or shutter speed we cause a change in the amount of exposing light. If such a change is made in a 2x increment i.e. in such a way that we either half or double the exposing light energy, we say the change equals 1 f/stop.

Our camera lens is equipped with an adjustable aperture. This aperture is a design copied from the human eye. Our eyes are equipped with a pupil (aperture) that opens up and closes down at the command of our brain. Opening up makes the pupil bigger thus allowing more light to enter for dim light conditions, and closing down reduces the amount of light entering the eye under bright conditions. We call this pigmented opening in the human eye, the “Iris”. This is the colored portion of the human eye named for the Greek god of the rainbow.

Now the opening in the camera’s diaphragm is a circular hole. Long ago it was deemed best to make adjustments to the diaphragm using an increment that either doubles or halves the amount of light that passes. Years ago no mechanical iris was available so lens makers used interchangeable metal slides each with a different size hole; these were slipped in just behind the lens and called “stops”. Modern lenses use a mechanism that allows the hole size to be adjusted easily. We call this the aperture or better yet, the iris diaphragm.

Because the aperture is a circular hole we must use the math of circles to calculate the size of the hole to make it twice as big or half as big. I will spare you the math but when we are done, the sequence is: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22. These values are called full stops because they change the hole size in such a way that each changes the amount of light transmitted by a factor of 2 (double or half) or more simply 1 stop between each. The tiny numbers are large openings (f/1 the biggest in my list) – the big numbers are tiny openings (in my list f/22 is the smallest).

Now the shutter is a clock mechanism used to control the exposure time i.e. how long the image will be allowed to play on the film or chip during exposure. . Long ago it was deemed best to cause the increment of change to follow the same pattern as lens aperture i.e. doubling or halving. Most exposures are quite short and thus the duration will be a fraction of a second. The sequence is: 1 - 1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/500 - 1/1000.

Since each shutter speed change is a doubling or halving (increment is 2), each the ebullient of a 1 stop change.

Alan Marcus
ammarcus@earthlink.net

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March 18, 2007

Re the original question:
f/16 @ 1/15 is equivalent to f/11 @ 1/60. + 2 stops in aperture, -2 stops in shuttter speed for 0 change in exposure. Then the change from 1/60 to 1/500 is -3 stops. The speed is doubled 3 times: 1/60 to 1/125 to 1/250 to 1/500.

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March 18, 2007

Now here's how it works. Say you use your light meter and it calls for a general exposure of EV13. Well you could use an aperture rating of 2.8 (3) and a shutter speed of 1/1000 (10). 3+10=13. This would be good for either a fast moving subject or where limited hyperfocal length (depth of focus) is desired. Or, if you want to blur motion or have a very deep DOF, you might try an aperture of f16(8) and a shutter speed of 1/30 (5). 8+5=13. These and other equal combinations would be exposure values equivalent to EV13.

You can apply these measurements to any format (film or digital) where you have manual control over your exposure settings.

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March 19, 2007

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March 20, 2007

Alan Marcus
ammarcus@earthlink.net

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March 20, 2007