shutter speeds

[wager123]

ok stupid questions but i don't know the math so any help greatly app. my 90mm has a sloww shutter speed at 125 how do i figure out how to adjust for it by using 1/3 stops. or other wise what speed is 125 - 1/3 of a stop.
hope i asked this right.
mitch

[2F/2F]

Same as film speeds: '100-'125-'160, and so on. Double or half each for one full shutter speed difference.

[wager123]

thanks
let me ask the question another way , a 1/125 shutter speed in mil secs is7.81ms if the shutter timer show that the shutter speed at 1/125 is 11.66 ms how do you figure how to compensate for it.
thanks
mitch

[rob champagne]

If the time measured was 11.66ms the speed of that exposure in normal notation is:

1/(11.66/1000) = 1/86 of a second (I think).

The amount to adjust the speed by would be

1/((11.66-7.81)/1000) = 1/260th of a second less exposure.

I would get someone else to confirm my maths because I'm not 100% sure about it.

[Ray Heath]

are you wanting to know the full and 1/3 shutter speeds or what to adjust a wrong shutter speed to?

[Fred Aspen]

Mathematically, roughly 8 ms represents 1/125 second shutter speed. 16+ ms represents 1/60 second shutter speed. 11.66 ms is about half way between (12ish to be exactly precise) so I would say your shutter is about 1/2 stop slow, about as precise as one can be with photographic equipment.

HTH,

Fred

[rob champagne]

To find a 1/3 stop increase you divide by ³√2 = 1.2599
so 1/86 / 1.2599 = 1/108th of a second which is till to slow.

To find a 1/2 stop increase you divide by ²√2 = 1.4142
so 1/86 / 1.4142 = 1/122th of a second

So in your case with the shutter speed being slow, you need an additional half a stop exposure of the slow shutter speed which should be close enough.

[rob champagne]

But given your shutter speed is slow then it could be sticking or it may be out of adjustment. If its sticking then an extra half stop may or may not work.

[jnanian]

i love threads like this ..

if you are using an olde betax lens, you would can
set your shutter inbetween your shutter speeds, but seeing
you are probably using a modern shutter,
my suggestion would be to just adjust your fstop a little bit,
and not worry/ stress too much about it.

have fun!

john

[sun of sand]

or just dont use 125