Film ISO help.

[MattTyler]

Sorry to be asking what is probably a stupid question, but i recently acquired a Minolta X700 film slr and some ilford fp4 (125 ISO) film and i can't work out how to set the correct iso setting on the camera. The dial seems to double the previous number but has 2 increments between these (25,?,?,50,?,?,100...1600) and my maths, which is pretty bad, is struggling here. Can i actually set this correctly?
My apologies for my stupidity and also thanks in advance.

Matt.

[another view]

The setting on the ISO dial for 125 would be one increment past the 100 mark, towards 200 (those two are 125 and 160).

Once you get some experience with that film and particular camera, you might find that you want to overexpose the film a little for more shadow detail - like setting it at 100 or 80 (first stop below 100). Use it at 125 until you know what you'll get, and then experiment from there. A lot of Kodak Tri-X shooters expose at 250 even though that film is rated at 400 (35mm version).

The other stops on the ISO dial are:
25 32 40 50 64 80 100 125 160 200 250 320 400 ??? 640 800 - can't remember from there...

[MattTyler]

Thanks for the help.

Any reason for this order or am i missing some maths problem?

Matt.

[another view]

Any reason for this order or am i missing some maths problem?

One very important reason: That's how it's always been.

I'm not sure what the technical explaination is for the numbers - one of the Kodak books might tell though. Or someone else?

[Michael Fanelli]

Thanks for the help.

Any reason for this order or am i missing some maths problem?

Matt.

They are each 1/3 stop more/less sensitive than the ones next to them.

[Chunk]

It's like an inverse exponential function in all those exposure formulas. Don't look too hard at this stuff unless you really need and excuse for a headache. Just be aware that each doubling of the iso is a doubling of the sensitivity so you need half as much light let in, either by halving the time the shutter is open or shutting the aperture a stop (half as much crossectional area).

[You might notice that the aperture numbers is also an exponential series inversely proportional to the shutter speed - seems like whoever started writing down all the calculations for exposure started from shutter time and let all the rest fall where they may]

It's been a long time since I've looked at this stuff so I may have something abit cornfused.

[another view]

They are each 1/3 stop more/less sensitive than the ones next to them.

Yes - and this is the important part to remember. What I meant was that I'm not sure why, for example, the two steps between 100 and 200 aren't 133 and 167, not 125 and 160. I'm sure there's a mathematical formula for it like there is for lens apertures (f1.4 is twice as much as f2, etc).

[Dzerzhinski46]

Yes - and this is the important part to remember. What I meant was that I'm not sure why, for example, the two steps between 100 and 200 aren't 133 and 167, not 125 and 160. I'm sure there's a mathematical formula for it like there is for lens apertures (f1.4 is twice as much as f2, etc).

When they were calculating the steps in between they probably interpolated (mathematical speak for figuring out the in between spots). But yeah, there probably is some formula for film sensativity. Or the other reason is that shutter speeds go 1/8th, 1/15th (instead of 1/16th), 1/30th vis 1/32th, 1/64th vis 1/60th, etc. It probably is connected with the shutter speeds used now on cameras. Another connection is Sunny 16. Now if you were using DIN, Sunny 16 wouldn't work. So it probably is connected with shutter speed. Hope that makes a little sense. Actually that was just me thinking out loud.

As for FP4, it is my favorite. I have been pretty happy rating it at the suggested ISO 125. I like the combination of FP4 and Ilfosol S. Fantastic stuff.

Sorry for the ramble.

Dzerzhinski

[Lemming51]

...
The other stops on the ISO dial are:
25 32 40 50 64 80 100 125 160 200 250 320 400 ??? 640 800 - can't remember from there...

The factor change for a full stop = x2.
25 - 50 - 100 - 200 - 400 - 800 - 1600 - 3200 - 6400 - ...
32 - 64 - 125 - 250 - 500 - 1000 - 2000 - 4000 - 8000 - ...
40 - 80 - 160 - 320 - 640 - 1250 - 2500 - 5000 - 10000 - ...

For 1/3 stop the factor is the third root of 2 = x1.259921. The intermediate values are rounded (eg. 640 instead of 634.96).

[handstrap]

Thanks for the help.

Any reason for this order or am i missing some maths problem?

Matt.

The order is derived from the square root of 2. Multiply an f.stop by 1.4 and you will get the next aperture in the series. The URL gives some additional information about apertures...

http://www.great-landscape-photograp.../aperture.html