Shutter Speed In 1/3 Stop Increments

68lbs

68lbs

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April 2008
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My camera's shutter speed only goes as low as 30". Can anyone tell me what the speeds would be in 1/3 stop increments from this point? I can only find reference upto 30" or slower.

I'm trying to work out if a 5 minute exposure is under-exposed, how long would I need to increase it by to gain a full stop, etc. I know it's exponential so the difference between say 10 and 11 mins isn't that much, but am unsure at what rate it changes.
 
A full stop difference means the shutter speed changes by a factor of two. (You did know that, didn't you?)

So a 1/3-stop difference means that the shutter speed changes by a factor of the cube root of two, which is 1.26. (Or you could make the calculations easier by using 1.25, which is close enough and much easier to do in your head.)

So, starting at 30 seconds:
plus 1/3 stop => 30 * 1.26 = 37.8 seconds
plus 2/3 stop => 30 * 1.26 * 1.26 = 47.6 seconds
plus 1 stop => 30 * 1.26 * 1.26 * 1.26 = 60 seconds (see, it works!)
and so on.
 
StewartR said:
A full stop difference means the shutter speed changes by a factor of two. (You did know that, didn't you?)
Click to expand...

Nope, I just knew my camera changed in 1/3 stop increments, that's it. Didn't realise it was so simple... it looks a lot more complicated when you see 1/250, 1/320, 1/400 etc.
 
Or, if you want to know how many stops difference there are between two exposures:

Unfortunately most people can't do this in their head. You'll need a calculator (or a spreadsheet) which can handle logartithms.

Stops Difference = ( log (Exposure 1) - log (exposure 2) ) / log (2)

Say you've got one exposure at 30 seconds and another exposure at 11 minutes, which is 660 seconds. Then the number of stops difference is:
(log(660)-log(30))/log(2)
= (2.820-1.477)/0.301
= 4.46 stops difference - call it 4 and a half stops

OK?
 
68lbs said:
Didn't realise it was so simple... it looks a lot more complicated when you see 1/250, 1/320, 1/400 etc.
Click to expand...
Mmm. And did you notice that 320/250 = 1.25 (approximately), 400/320 = 1.25, and so on ...?
 
daftbugga said:
that is my maffmatics anyway , i could really do with a calculator..
Click to expand...
A calculator wouldn't help you, I'm afraid, because your "maffmatics" is wrong.

300 + 1/3 stop = 300 * 1.26 = 378 or if you prefer 6 minutes 18 seconds

300 + 2/3 stops = 300 * 1.26 * 1.26 = 476 seconds or if you prefer 7 minutes 56 seconds

300 + 1 stop = 600 or if you prefer 10 minutes
 
StewartR said:
Or, if you want to know how many stops difference there are between two exposures:

Unfortunately most people can't do this in their head. You'll need a calculator (or a spreadsheet) which can handle logartithms.

Stops Difference = ( log (Exposure 1) - log (exposure 2) ) / log (2)

Say you've got one exposure at 30 seconds and another exposure at 11 minutes, which is 660 seconds. Then the number of stops difference is:
(log(660)-log(30))/log(2)
= (2.820-1.477)/0.301
= 4.46 stops difference - call it 4 and a half stops

OK?
Click to expand...

Or you could just keep doubling from the 30 secs until you get to 11 mins which you can do in your head and arrive at 4.5 stops ish in a few seconds.

I haven't really thought about 3rd stops. I set my camera to half stop increments to avoid confusing myself ;)
 
StewartR said:
A full stop difference means the shutter speed changes by a factor of two. (You did know that, didn't you?)
Click to expand...

I did! What's my prize? ;)

Similarly for apperture; to increase a full stop you double the area of the apperture, hence increasing the diameter of the apperture by a factor of the square root 2. This is why appertures follow the sequence 1, 1.4, 2, 2.8, 4, 5.6, 8, 11.2, 16
 
LOL. Why not just change the shutter increments to half?
 
do digitals suffer from reciprocity failure , or some sort of sensor equivalent ?
 
donutagain said:
do digitals suffer from reciprocity failure , or some sort of sensor equivalent ?
Click to expand...
Nope. To all intents and purposes digital is RF free, it's one of the dubious pleasures just left for film users to deal with.
 
at least you digital chaps dont have to take that into consideration when making longer exposures then .
 
Ah but..

reciprocity failure in long exposures - yes

noise in long exposures - nope

the Lord giveth....and taketh-eth-eth away..:lol:
 
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