"Real" ISO values of sheet film.

[andysig]

In a book on large format photography which I have just read (Großformat-Fotografie by Hermann Brix), the author in his chapter on the zone system describes how he assessed Kodak T-Max 400 and determined that the actual ISO rating is 200 i.e. half the official value. He then determined that the optimal development time (in Kodak HC110, 1 + 10 dilution) was 9 mins 30 secs.

My questions are as follows:

a. While it's clear from the above that films allow a degree of exposure latitude, why do manufacturers not give the "real" ISO or is the ISO value somehow arbitrarily selected?

b. Is there a table somewhere of the "real" values of the commoner sheet films along with the ideal development times for various developers?

By the way, although I have no intention of getting into the zone system, I would like to be able to get optimal results in terms of what I can get out of the film. I've just started developing my own sheet film and am currently using T-Max 100 (exposed at that ISO value) developed in Tetenal Ultrafin according to the manufacturer's instructions.

This leads me to my final question: are there any B & W film and developer combinations which are particulary suited to landscape work, detail shots e.g. carved wooden doors in old houses and portraiture? Or is it all just a matter of experimentation until you find something you like?

 

[Steve Smith]

why do manufacturers not give the "real" ISO or is the ISO value somehow arbitrarily selected?

The ISO stated is the real ISO as it is defined by an international standard. Any other figures are EI (Exposure Index).

It is fairly common that when people do tests to find their ideal or 'personal' film speed, the vast majority conclude that they will use it at half the manufacturer's rated speed and reduce development by about 20%.

This was so common that I didn't bother doing the tests myself. I just tried it (with HP5+ at EI 200) and decided I liked it.



Steve.

 

[Ian Grant]

The ASA/BS part of the ISO speed is determined under laboratory conditions which don't reflect reality. A manufacturer can use the ASA/BS method of testing or the more practically based German DIN method. So an ISO speed of 400/27º is actually 400 ASA/BS and 27ºDIN

With the Introduction of Tmax films the ISO standard was loosened because Tmax films couldn't pass the old ASA part ISO standard as 100 & 400 speed films.

Now manufacturers are supposed to state how they determine the film speed. So Ilford state that their film speeds are based on practical testing:

The ISO speed rating was measured using ILFORD ID-11 developer at 20°C/68°F with intermittent agitation in a spiral tank.

It should be noted that the exposure index (EI) range recommended for HP5 Plus is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard.
(from the HP5 Datasheet)

Choice of developer makes a significant difference, as does teh range of tones and amount of shadow detail required and when Tmax films where released Kodak' Datasheet recommended using them at half the box speeds if you wanted a greater tonal range.

This is why it's important to do your own tests to find the speed/development times that suit you best.

Ian

 

[Stephen Benskin]

The ASA/BS part of the ISO speed is determined under laboratory conditions which don't reflect reality.

They actually do reflect real world conditions, "reflect" being the key word in the sentence.

The reason why the author found his personal film speeds to be different from the ISO speed is because they are different testing methods. The Zone System assumes a different relationship between the metered exposure point and the speed point than the ISO method. I'm just about to cover the relationship between the sensitometric exposure and camera exposure on the K factor thread. The reason why people are happy with both is do to the range of acceptable variance exposure with b&w film.

As a litmus test, anyone using "real film speed" when talking about personal testing doesn't know what they are talking about. Find a better author.

 

[Monophoto]

As Steve says, the rating assigned by the manufacturer is derived from a set of standardized tests ('type tests') conducted by the manufacturer on samples of the film. That is in fact the 'actual' rating.

What the author of your book did was to conduct a different set of tests that took into account his lens, his camera, and his meter. True Zonies conduct that test each time they change emulsion number so that they can also account for manufacturing variations in the film. I've done that test on both sheet film and roll film, and my experience with 35mm TMY is that the measured EI ranges from 160 to 240.

It is typically the case that when a Zone System test is done, the measured EI is about half the rating assigned by the manufacturer. But there can be no such thing as a standard table that lists these EI values since they take into account variables that change with photographer.

It's also quite common for the Zone System film development test to result in a time that is different from what the manufacturer recommends. Again, it comes down to the fact that the test accounts for variables that are unique to each photographer and his darkroom. In my case, I found that the optimum development time for TMY in HC-110, dilution B is 4.5 minutes - half what your book reported. And when I use sheet film, I use a time of 11 minutes in HC-110, dilution H.

