iso vs. EI

<p>when people say that a certain film performs best at a different EI (say panF 50 at 25 EI), do they mean to basically set the meter to the EI (not the films ISO) and then take a exposure reading, and process the film as if it was the EI....<br>

ex. of my thinking....<br>

panF ISO 50, but will shoot at 25 EI<br>

-set meter to 25 (iso) and take reading<br>

-adjust camera to what meter says<br>

-then process film as if was a 25 iso, not a 50 iso</p>

<p>this seems to me like it would overexpose the film. WOULD i be right to set meter at 25 iso take reading and adjust camera according, then Pull process the film (N-1) according to PAN F instructions???<br>

thanks</p>

<p>Yes, the term EI is used in this context to mean what you tell the meter your film speed is. So if you set your camera's ISO dial to 25 when using Pan F 50 film, you were using EI 25. Now if you develop normally you're overexposed by 1 stop, but you're correct that you can use a 1-stop "pull" to compensate.</p>

<p>If you go here: http://www.digitaltruth.com/devchart.php?Film=Pan+F&Developer=&mdc=Search you can see dev times for that film for a lot of EI/developer combinations.</p>

<p>More reading here as well:http://www.rogerandfrances.com/subscription/ps%20iso%20speeds.html</p>

<p>There are probably some legitimate differences of opinion on this but...</p>

<p>ISO speed is determined specifically according to ISO standards. Any deviation from ISO standards, including using a different developer, may produce a slightly different true film speed - as determined by the usual minimal measurable density over base plus fog. Even the ISO standard acknowledges that using different techniques and different developers may indeed produce a difference in the true film speed.</p>

<p>Using a different technique and/or developer may produce a slightly different true film speed. This is usually referred to as the EI or exposure index. Diehard zonies will test and measure their results to determine the EI for their personal preferences, including their preferred developers.</p>

<p>Some folks, including me, will take rather generous liberties with the term EI and apply it to push processing. This is probably a sloppy and incorrect usage of the term, since pushing Tri-X to 1600 is not even remotely close to its true speed (per the usual standard for determining true film speed). I'm trying to break myself of the habit of using EI when referring to pushing. The true EI of TMY or Tri-X might indeed be something like 500 in a speed enhancing soup like Microphen. But I don't know whether it's appropriate to use the term EI in reference to exposing Tri-X at 1200-1600 and souping it in Diafine, since there is no true shadow detail per the usual standards.</p>

<p>Occasionally you'll hear claims that the ISO standard is the only true measure of film speed. Not so. The ISO standard is simply a consistent, repeatable means by which a film's true speed can be evaluated. But if you read the actual text of the standard carefully it does make allowances for deviations from the ISO method:</p>

<blockquote>

<p>From ISO 6:1993(E)<br /> Black and white films will generally provide excellent<br /> results in several different developers and processing<br /> conditions. At the same time, it is realized that the speed<br /> of a film depends on the process used. Therefore, this<br /> International Standard specifies a method of determining the<br /> photographic speed of film/process combinations. <em>This means<br /> a particular film may have several ISO speeds associated<br /> with it depending on the processes used. For this reason, it<br /> is important that manufacturers indicate the processing<br /> conditions for which ISO speed values are quoted.</em> <br /> <br /> This International Standard recognizes that black and<br /> white films do not generally have a unique speed if several<br /> different processes are recommended. This conflicts with the<br /> tradition of associating a specific speed value with a<br /> particular product. In the future, the process used for<br /> determining sped values should be unequivocally described to<br /> avoid misinterpretation. Since users often do not know how<br /> these films well be processed, manufacturers have an<br /> obligation to proved a speed value for this situation which<br /> will ensure good results. Usually they will take advantage<br /> of the overexposure tonal latitude of the film and give it a<br /> conservative speed value to protect users from underexposure<br /> effects in case the film is put through a process which<br /> yields low speed.<br /> <br /> It is recognized that the speed at which a film can be<br /> exposed is depends on the extend of development, scene<br /> luminance range, subject matter, printing paper, etc. This<br /> International Standard specifies that film/process speed is<br /> determined with the film is processed to obtain a specified<br /> contrast level. Then relative ISO speed ranging of various<br /> films in different process system s will generally differ.<br /> The ISO speeds will provide correct exposures for average<br /> scenes with exposure metes conforming to ISO 2720 or ISO<br /> 2721 when the film is processes as specified in this<br /> International Standard.</p>

</blockquote>

<p> </p>

<p><em><strong>This International Standard recognizes that black and<br /> white films do not generally have a unique speed if several<br /> different processes are recommended.</strong> This conflicts with the<br /> tradition of associating a specific speed value with a<br /> particular product. In the future, the process used for<br /> determining speed values should be unequivocally described to<br /> avoid misinterpretation. Since users often do not know how<br /> these films will be processed, manufacturers have an<br /> obligation to provide a speed value for this situation which<br /> will ensure good results. </em> <em> </em><br>

<em><strong>Usually they will take advantage<br /> of the overexposure tonal latitude of the film and give it a<br /> conservative speed value to protect users from underexposure<br /> effects in case the film is put through a process which<br /> yields low speed.</strong> </em></p>

<p>What the ISO-6/1993 is saying, is that the film manufacturers can use any developer they choose to arrive at the ISO speed rating, provided that they state which process was used. Ilford`s ISO ratings are usually based on development in ID-11.<br>

If the user chooses a different developer such as Perceptol which loses speed compared with ID-11 or Microphen which slightly increases speed compared with ID-11, then a different exposure index will probably be required. This becomes your personal exposure index through personal testing, although the exposure index might also differ from the ISO even if the user selects ID-11 as their developer.</p>

<p>The film manufacturers will usually give the film an ISO rating slightly lower than that which was found in ISO testing due to the films latitude towards over exposure, to help prevent the film user from under exposing their negatives.</p>

<p>I look at this ISO adjustment issue a little (maybe a lot) differently. The film rating number on my light meter and/or camera is the highest number that gives me the shadow separation (detail) that I'm looking for. For my uses that number is usually lower than the manufacturers rating. For me that is not overexposing - it is exposing enough. Adjusting the film rating down increases the density of the negative but has little or no effect on the contrast. If a good percentage of my exposures print on a grade 2 or 3 paper (or w/ VC filters) I will not change the development time. In short, I rate the film for shadow detail and pick development times to control contrast keeping the two as separate issues. Some have developed very sophisticated methods for adjusting film ratings and development times using a densitometer. But you can do very well by simply looking at your prints critically making subsequent adjustments to exposure and development based on what your eyes alone tell you.</p>

<p>Andrew is correct that shooting a 50 ISO film and 25 EI and pull-processing it amounts to treating it as a 25-speed film. But I don't think that's what most people mean when they say a film performs best at a certain EI other than its official ISO. To me, what that means is that the film isn't really as fast as its ISO claims it to be. In other words, a 50 ISO film might "really" be only 25 and therefore needs to be exposed at 25 but still developed according to the manufacturer's specifications to get good results. Yes, you are overexposing compared to the official ISO. But if the ISO is inflated, you are actually exposing correctly. ISO speeds are supposed to be determined very objectively, but over the years there have been plenty of times when film companies have been accused to rating their film too high. The other issue is that photographers meter differently so one person might get good results with his meter set at 50 and another with his set at 25. But generally the development stays the same. In slides, the processing is uniform but lots of people rated Velvia at 40 because they thought it wasn't "really" 50. Others rated it at 64 because they liked their slides a little darker.</p>