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The exposure index (EI) of a film speed is the rating of a particular film that varies from the recommended ISO film speed. For instance, 100 film speed may actually be used as a 400 film speed in what is called pushing film. Conversely, a 400 speed film may be used as a 100 speed film in what is called pulling film. By changing the ISO film speed to the EI speed, the photographer is speeding up, or slowing down, a particular film. When pushing film, the photographer can gain an extra one or two stops to obtain good negatives in low-light or motion conditions. For instance, if a photographer rates an ISO 100 film at EI 200, they gain one stop in exposure. Read more about this in pushing or pulling film.

The EI film speed setting may also be used in cases where the shutter of a camera is off timing and always overexposes or underexposes the film. There may also be instance where the light meter is inaccurately reading. In these cases, the photographer may adjust the EI rating to compensate for these errors. By forcing the EI rating faster than the ISO speed (pushing film) the photographer is underexposing the film. Forcing the EI rating slower than the ISO speeding (pulling film) causes the film to be overexposed. It should also be noted that when a photographer changes the EI rating, the film will require a different development time to properly develop the negatives. In addition, you cannot change the ISO speed midway through a role or you will under/overexpose half the roll and will have no way of fixing this.

Why Adjust the EI Film Speed?

Accidents happen. If by chance you load 100 ISO film into your film camera and forget to adjust the ISO setting and leave it set to 400, you can still continue to shoot film and save it.

Don’t have the right film. There may be many cases where you need a faster film but are only equipped with slower films. You can push the slow film to a faster EI speed to improve exposures indoors or help with motion photography. There may also be instances where you want to slow the film to capture blur motion or other effects.

Contrast. In high contrast areas where there is an abundance of shade and bright light, lowering the film speed can help to reduce the dramatic difference in contrast. Conversely, increasing the film speed can add contrast.

Grain. This is not for everyone, but pushing film will help achieve grain that is associated with faster films. Some photographers like the look.

You must also understand that pushing or pulling film with different EI film speeds will require you to compensate when developing the film. Pushing the speed will require over-developing the film while pulling the speed will require under-developing it. Read more about EI processing in the push and pull processing page under the film processing section.

via Exposure Index – EI Film Rating | Gudie to Film Photography.

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Zone system film speed test.

 

Black and white film speed is found by testing for the minimum exposure that will produce enough density in the negative to print as a tone just visibly lighter than maximum black. Zone I. You can shoot this test on a roll of film that you plan to develop normally. It only takes six frames.

 

Making Test Exposures

 

1. Set up a target, white, gray or any medium color will do. It should be large enough to fill the frame from six feet away. The target should be smooth – wrinkles can cause shadows or uneven brightness.
2. Light the target evenly. Meter readings in the center and at the edges should not vary more than 1/3 of a stop.
3. Set your meter at the film’s ISO rating (e.g. 400 for Tri-x). This will be called the test speed.
4. Take a reflected meter reading of the target; choose a shutter speed that will allow you to bracket without using the slowest or fastest speeds.
5. Set your camera for manual exposure; any auto-exposure setting will make the test meaningless.
6. Record your exposures as you make them.
7. Make the following exposures

 

1. Meter reading – Zone V
2. Blank frame – FB+F film base plus fog
3. 5 stops less thank the meter reading. (close down the lens, you may need to increase the shutter speed don’t forget to put it back at your original setting as soon as possible) – Zone 0
4. 4 stops less than the meter reading -Zone I
5. 3 stops less than the meter reading – Zone II
6. 2 stops less than the meter reading – Zone III

 

8. Develop the film normally. Using the developer, time temperature you usually use.

 

Evaluating the test

 

To get the true film speed you must determine which test exposure produces Zone I. There are two ways to do this: you could make a visual determination by judging prints or you could use a densitometer. The densitometer is more accurate but you need to have access to the equipment.

 

The Densitometer Method

 

 

1. Take a reading of the blank frame (exposure b)
2. Take readings of all other frames; the frame with 0.10 – 0.15 more density than the blank frame (b) is the speed frame.

 

If test frame c is the speed frame then your true speed is 2x the test speed.

 

If test frame d is the speed frame then your true speed is equal to the test speed.

 

If test frame e is the speed frame then your true speed is ½ the test speed

 

If test frame f is the speed frame then your true speed is ¼ the test speed

 

For example: You tested Kodak Tri-x (ISO 400) and your blank frame (b) reads 0.23

 

Frame c reads 0.23

 

Frame d reads 0.26

 

Frame e reads 0.33

 

Frame f reads 0.41

 

Your speed frame is e; if your test speed is ISO 400 then your true speed or EI (exposure index) is ½ that or 200.

 

The Visual Method

 

 

1. Print a test strip from the blank frame (b)
2. From the test strip choose the shortest exposure that turns the paper maximum black. If your not sure what maximum black looks like compare your test to a piece of paper that has been exposed to white light then developed.
3. Print the rest of the frames from your test (c,d,e,f,) using the exposure time you determined in step 2 which would maximum black in the minimum time.
4. When the prints are dry judge which print is a tone just visibly lighter than max black. That is the speed frame.

 

For example: You test Kodak Tri-x (ISO 400) and your blank frame (b) prints max black in 5 sec.

 

At 5 sec frame c is max black

 

At 5 sec frame d is max black

 

At 5 sec frame e is just lighter than max black

 

At 5 sec frame f is almost gray

 

Your speed frame is e; if your test speed is ISO 400 then your true speed or EI (exposure index) is ½ that or 200.

 

Another variation on the Visual Method is to place your target in a scene with recognizable tones. These tones will help you determine which is the correct exposure.

 

 

The Practical Method

 

This method is not as scientific or accurate as the densitometer tests however it works. You will need a gray card for this method

 

1. Choose an evenly lit stationary subject.
2. Set your meter at the film’s ISO rating (e.g. 400 for Tri-x). This will be called the test speed.
3. Take a reflected meter reading of the gray card; choose a shutter speed that will allow you to bracket without using the slowest or fastest speeds.
4. Set your camera for manual exposure; any auto-exposure setting will make the test meaningless.
5. Record your exposures as you make them.
6. Make the following exposures.

 

1. Blank frame – FB+F film base plus fog
2. 5 stops less thank the meter reading. (close down the lens, you may need to increase the shutter speed don’t forget to put it back at your original setting as soon as possible) – Zone 0
3. 4 stops less than the meter reading -Zone I
4. 3 stops less than the meter reading – Zone II
5. 2 stops less than the meter reading – Zone III
6. 1 stop less than the meter reading – Zone IV
7. Meter reading – Zone V
8. 1 more than the meter reading –Zone VI
9. 2 more than the meter reading –Zone VII
10. 3 more than the meter reading –Zone VIII
11. 4 more than the meter reading –Zone IX
12. 5 more than the meter reading –Zone X

 

 

Develop the film normally. Using the developer, time temperature you usually use.

 

 

Evaluating the test

 

 

Contact the film for the minimum time it takes to reach maximum black.

 

Choose the frame with the best detail in both high lights and shadows.

 

If frame g – zone V is correct then your EI is equal to the test speed.

If frame f – zone IV is correct then your EI 2x the test speed

If frame h – zone VI is correct then your EI half the test speed.

If frame i – zone VII is correct then your EI one quarter the test speed.

 

For example:

 

You test Tri-x ISO 400 and frame f looks best then your EI would be 200. ISO/2=EI