Archived posting to the Leica Users Group, 2004/01/27
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The International Standards Organisation (ISO) standard for black and white film speed is ISO 6:1993. You can buy a .pdf of it for 47 Swiss francs from www.iso.ch. It stipulates that the 'real' speed of a film is that at which the film in question, developed in a certain developer at a certain temperature gives a closely specifid sensitometric output (i.e. range of densities). IMPORTANT! and as Luc mentioned: The basis for the current ISO standard was developed by the American Standards Association (ASA) before the zone system was popular (and possibly even before Ansel, Fred and whoever else was involved had even formalised it themselves - I've had problems getting a *precise* history regarding exactly _when_ all this happened). It was developed by Kodak and the ASA based on a system that should be employed more. They shot a lot of film at varying 'speeds' (i.e. exposed it to varying amounts of light) and developed it all the same way, then printed it on a specified paper (obviously they did this a number of times to get in the right ballpark). They then showed the photos to a large number of people and asked them to rank them by which 'looked' best. They then analysed these negatives that the people regarded as 'best' and described them sensitometrically. So when you see the ISO standard *remember that the initial sensitometric basis was generated qualitatively*. People's eyes are more sensitive to highlight detail, so that bears some relevance to the standard, it was done (I have read) with mainly portraits of caucasian people on a dark background - which is also very important. It's also good to remember that the standard uses one developer one temperature and tightly prescribed agitation etc etc. To be an accredited lab that can test film requires a lot of equipment and a heck of a lot of calibration and testing equipment. Luckily, to make good photos you don't need much of that equipment at all. This is why Tri-X can have a 'real' speed of anywhere from 125-1000, because we use different developers, temperatures, agitation, printing paper (or these days scanners etc.) from the ISO standard. The ISO standard is a guide, but as at the very start of the standardisation process, your eyes are better. If a certain speed (given your developer and other variables) looks 'right' (and this very much depends on whether you [a] know what you want and/or [b] know what is 'good') then it probably _is_. For what it's worth the 'real' ISO speeds of the superspeed films currently on the market are as follows: Fuji Neopan 1600 ~640 Ilford Delta 3200 ~800 Kodak TMax 3200P ~1000 I use the ~ because these are rounded to the nearest third of a stop. It's useful to keep in mind that these speeds mean more on the box that the film comes in than they do in your darkroom. Plenty of people have taken great photos without even knowing what speed their film was, so find a message from one Kyle Cassidy and read the last line . . . off you go . . . Marty PS I don't know if CCDs and other digital capture media have their own ISO for sensitivity. Maybe they do. Maybe one of the electrical gurus can tell us?!!! >The fullest tonality range is reached at a certain ISO, below or above >there's no shadow detail or the highs are blocked. This is essentially >based on the Zone System, and the 'ranges' of the film are usually >tested using a densitometer on the film itself. From there you then >adjust Film-Dev-Time-ISO to calibrate to your methods and taste. > >LB > >On Lundi, janvier 26, 2004, at 08:49 PM, Adam Bridge wrote: > > On 2004-01-26 <feli@creocollective.com (Feli di Giorgio)> thoughtfully > wrote: > >> >> I have read that the true speed of TMX3200 is about 800-900asa, Delta >> 3200 is 1200asa and Neopan1600 is really about 640asa. >> > > Now what does this mean? I'm never clear on just what a films "real" > speed is. > > How do you test it? How do you know? > > Thanks > > Adam > -- > To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html > _____________________________________________________________ Get your Free Global name@sharkattacks.com e-mail address at http://www.sharkattacks.com - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html