What are ISO film speeds based on?

[raczoliver]

As I have been reading these forums and experimenting in my

own "darkroom" (well, a bathroom actually) I realized that almost

everyone likes to overexpose and underdevelop their negatives by 2/3-

1 stop. This decreases the apparent film speed, but often makes the

negatives have much richer tones, and prints look much better than

from negatives exposed according to the ISO rating and developped

according to the manufacturer's recommendations.

 

My question is, why do they write "ISO 100" on a film that really

should be ISO 64, and why ISO 400 on a film that works much better at

200? What is the ISO standard based on, if not practical evaluation

of a certain film in a certain developer?

[chris_waller]

You'd need to look up the exact details in a reference book but in essence the speed is based on the exposure necessary, under a given development regime, to give a density of 0.1 above fog. It is a rigourous method of determining the relative sensitivities of films but the application of those films to 'real' photographic situations varies greatly, hence the decision to expose at diferent speeds and develop accordingly to adjust the contrast of the negative.

[rick_jones5]

A manufacturer's ISO rating is quite accurate based on the particular developer used in testing with development to a particular contrast index. Also keep in mind the manufacturer is going to rate based on the very minimum exposure needed for adequate shadow detail. Do you think they would sell more film by allowing a more generous safety factor and therefore a lower speed rating? In practice there are many factors which could make more exposure and less development beneficial. Some developers will give up speed for the sake of other benefits (ex. Microdol X). Some enlargers print with more contrast and therefore call for less development than manu suggests (ex. condenser enlarger). My principal shutter speeds may be running a little fast. My most used apertures may be just slightly off giving less exposure. All these factors and more make it impossible for the manu to give a prcise ISO #.

[hans_beckert]

You can order the ISO standard from this site:

 

http://www.iso.ch/iso/en/ISOOnline.frontpage

 

Here is the one for film speed:

 

http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=3580&ICS1=37&ICS2=40&ICS3=20

 

You have to pay for it.

[david_goldfarb]

If you look at Ilford's technical data sheets available from their website, they are a little more realistic about this. They list a range of development times and exposure indices with some information about the advantages of one particular combination or another (e.g., fine grain vs. speed vs. convenience, etc.). You still might make adjustments from their recommendations (for instance I use the recommended development time for Delta 400 Pro in Perceptol at EI 250, but I rate the film at EI 200 for a little better shadow detail or perhaps because my meter or shutter isn't quite accurate), but this way of presenting the options is a much better reflection of the kind of decisions that B&W photographers make than simply saying "Blahblahpan-X ISO 400."

[dennis_oconnor2]

You have been given excellent sources to read on how the ratings are derived... I want to discuss the real world... Let's assume that a film is rated as ISO/ASA 100 by the manufacturer... Now assume that your light meter is reading a half stop fast <not unusual> and that the light transmission of your lens is actually a half stop slower compared to whatever the f number shows on the barrel <also not unusual>... This means that your roll of film will be underexposed by one stop at a nominal 100... So you will have to rate the film as ISO/ASA 50 in order to get properly exposed negatives with your equipment... The film is still ISO/ASA 100, but 'your' exposure index is 50.. Another photographers index might be 40, or 64, or 80, or even 125, in order to get the appropriate negative density...

 

Also, the rating will change (black and white films, primarily) depending upon the developer... Some developers will cut the rating by one stop or even two stops, others will increase it two thirds of a stop... In my hands Tmax 100 is a 125 film developed in Xtol, a 100 in D-76, and 80 in my homebrew developer..

 

Cheers ... Denny

[hans_beckert]

In agreement with the last two posters:

 

If one is concerned with the basis for the ISO speed, the relevant material is obtained from the ISO itself. As far as applicability to photographic practice, many variables influence the speed attained in use.

[andy_piper2]

1) ISO speed, as mentioned, is determined by a specific exposure/development/sensitometer-reading regime established by the International Standards Organization.

 

2) Equipment vagaries, developer preferences and personal taste play a role in whether the "ISO" speed works for any particular person.

 

3) Metering preferences can play a big role as well

 

The ISO speed may work fine if you carefully meter the deep shadows and then adjust exposure to darken them 2, 3, or 4 stops (depending on which flavor of the Zone System you prefer) below the 'medium-gray' meter reading.

