Exposure Index

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Can someone please explain to me in easy terms what an exposure index is and how to obtain it?

-- Ben Langley (benjaminlangley73@yahoo.com.au), September 27, 2000


In order to know how to expose a film under given lighting conditions, you need some kind of index to translate the brightness measured by your meter into suggested combinations of shutter speed and aperture on your camera. Typically, this index will be the ISO speed of the film as given by the manufacturer. It is determined by a standard (ISO) method and applies to standard conditions. Sometimes, however, there is good reason not to use the standard speed. Examples are personal preference or experience (let's say you found that using the ISO speed gave you images that are a bit too dark), non-standard lighting conditions (such as shooting in a bar, where there is much more red light to which the film is less sensitive than the meter), and only having an ISO 100 film in your camera when the lighting requires an ISO 400 one. In such cases, you set your meter to a different speed that is more appropriate. As it is non-standard, it's not an ISO speed, but just an exposure index (EI).

So, to cut a long story short: The ISO speed is a laboratory value for the speed of the film under standard conditions. The EI is the speed index you actually use to account for personal factors that are not standard.

Now for obtaining it the easy way:

1) Expose a film at its ISO speed, process as specified by the manufacturer.

2) Critically assess the images, particularly the tones you are most interested in (usually the shadows).

a) If they are fine, you have found your EI. It happens to equal the ISO speed.

b) If they seem underexposed, go back to 1), but use the ISO speed minus two-thirds or one f-stop. DON'T VARY ANYTHING ELSE. If the images are fine now, you've found your EI. Otherwise, repeat at lower speeds, increments becoming smaller as the results get closer to what you want.

c) If the images seem overexposed, do the same as in 2) b), but at ISO speed PLUS something.

Now for the complications:

It may be that using an EI that is reduced w/ respect to the ISO speed you find the highlights too dense. In that case, you have to adapt development time. Start by reducing it by 10 to 15%. This will not usually have a great effect on the shadow detail, but it may affect your EI if you are used to metering off midtones instead of shadows. Then you have to repeat the process using the adapted development time. It is therefore recommended that you determine your EI on the basis of the desired shadow detail, or rather density.

Let me repeat my waring: NEVER VARY TWO PARAMETERS AT THE SAME TIME! The only thing you achieve by doing so is total confusion and a waste of time and material.

-- Thomas Wollstein (thomas_wollstein@web.de), September 27, 2000.

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