# calculate sensitivity to set...

### #1

Posted 10 January 2006 - 04:23 AM

How do you usually calculate the sensitivity to set if a filter cuts 1/x of a stop. I mean, if I have a 400 ASA film and a filter I use takes 1/3 of a stop then I know that I have to set the filmspeed to 320. The math for this would be: 200 x 2^(2/3), right? Isn't there an easier way to do calculate it? I hope you understand what I mean, sorry for my lack of English....

Regards,

Michael

### #2

Posted 10 January 2006 - 04:47 AM

Hi,

How do you usually calculate the sensitivity to set if a filter cuts 1/x of a stop. I mean, if I have a 400 ASA film and a filter I use takes 1/3 of a stop then I know that I have to set the filmspeed to 320. The math for this would be: 200 x 2^(2/3), right? Isn't there an easier way to do calculate it? I hope you understand what I mean, sorry for my lack of English....

Regards,

Michael

Hi,

Keep things simple, take a reading then open by 1/3 stop!

Stephen

### #3

Posted 10 January 2006 - 10:31 AM

I have a 400 ASA film and a filter I use takes 1/3 of a stop then I know that I have to set the filmspeed to 320. The math for this would be: 200 x 2^(2/3), right?

Since your starting point is "what value is 1/3 of a stop under 400 ?", the logical way to calculate this would have been 400/2^(1/3)

But it is also true to say that the correct result is 2/3 of stop over 200 ASA and therefore your calculus is good.

Since you know that 2^(1/3) = 1.25, you can just remember the series : 100, 125, 160, 200 etc. and multiply these figures by whatever factor you like to get the values you're looking for. For instance : 200, 250, 320, 400 are also the values by 1/3 of a stop steps...

Look at your light meter you will see the sensitivity values are in that order

### #4

Posted 10 January 2006 - 06:06 PM

look here http://en.wikipedia....wiki/Film_speed to get a list of the most common film speeds.

Each step equals a 1/3 stop.

Just learn the common steps as Laurent says and forget about calculating on set :-)

Greets,

Dan

### #5

Posted 11 January 2006 - 03:09 PM

Michael,

look here http://en.wikipedia....wiki/Film_speed to get a list of the most common film speeds.

Each step equals a 1/3 stop.

Just learn the common steps as Laurent says and forget about calculating on set :-)

Greets,

Dan

It's easiest to just remember the sequence of film speeds (or apply the compensation in your meter as such, as many meters can do that) rather than calculating anything on set.

### #6

Posted 12 January 2006 - 12:41 AM