# shutter speed and exposure time

Discussion in 'Australia Photography' started by Don, Oct 25, 2003.

1. ### DonGuest

When playing around with a particular file in windows explorer I was able to
read the exif data that included the following figures:

exposure speed 1sec
exposure time 1/6 sec

I have always thought that you set your exposure by establishing the size of
the aperture (ie F4) AND utilising the time the shutter stayed open (ie
1/6th of a second). What is the exposure speed.

regards

Don, Oct 25, 2003

2. ### KenGuest

A total guess, but exposure "speed" = ISO? 1 being 100 not sure what the
'sec' would indicate, assuming a bug?.

KC

Ken, Oct 25, 2003

3. ### kGuest

from:
ca9fb340&rnum=4

ExposureTime
Exposure time, given in seconds (sec).
Tag = 33434 (829A.H)
Type = RATIONAL
Count = 1

ShutterSpeedValue
Shutter speed. The unit is the APEX (Additive System of Photographic
Exposure) setting (see Annex C).
Tag = 37377 (9201.H)
Type = SRATIONAL
Count = 1

The relation between the two values is given by the
formula:

ShutterSpeedValue = -log_2( ExposureTime)

(where log_2 denotes the logarithm of base 2).

So one should have the following table:

ShutterSpeedValue ExposureTime
-3 8
-2 4
-1 2
0 1
1 1/2
2 1/4
3 1/8

k, Oct 25, 2003
4. ### UbiquitousGuest

Sounds overly complex, but still doesn't explain why speed of 1 sec = time
of 1/6 sec.
By that table, the time should have been 1/2 of s ec?

Ubiquitous, Oct 26, 2003
5. ### kGuest

I'da thought so to, but that was the result of a quick google search (hence the
referenced link) - maybe ISO has something to do with it. hmmm. at least now
be both know that the seemingly contradictory figures of speed and time have
some actual correlation, something I too did not know before

k

k, Oct 26, 2003
6. ### DonGuest

I just get further confused. Yes, its with my 10D.

Regards and thanks for the responses. I will post anything that comes close