What happens when shooting iso100 film at iso50, or iso25?

[Treymac]

Hey guys. What happens when shooting iso100 film at iso50, or iso25? Does it allow you to keep your shutter open longer? If it does, how much longer does it give your shutter?

 

[Plato]

Hey guys. What happens when shooting iso100 film at iso50, or iso25? Does it allow you to keep your shutter open longer? If it does, how much longer does it give your shutter?

ISO 50 - twice as long. Photo too light.
ISO 25 - four times as long. Photo much too light.

Find a book on basic photography.

 

[epp_b]

You under or over expose when you set your ISO dial to something other than your film ISO. If you have ISO 100 film and set it ISO 200, the meter will underexpose by one stop. If you have ISO 100 film and set to 50, the meter will overexpose by one stop. If you have ISO 100 film and set to 25, the meter will overexpose by two stops.

And so on and so forth...

 

[molested_cow]

Not sure if I am right, but if you set your camera to iso50 when using a 100 film in Aperture priority mode, your shutter speed needs to be 1/2x. At ISO25, it should be 1/2X1/2=1/4 times.

The reason is your camera thinks that you need more light, therefore you actually need to underexpose according to what your camera's light meter is telling you.

 

[UUilliam]

the feature to change your film speed on Film camera's is basically to help the Inbuilt light meter to guess a closer to accurate measurement.

 

[NateWagner]

well, if you want a little light reading... you can check this out
ISO-WIKI

 

[chris]

Don't forget that if you reduce the film speed then you will have to reduce the development time to compensate; this is easily done if you process the film yourself or send it to a professional laboratory, a in-store auto lab will probably not be able to adjust the development time and you will then just end up with over exposed negatives. Also, you need to expose the whole film at the lower film speed, not just odd frames.

The easiest way to get long exposures if just stopping the lens down is not enough is to use a neutral density filter.