What is Exposure Index

[jsmithphoto1]

Hey guys! I have a stupid question...

I have NEVER used EI before (just ISO), and I want to know what it is so I can better understand and explain it. The info I seen was a tad confusing. I have been using film all my life and have been developing for 10 years, and believe me... I had to humble my proud self to ask, but I figured you guys were the best to ask! Thanks in advance!

 

[David Brown]

ISO is the film's "box speed" determined by testing to the "ISO" standard. EI (exposure index) is the "speed" rating that you actually use. They can be the same or different. For instance, if one shoots TriX at 200, the ISO is still 400, but the EI is 200. If one shoots TriX at 400 (or any film at "box speed") the ISO and EI are the same. That's really all there is to it in practice.

Folks often say that they shot a 400 speed film at "ISO 200". No, they shot it at EI 200, but we know what they mean ...

 

[Sirius Glass]

I have always used the box speed. Some cut the ISO by 1/3, 1/2. or 2/3 f/stop to get a denser negative or because they think that they will get more shadow detail. I suspect that many do it because:

1. They do not know how to meter correctly and regularly take light readings of the sky.
2. Their light meters, cameras or lenses need a CLA.
3. They think they are smarter than the film manufacturer.
4. They love to test and would rather test than actually take photographs.
5. Some are Zonistas.
6. They are compensating for not knowing what they are doing. Related to #1 though #4.

 

[MattKing]

For clarity, when we refer to someone "shooting" a film at an EI of, e.g., 200 we are actually saying that we are setting a meter to an EI of 200, measuring the light with that meter, and then using the results to set the camera.

I point this out, because in many cases the EI that people choose reflects metering technique as much or more than the characteristics of the film.

On the other hand, films like Ilford Delta 3200 are designed to behave in different ways when you use different EIs. So the choice of EI with that film is a choice to take advantage of one of the sets of characteristics of that film.

 

[Poisson Du Jour]

In simple terms, if you expose your film at ISO 100, it's ISO 100.
It is the educated photographer who actually knows what he/she is doing, correctly predicts the outcome and is satisfied with the results!

If you select ISO 125, 200 or something else, it is sometimes known as a "speed shift", but more generally termed as an Exposure Index — EI.

There is usually a very good reason for this change, such as to provide a bit more detail in shadow areas, or to reduce the risk of highlights burning (rarely both at the same time). Sometimes it can be used as a form of under- or over-exposure.

You should know how the film you are using will respond to the change, especially when using chrome film. With negative film quite large changes in EI are more common than the small changes that are frequently used to tame temperamental chrome films when/if lighting and the scene are troublesome or needs help.

You should jot down any change applied to the film's speed (EI, then!) so you can make value judgements once the images are processed. No harm is done in using one or several EI values on a single roll of film, just keep notes of what you are doing.

 

[jsmithphoto1]

LOL! I have always used the box speed as well, which has hindered me from learning more about EI, because I don't really push or pull. I just set my camera to whatever speed the film is (ISO) and use the TTL meter to get the exposure or effects I want. The only time that I have seen a need to expose at a different ISO is when I am using a super out-dated film (such as some old Kodak Vision I occasionally use) or if using Modified Windisch developer.

 

[jsmithphoto1]

Awesome! That makes a lot of sense! So basically EI is shooting an ISO 400 at an ISO 200 or vice versa? Or, more technically, metering the exposure at, as you said, 200, but shooting on a 100 speed film? This is starting to make a little more sense now. Thanks!

 

[jsmithphoto1]

Thanks for your replies, everyone! This is beginning to help and make more sense!

 

[Sirius Glass]

Thanks for your replies, everyone! This is beginning to help and make more sense!

As your understanding increases, you sanity will leave to provide space for the new understanding. Once you fully understand EI and the Zone System sanity and logic will completely leave your mind. You will be a member of a large group of film photographers.

​

 

[AgX]

The EI not only relates to one's own idea of exposing of a film but also relates to manufacturers' proposals.

This goes as far as films being sold not with ISO but an EI rating (e.g. Ilford 3200 Professional)
Typically this is the case for a film intended for push-processing, which is not conform ISO standards of speed measuring, but still yield acceptable results.

 

[Bill Burk]

The definitions given so far are all good.

Sirius Glass listed a lot of good funny reasons people use a different EI number than the ISO...

And maybe I fall into one of those categories, but I think of it as...

EI is what you set your light meter to, so that you will get the results you want when you meter the way that you do... develop the way you do... and print the way you do.

Sometimes the reason you need to set that meter to a different number is because you use a different developer that doesn't get the ISO speed - or gets more than the ISO speed (reversal processing to get transparencies from black and white film and other specialty developers like DR5 come to mind).

Sometimes you are using expired film.

Sometimes you have your own idea of what a quality negative entails. An example I often keep in mind is how much I enjoy a negative that takes 32 seconds to print and it would be nice to use the enlarging lens at its sharpest f/stop. So I might choose an EI that accomplishes that goal.

 

[Gerald C Koch]

The ISO speed is determined by the manufacturer using a specified method dictated the International Organization for Standards. Hence the acronym ISO.

https://en.wikipedia.org/wiki/International_Organization_for_Standardization

Formally film was rated by the American Standards Association ASA. The two methods were slightly different in defining the portion of the Characteristic Curve that is used for the calculation. In addition the ASA standard demanded the use of a specified developer. The ISO standard allows the manufacture to use a developer formulation of their choosing.

If you are using any speed other than the ISO rating then you are using an Exposure Index abbreviated as EI also known as the effective speed. Careless use of the acronym ISO only leads to confusion.

