Knowing which ISO film to load next can be really difficult. Here is a guide along with an downloadable excel file for you to add your own cameras and lenses.
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1. The Exposure Table
The exposure table is made up of the most common shutter speeds (1/1000 up to 2 second) incremented in full stops only and a range of apertures (1.4 up to 32) also incremented in full stops only. The table could have been presented as a block of boxes without being diagonally oriented but the reasoning for this is crucial. Pick any column and look at the shutter/aperture values and their tone. Starting at the top the shutter is fast with the aperture large and as you work your way down the shutter is increasing by 1 stop as the aperture is decreasing by 1 stop. This means that the shutter/aperture combinations in that column are all equivalent exposures!
2. In More Detail
The top row of the exposure table sets the shutter speed at 1/1000th of a second. On the scale of shutter speeds from 1/1000th up to 2 seconds, 1/1000th will produce the least exposed image and as such it is displayed in black and 2 seconds will produce the most exposed image and as such it is displayed in white. The same goes for the apertures 32 to 1.4. This is useful as a visual representation for the graph as a whole as seen in the column below.
Looking at this column with a shutter starting at 1/1000th and an aperture of f1.4 each step down the column produces an image of the same exposure.
This is important because unlike a digital camera, I do not have constant access to change the ISO setting to correctly expose my image . Once the film is loaded - that's it. So let's take a look at how you can add your own camera and lenses to the sheet for your own use.
3. Fill Out The Data
First name your camera and your lens next to each environment. Add as many environments as you like next to each lens. At the top have the brightest environment and as you go down the environments should become darker until you get the the darkest environment you can shoot on that camera and lens combination.
4. Figure Out Your Own Settings
Regardless of what film you currently have loaded you can still change the ISO setting and get a reading from the internal light meter. Go out to different environments with each of your lenses and pick an ISO and aperture that puts you somewhere in the middle of the f-stop reading. In my case I took a reading in my living room one dark evening (Dim Incandescent No Natural Light Interior) at 3200 ISO and 1/30th shutter speed and got an exposure reading of f5.6. This means I have the full range of shutter speeds and apertures from 1/1000th at f1.4 all the way to 1/2 second at f32. At 1600 ISO I lose the ability to shoot at 1/1000th at which point the image would be 1 stop underexposed.
5. Closing Points
Shooting in direct sunlight shouldn't require testing if you base it on the sunny 16 rule which states that in direct sunlight your shutter speed should be 1 over (/) your ISO meaning that at 400 ISO your shutter should be 1/400th. If you want to shoot in a range of environments you should pick an ISO that can accommodate the two. If I want to shoot in broad daylight and an overcast day then I'd be better off with a 400 or 800 ISO.
Also remember that reciprocal rule states that you shouldn't shoot handheld at shutter speeds less than 1 over (/) your lens length. If you're shooting a 50mm lens on a full frame sensor then it's not advised to shoot shutter speeds slower than 1/50th due to camera shake and resulting blur.
Good luck!
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