Shutter Speed

JC
Posted By
Jack_Cane
Oct 4, 2008
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535
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3
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Closed
A bit off topic, but maybe not. Information comes from file | file info in CS3.

Metadata from a jpg image show different exposure times.

First: "Exposure", giving the traditional expression in decimal form, e.g., 0.017 s for 1/60s.

Second: "Shutter Speed", expressed as NNNN/256. I am unfamiliar with this convention. How does it relate to the traditional expression?

There is also a Brightness parameter, also expressed as NNNN/256, and can be negative.

Any insight and/or reference pointer would be appreciated.
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MD
Michael_D_Sullivan
Oct 5, 2008
You don’t say where you are getting these values in the PS CS3 file|file info screen. The "Camera Data 1" information will probably differ significantly from the information from the "Advanced"->EXIF Properties, even though the Camera Data 1 page is using EXIF data.

There are several EXIF tags that represent information in different ways.

There is no Exif tag named Exposure. There is ExposureTime, which gives the exposure time, in seconds, as a "rational" number, which is represented by a fraction with a 32-bit number as the numerator and the denominator. Accordingly, it could be anything from 4,294,967,295 seconds down to 1/4,294,967,295 seconds. An application can display this as a fraction or in any other way it wants, such as a decimal fraction of a second. In my experience, PS CS3 expresses this as a fraction such as 1/30, not as a decimal.

There is also an Exif tag named ShutterSpeedValue. This represents the shutter speed as an APEX value, not in seconds, stored as a ratio of two signed 32-bit numbers. APEX (Additive System of Photographic Exposure) units are logarithmic units such that an increase by 1 unit represents a halving of the amount of light, such as a one-stop increase in aperture or a halving of the exposure length. The ShutterSpeedValue APEX value of 0 represents 1 second. Thus, 1 represents 1/2 second, 2 represents 1/4 second, etc. In one JPEG camera image I looked at, which had a 1/30 second exposure, the File|File Info > Advanced > EXIF Properties showed ShutterSpeedValue as being 4906981/1000000, which comes out to 4.9 APEX, and 5 should represent 1/32, so the 1/30 is right on. In your case, you are getting NNNN/256. Divide it out; say the result is X (round to the nearest whole number for simplicity). Your exposure in standard terms is 1/(2^X) seconds. If X is 9, it’s 1/512 second, or roughly 1/500. Etc.

Brightness is probably the EXIF BrightnessValue tag. This is also an APEX value represented as a ratio of signed 32-bit numbers. The EXIF standard says, "Ordinarily it is given in the range of -99.99 to 99.99." Zero represents an incident light brightness of 1 foot-lambert, and it grows with increasing brightness. My camera images did not contain this value.

The EXIF Standard v.2.2, is available for free here <http://www.exif.org/Exif2-2.PDF>.
JC
Jack_Cane
Oct 5, 2008
Thanks Michael. That was very helpful, just what I was looking for.

My actual example is 1513/256=5.9, 2^5.9= 59.7, so the exposure was 1/60 s. Now "shutter speed" makes sense; the number is actually reciprocal seconds which is a unit of speed, actually the time to travel 1 unit of distance.

Since this activity revolves around a college course, I’ll identify the number as the negative log to base 2 of the exposure time. Good exercise for students.

Again thanks for pointing me to the Exif standard.
MD
Michael_D_Sullivan
Oct 6, 2008
Glad to help.

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