You might also find this informative as you may come across Weston (used on Weston light meters) , DIN (German cameras and film), and GOST (used on Russian light meters and cameras).

https://en.wikipedia.org/wiki/Film_speed

 

[HiHoSilver]

I like Sirious' list. I'm working my way through - don't want to miss one. I know I'm near the mark by nagging uncertainties about metering. For all the articles & vids, I don't know that I've ever seen a physical image of a meter being used with any detail. Shade the sensor from any direct light that might hit the sensor, how to check different parts of a contrasty composition & meter each section - knowing you got a valid reading, etc. Maybe its just that those that know the detail - have done it a long time & forget what its like not to know it - and what the noob needs to get savvy about. Its also possible I fixate on any possible ambiguity in the descriptions/instructions. Two more rolls shot today. I need to concentrate on taking better notes so I don't have to wonder why the results are as they are.

 

[Doc W]

I have always used the box speed. Some cut the ISO by 1/3, 1/2. or 2/3 f/stop to get a denser negative or because they think that they will get more shadow detail. I suspect that many do it because:

1. They do not know how to meter correctly and regularly take light readings of the sky.
2. Their light meters, cameras or lenses need a CLA.
3. They think they are smarter than the film manufacturer.
4. They love to test and would rather test than actually take photographs.
5. Some are Zonistas.
6. They are compensating for not knowing what they are doing. Related to #1 though #4.

Sirius, you crack me up, and with this set of prejudices, you generate a lot more heat than light. Lots of us test our film for best results with different developers, etc. It's not a big deal. It was not invented by the Illuminati. The proof is in the final result, the print on the wall, and lots of people use lots of different approaches to get there.

 

[Bill Burk]

Sirius, you crack me up, and with this set of prejudices, you generate a lot more heat than light. Lots of us test our film for best results with different developers, etc. It's not a big deal. It was not invented by the Illuminati. The proof is in the final result, the print on the wall, and lots of people use lots of different approaches to get there.

I agree! Sirius' list is a hoot.

We could ad to it ad infinitum - invite 'em - the Illuminati to add more light to 'em.

 

[Poisson Du Jour]

I have always used the box speed. Some cut the ISO by 1/3, 1/2. or 2/3 f/stop to get a denser negative or because they think that they will get more shadow detail. [...]

Surprise, surprise. That is what I teach. It's what I've been doing since Nelson lost an eye. And I'm not flying solo. True dinks, sweethearts, swing the other way for highlights.

Should I be further surprised that a flogging with a warm lettuce leaf is now heading my way?

 

[georg16nik]

I have always used the box speed. Some cut the ISO by 1/3, 1/2. or 2/3 f/stop to get a denser negative or because they think that they will get more shadow detail...[]...

The optimistic ISO speed numbers works for baby diapers contrast or if you shoot polar bears on snow with the sun at it's zenith.
The ISO is relevant to real world photography as organic food is to McDonald's burgers.

 

[benjiboy]

I have always used the box speed. Some cut the ISO by 1/3, 1/2. or 2/3 f/stop to get a denser negative or because they think that they will get more shadow detail. I suspect that many do it because:

1. They do not know how to meter correctly and regularly take light readings of the sky.
2. Their light meters, cameras or lenses need a CLA.
3. They think they are smarter than the film manufacturer.
4. They love to test and would rather test than actually take photographs.
5. Some are Zonistas.
6. They are compensating for not knowing what they are doing. Related to #1 though #4.
   
   I always shoot at box speed unless I consistently get incorrect exposures at that speed, I figure the film manufacturers know more than I do about their films.

 

[RalphLambrecht]

I have always used the box speed. Some cut the ISO by 1/3, 1/2. or 2/3 f/stop to get a denser negative or because they think that they will get more shadow detail. I suspect that many do it because:

1. They do not know how to meter correctly and regularly take light readings of the sky.
2. Their light meters, cameras or lenses need a CLA.
3. They think they are smarter than the film manufacturer.
4. They love to test and would rather test than actually take photographs.
5. Some are Zonistas.
6. They are compensating for not knowing what they are doing. Related to #1 though #4.
   
   I always shoot at box speed unless I consistently get incorrect exposures at that speed, I figure the film manufacturers know more than I do about their films.

1. 
   
   To me, ISO is just a reference number for my film test. My EI slightly changes with contrast indexISO is measured at a fixed contrast index.

 

[Photo Engineer]

This is an old reference and not everything is correct, but it is interesting.

http://www.photomemorabilia.co.uk/Ilford/Film_Speeds_1960.html

 

[Stephen Benskin]

This is an old reference and not everything is correct, but it is interesting.

http://www.photomemorabilia.co.uk/Ilford/Film_Speeds_1960.html

The more things change...

Good find Ron. A nice bit of historical contemporary perspective.

 

[Photo Engineer]

Actually, I forgot to give the finder recognition.

Bill Troop sent this to me recently and so thanks go to Bill.

PE

 

[DREW WILEY]

God originally created light in exactly eight zones of gray. But then Super-XX film came along and somebody thought they detected twelve zones. But then Ansel Adam's discovered that if you simply developed Super-XX for less time, you got back to eight zones, which made Minor White happy because he had already founded a religion which only allows eight zones to be contemplated while sitting crosslegged on a bamboo floormat. Kodak taught a different religion entirely, based on a curved universe.

 

[Photo Engineer]

Love that Drew. Einstein would agree.

PE

 

[cliveh]

Although I always shoot at box speed, I may expose at variation to this.