 

Whereas the ISO speed may NOT work as well if you meter more generally (averaged metering or incident-light metering). In a contrasty scene the shadows may drop below the 'official' 0.1 density; in a 'flat' scene they may be too dense.

 

I actually usually expose most B&W films at an EI 2/3rds stop HIGHER than the ratings (e.g. Pan F at 80, Delta 400 at 640 or 800). But that's because I scan my film, and my scanners a) can pick up very tiny levels of silver in the shadows, and b) tend to compress and muddy the midtones and highlights unless they are well down on the straight-line portion of the films' H&D curves.

 

4) Lens contrast also has an impact - a modern, contrasty lens may require cutting the film speed to hold shadows within the film's range, while an older, softer lens may do just fine at the rated ISO.

[sjmurray]

I think an important, related factor is the way you meter. Many people use the built in camera meter, which is usually center weighted, but still averages to 18% gray. Aiming closely at someone's face will place the skin tones at 18% gray, which is already one stop underexposed for Caucasian skin. If you want to compensate for this and expose more correctly, you could rate your film on that camera's setting as one stop less than the box speed. On the other hand, you wouldn't necessarily want to do this for a landscape. In other words, it gets complicated. Using a spot meter or hand held incident meter isn't always the best/most convenient way to go for sports or street photography, so you have to know just what your camera is seeing and metering so you can make adjustments accordingly.

 

In other words, simply rating your film more or less than the box speed is not necessarily really changing the effective ISO rating, its just telling your camera to on the average expose more for the shadows or highlights, depending on which way you go.

[hans_beckert]

In response to the second part of your question, many photographers find that modest amounts of additonal exposure and modest decreases in development provide results preferable to what the ISO standards of exposure and development call for.

 

The ISO standard is designed to provide acceptable results with a wide variety of apparatus and formats. Miniature workers and large format workers often, though, have significantly different problems and photographic goals, which means they handle exposure and development of film in different ways. Thus, the standard has to be tailored for each application.

 

Almost all films have a great deal of over-exposure latitude, but very little underexposure latitude when following the current ISO standard. Previous versions of film speed standards called for more exposure as a safeguard for underexposure. In the 1950's, a safety factor of 2.5 was applied to film speed values over the absolute minimum required to achieve a normal exposure. If you ever watch black and white movies from the 1950's, before the standard was changed, you can see the kind of almost washed-out look that this extra exposure factor gave.

 

The safety factor has been reduced to almost nothing in the current standards. Thus, the current standard provides little underxposure latitude.

 

Development recommendations are, however, a little generous according to some experts.

 

Thus, the current situation is that a little additional exposure and a little less development are not harmful and may be prefered by some.

[kelly_flanigan1]

The formal legal ISO/asa defintion is based on the speed point; where the film MUST be developed to have a certain slope. Over and under developing change the slope; and thus the film has no legal ISO rating.<BR><BR> This is akin to rating dirt bikes ability to climb hills; in a certain time. A standard slope of the hill MUST be defined to make the test useable and valid. If one tests bike A versus B; and uses different hills; then the test is bull dung/poop; ie bogus. Often people get Exposure index and ISO/ASA confused. ISO/ASA is measured to a slight exposure above fog; with the film developed to a rigidly strict slope. Unlese the slope/hill is defined; the speed of the film/bike will vary with the slope/hill. The Kodak books; and SPIE handbook; and ISO/ANSI specs define ISO for all types of film types. <BR><BR>The consumer needs a defined product; and a legal test method; and not some marketing BS. The ISO spec is made to compare film A versus B; with a standard contrast. One can develop tri-X for hours in a developer; and get a super contrasty film; that has no formal ISO number. This is because the film was over developed; and has the wrong slope; and thus is not defined. Several of the ASA specs went thru a formal change about 1960; with a 2x "safety factor" removed on B&W products.

[gary_reese2]

The ASA standard calls for development to a 0.8 gamma. That is considered overdevelopment with most any general purpose, pictorial film. Most of us know that effective film speeds inch upwards with overdevelopment. Which leads us to the inescapable conclusion: ISO speeds (ASA/DIN) are overstated for films developed to normal, lower gammas/contrast indicies/average gradients. It is no wonder most folks who bother to test for their own Exposure Index (E.I.) come up with a slower speed.

[jim kerr]

For practical purposes-zone I.