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Difference between EI and ASA/ISO?

Kodak Exposure Index ASA Exposure ISO

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#1 Connor Adam

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Posted 05 February 2015 - 06:04 PM

Hey, 

 

I was reading through Kodak's 'The Essential Reference Guide for Filmmakers' and came across the following:


A word about film speeds

You probably know that motion picture films use exposure index (EI) to indicate speed. Although similar, EI is not the same as the ASA or ISO speed used for still films. EI denotes a somewhat conservative figure related to the higher quality requirements of motion picture film that must be projected onto a large screen. Typically the EI speed is about one stop lower than ASA or ISO. EI 500 film, therefore, is the equivalent of ASA/ISO 1000. 

 

Is anyone able to develop further on this? It is the first time I've come across the concept that EI is not the same as ISO. If I take what is written literally, does this mean that I should be setting my light meter to read at 500 instead of 250 (assuming I am shooting on say 250D stock) to get correct exposure?

All the best,
Connor


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#2 aapo lettinen

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Posted 05 February 2015 - 06:43 PM

exposure index means the actual speed where you rate the film. You can rate for example ISO500 film to EI 1000 by under exposing 1 stop and push processing  ( EI is "speed setting" as opposed to ISO which is "speed rating" ) .

 

You can also rate for example a 200 ISO film to EI 400 and process normal, then you would lose some shadow information and gain some highlight information because you would underexpose the film by 1 stop. If you correct it later in scanning/printing/color correction to normal brightness (pushing one stop in printing) you would have more visible grain and maybe lost more or less one stop of shadow information

 

if the film manufacturer rates the 1000 ISO film EI500 it means they recommend to expose it to that speed to get well balanced image (desired color/grain, shadow/highlight balance, etc. if normal processed) . you can rate the ISO1000 EI500 film to which ever setting you want but that affects hugely the film's response . 

If you want the basic image the film manufacturer intended you will set the meter to the EI they recommended, if for example EI500 then you have to set your meter to ISO500. 


Edited by aapo lettinen, 05 February 2015 - 06:47 PM.

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#3 Josh Gladstone

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Posted 05 February 2015 - 06:58 PM

Well that is exceedingly confusing. The short answer is no. If you look up the technical information for 250D (http://motion.kodak....data/TI5207.pdf), under Exposure it reads:

Exposure Indexes

Daylight (5500K): 250

Tungsten (3200K): 64 (with 80A filter)

Use these indexes with incident- or reflected-light exposure meters and cameras marked for ISO or ASA speeds or exposure indexes. These indexes apply for meter readings of average subjects made from the camera position or for readings made from a gray card of 18-percent reflectance held close to and in front of the subject. For unusually light- or dark-colored subjects, decrease or increase the exposure indicated by the meter accordingly.

 

 

Further down it has a table of exposure corrections for color balance and clearly lists the Exposure Index at 250. So I don't know! I do know that box speed for 250D is 250, but hopefully someone else has an idea about what the deal with everything else is.


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#4 Connor Adam

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Posted 05 February 2015 - 07:21 PM

Many thanks for your help. I think I understand the concept now, although still a little confused as to the statement they make saying 'EI 500 film, therefore, is the equivalent of ASA/ISO 1000'.

Perhaps Kodak are referring to a recommendation to overexpose film by a stop?


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#5 Josh Gladstone

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Posted 05 February 2015 - 07:45 PM

Right, but wouldn't you be underexposing a stop if you rated it at 1000?


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#6 John E Clark

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Posted 05 February 2015 - 07:55 PM

Many thanks for your help. I think I understand the concept now, although still a little confused as to the statement they make saying 'EI 500 film, therefore, is the equivalent of ASA/ISO 1000'.

Perhaps Kodak are referring to a recommendation to overexpose film by a stop?

 

ISO is the modern standard, ASA was the older standard, and the standard states 'thus and such' about how to determine the speed.

 

An 'exposure index' is the value one uses for setting the meter. It can match the ISO/ASA value, but can be used to 'change' the camera settings, such as 'under expose', or 'over' depending.

 

So, if I use a film with an ISO rating of 400, and I set my Exposure Index to 800, I have 'under exposed' the film by 1 stop. Or if I notice that my meter yields exposures that are 'too thin', I can set my Exposure Index to 200 and compensate for the apparent underexposure.

 

Underexposure could be due to a creative choice, or just because the f-stop on the lens or the shutter speed is off, and heck, the meter could be off as well... if each item is off by 1/3 stop in the same direction I could end up with a full stop of 'underexposure' due to that.

 

In 'creative choices' one can chose to 'underexpose' and push the development process, yielding a specific effect. Likewise one can 'over expose' and 'pull' the development by one stop, yielding a different 'look'...


Edited by John E Clark, 05 February 2015 - 07:58 PM.

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#7 Connor Adam

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Posted 05 February 2015 - 08:07 PM

Josh, of course. Typo on my part!

Thanks John - a clear explanation. I just wasn't aware that exposure index describes where the exposure is set, opposed to the sensitivity (or ISO/ASA rating) of the stock/sensor.
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#8 David Mullen ASC

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Posted 05 February 2015 - 09:54 PM

Exposure Index is more or less the same as ISO except that the value has been tweaked by the manufacturer so that the customer will get optimal results.  In other words, E.I. freed the manufacturers from listing the actual measured ISO value so that they could suggest a rating that they thought would work better for that stock.


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#9 Bill DiPietra

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Posted 06 February 2015 - 02:31 AM

At the end of the day, if you use the tungsten or daylight ratings on the film can as your reference and compensate (for filtration, overexposure, etc.) from there, you'll be fine.
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#10 John E Clark

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Posted 06 February 2015 - 01:19 PM

At the end of the day, if you use the tungsten or daylight ratings on the film can as your reference and compensate (for filtration, overexposure, etc.) from there, you'll be fine.

 

I use to rate Tri-X at ASA 200... but I always shot color at the manufacturer's rating... I never developed color, except as a school exercise. With color there were just too many variables, chemical temperatures, then in the printing dialing in different amounts of color correction, with out the benefit of a analyzer... for the few prints I did do, it was hours of waving filters of various strengths over the test print... ok... I'd burn up 20 sheets of B&W paper on a print... but color struck me as more tedious...


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