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While reading a book that was recognized as a good one by many readers, I found the following table:

This means that any of these settings would each result in the same amount of light hitting the camera’s sensor to make the exposure.

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On the other hand, there is F/16 Sunny rule, that says:

The Sunny 16 Rule is a way to meter for correct exposure during daylight without using the camera's meter. So for example, if your ISO is 200 at f/16, then your shutter speed will be 1/200 seconds. If your ISO is 100, then your shutter speed will be 1/100 seconds.

According to this rule, on F/16, ISO 100, the SS will be 1/100, and not 1/50 as in table above.

Which combination is the correct one? Or all this is about the relative values?

(forgive me for asking stupid questions, but that is the only way to learn)

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    The "Sunny 16" rule is for sunny conditions. Does the book say what lighting conditions the table is supposed to be for? Or is it just referring to the inverse relationship between aperture and shutter speed in a general sense? – osullic Nov 24 '15 at 13:07
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Like Michael says, the top chart is just hypothetical, and it only uses some random exposure, and is NOT about any specific exposure condition. Instead, it tries to show two concepts.

It is an Equivalent Exposure guide, indicating that f/2.8 at 1/1600 sec, or f/4 at 1/800 sec, or f/5.6 at 1/400 sec, etc, etc, are Equivalent Exposures. You can choose any one of the combinations to keep the same exposure (but as is, the chart itself is NOT about your specific local exposure). You might prefer one aperture for depth of field, or one shutter speed for stopping motion, etc, etc. You choose the Equivalent Exposure best for your purpose.

Also, it shows that "stop" relationship to both shutter speed and aperture. 2x shutter speed is one "stop", same as one aperture stop is one stop. If you think of the chart as a slide rule, and imagine one scale is moved to slide it one step horizontally relative to the other, then the way it lines up then shows Equivalent Exposures of that new exposure, shifted "one stop".

The chart is about the Equivalent Exposure concept, an extremely important concept, but then we don't need the chart. Our camera viewfinder shows the numbers being used. In camera modes other than Manual, when we change shutter speed or aperture, the new value of both values is shown.

Sunny 16 is about some actual exposure, but you should look up Sunny 16 again. Bright sunshine is a constant, and Sunny 16 also tries to account for degrees of overcast (judged by the shadows cast). In the old days before light meters, Sunny 16 is all we had. But it is difficult for us to judge degrees precisely (like 1/3 stops). It worked well with negative film, which had wide latitude, but our guesses were typically just "ballpark", and digital really needs to be more precise. Your camera light meter should work better than Sunny 16, BUT Sunny 16 is always a good guide to know to decide if your meter value is reasonable or not. In reality, bright sun is normally EV 15, which is ISO 100, 1/125 second at f/16.

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The quote from the book does not state that the numbers are intended for any specific lighting situation, so it appears to be indeed only concerned with showing how aperture and shutter speed can vary while maintaining the same exposure.

And anyway, the "Sunny 16" rule is only a broad rule of thumb and one f-stop difference is not that big. Maybe the author of the book simply looked at his camera on a slightly overcast day rather than a sunny one.

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The “sunny 16 rule” : Set 1/ before the ISO for the shutter speed and set the aperture to f/16. For 100 ISO this translates to shutter speed 1/100 of a second @ f/16. Keep in mind that many cameras do not sport a 1/100 of a second setting. In this case, use the closest, likely 1/125 of a second. Also, keep in mind that photo systems, be they film or digital, allow some latitude in the exposure setting. The closest we can measure with a light meter is about 1/3 of an f/stop. The closest we can set our cameras exposure with accuracy is no better than 1/3 f/stop. This is due in part to mechanical limitations that prevent exact adjustment of the diameter of the iris and mechanical limitation in the escape mechanism of the shutter.

The “sunny 16 rule” pre-dates 1960, when the American Standards Association (ASA) revised their testing methods. After 1960 previously applied safety factors against under-exposure were cast aside. This action effectively doubled the published ISO setting. Thus tables for 100 ASA (now International Standards Origination ISO) must be revised. What was 1/50 of a second for 100 ISO is now 1/00 of a second.

  • I don't think the first use of the term "Sunny 16" is known, but Daguerreotype days in 1840 published tables of Daylight, Cloudy and Overcast exposure times. And Kodak published it in each film box, before and after light meters came along. Weston meters did not switch to ASA until the mid-1950s, after which ASA 125 corresponded to older Weston 100. But Sunny 16 did not change with them, which I think is why it is still 1/100 at f/16, instead of the more accurate 1/125 at f/16. – WayneF Nov 24 '15 at 21:43

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