# question on effects of ISO on exposure

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#### Castlesinthesky

##### New Member
Hey guys, i've got this question on using ISO to affect exposure.

I understand that upping the ISO rating say from 100 to 400 would mean that the film is 4 times more sensitive to light.

so in short, this allows me to set a shutter speed 2 stops faster say 1/250 to 1/1000 at a give F-stop value right?

so flowing with this logic, if i up the rating of film from 100 to 1600, would it mean that the film is 16 times more sensitive to light and therefore would allow me to set a shutter speed 8 times faster at the same given F-stop value to give me the same exposure if i had originally stayed with the film rating of ISO 100?

would appreciate any confirmation on this...thanks guys!

#### darkness

##### New Member
Yes, except that as you mentioned, since it's 16 times as sensitive, it should be 16 times faster, not 8. So a 1s exposure becomes 1/15s (I think there isn't 1/16s).

#### Castlesinthesky

##### New Member
darkness said:
Yes, except that as you mentioned, since it's 16 times as sensitive, it should be 16 times faster, not 8. So a 1s exposure becomes 1/15s (I think there isn't 1/16s).
which means i set the shutter speed 16 times faster and not 8 times?given that i up my iso from 100-1600 shouldn't the equivalent be 8 times instead since an incremental of one stop would mean the doubling of light let in?

was going through this thought process...so hope my doubts on this can be cleared up

#### Prismatic

##### Senior Member
100>200>400>800>1600
Increase in exposure = 2x2x2x2 = 16 times more sensitive.
But you get only 4 stops faster shutter speed.
So if originally you were shooting at 1/30, 4 stops will be 1/60, 1/125, 1/250, 1/500.

However, that is IF you can find a film that can be pushed 4 stops and still give reasonable results.

#### dbcs

##### New Member
Since we are on this...

anybody know if there is a mathematical relation btw Aperture & Shutter Speed?

e.g. F4, 1/60s, F8 ???s

Also, for "normal" 35mm film eg Kodak, Fuji etc. how is the push performance?

100 -> ??? ISO

400 -> ??? ISO

#### mpenza

##### Senior Member
every change of aperture by 1.4x, is a stop difference. so f8 is 2 stop smaller than f4 and the shutter speed will be 1/15s for the same scenery.

#### coke21

##### New Member
Castlesinthesky said:
which means i set the shutter speed 16 times faster and not 8 times?given that i up my iso from 100-1600 shouldn't the equivalent be 8 times instead since an incremental of one stop would mean the doubling of light let in?

was going through this thought process...so hope my doubts on this can be cleared up
Just to add, when you push your film like you mentioned, the grainier your shots will become. Not to also mention that when you push process your film, its gonna cost more.

#### Castlesinthesky

##### New Member
Prismatic said:
100>200>400>800>1600
Increase in exposure = 2x2x2x2 = 16 times more sensitive.
But you get only 4 stops faster shutter speed.
So if originally you were shooting at 1/30, 4 stops will be 1/60, 1/125, 1/250, 1/500.

However, that is IF you can find a film that can be pushed 4 stops and still give reasonable results.
how did you derive 4 stops faster looking at it?it's done by counting number of times it's doubled which in this case is 4 times right?

so if i up the iso from 100-800, which would be 8 times more sensitive

2x2x2=8 times more sensitive

would me that it would give me 3 stops faster shutter speed right?

#### showtime

##### New Member
mpenza said:
every change of aperture by 1.4x, is a stop difference. so f8 is 2 stop smaller than f4 and the shutter speed will be 1/15s for the same scenery.
to be really acurate, its not 1.4 but rather root 2... (square root of 2)
just for everyones info...

#### Prismatic

##### Senior Member
Castlesinthesky said:
how did you derive 4 stops faster looking at it?it's done by counting number of times it's doubled which in this case is 4 times right?

so if i up the iso from 100-800, which would be 8 times more sensitive

2x2x2=8 times more sensitive

would me that it would give me 3 stops faster shutter speed right?
Correct. But most film are only good for pushing up to 2 stops. Anything more, you get grainy pictures, unnatural contrast and colours.

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