The Zone System is a marvelous tool to calibrate your system. But the risk is that you can end up doing nothing but testing, and never make any real photographs. I suppose that's akin to the other disease that some photographers get - GAS.

As to your final question, my experience is that you can produce great images with any film - you just have to calibrate it for optimum response with your lenses, cameras, meters, water supply, developer, agitation cycle (everyone agitates differently), timer repeatablility, enlarge characteristics, and the photographers' esthetic preferences.

 

[Steve Smith]

I'm no expert but here are a couple of paragraphs I wrote more for my own reference than anything else.

http://www.freewebs.com/stevesmithphoto/personal.html

http://www.freewebs.com/stevesmithphoto/contrast.html

They may be of some use (or they may confuse the issue further!).


Steve.

 

[Chuck_P]

I test using ZS principles and have found my personal EI to be the actual "box speed" with TMX/d-76 1:1 and TMX/XTOL 1:1. Only with TMX and HC-110 1:63 have I found a 2/3 reduction off the box speed.

Although it is a very common notion, the notion that there is a risk of doing nothing but testing with the ZS and never photographing is bogus. A newcomer should not be influenced by those comments.

 

[Chuck_P]

By the way, although I have no intention of getting into the zone system, I would like to be able to get optimal results in terms of what I can get out of the film.

Then pay attention to your shadow areas of the negative. If you find that you routinely are dissatisfied with them with the "box speed", then you should consider rating the film at a lower speed.

 

[andysig]

Those are very interesting and illuminating replies. Thanks to all.

Monophoto: I am stunned that there are people who recalibrate for emulsion batch changes. Now that is dedication!

Steve Smith: Thanks for those two links as they sum it up quite nicely for a relative beginner like me.

CPorter: That is definitely the level of advice I can cope with! I shall abide by that until I can work my way up to the two summaries provided by Steve.

 

[Stephen Benskin]

What the author of your book did was to conduct a different set of tests that took into account his lens, his camera, and his meter.

I hate to have to say it, but if you carefully break it down, they really don't. All they really do is add a ton of variables to the testing procedure which helps to confuse the issue of film speed. Garbage in garbage out. If you don't understand and are able to control most or all the variables influencing the test, how can you be assured of the results? You want to factor in the lenses, have them bench tested. If you want to know the speed of the film, do a test for the speed of the film.

While there are differences in effective film speed from different developers, the consistent 1/2 to 1 stop difference between ZS and ISO speeds comes from the assumed difference between the testing speed point (sensitometric for ISO) and the metered exposure point. The ZS stops down four stops. The ISO speed point is 3 1/3 stops. That's the difference and the reason why the ZS results are so consistently different.

What exposure you like or works best for you is another issue and doesn't address the question of film speed determination and shouldn't be part of the discussion.

 

[Steve Smith]

I shall abide by that until I can work my way up to the two summaries provided by Steve.

Those are just my simplified views on the subject and may not agree with those of others!


Steve.

 

[removed account4]

iso/asa/box speed is determined in a lab not in "real life"
( as ian stated in his post )

box speed is just a number .. a starting point.
unless you are in the same emulsion testing lab, and have (their) perfectly calibrated ( or uncalibrated ) equipment
and are processing the film in the same cocktail under the same conditions your results will be different.

 

[Vaughn]

I believe it is one of Ted Orland's Photographic Truths that no two light meters will agree.

So "real-life" film speed will also be affected by which meter you use. If my meter consistantly reads one stop more than your meter, my film speed will be half your film speed (assuming everything else is equal).

 

[BetterSense]

I still don't understand why it's important to even know your film speed. Who cares? Why is it important to know what your film speed actually is? All that matters is how much you have to expose in order to get the pictures you want.

 

[Existing Light]

I still don't understand why it's important to even know your film speed. Who cares? Why is it important to know what your film speed actually is? All that matters is how much you have to expose in order to get the pictures you want.

I totally agree. When I shot HP5+ as negative film, I've found I got the shadow detail I liked with my tools and materials at around 200. You can call that the EI or the "film speed." Semantics really isnt my thing. Either way, when the print is finished and framed, it doesnt really matter, does it?

 

[brianmquinn]

The links by Steve are great. I have done about the same thing for years (after reading The Edge of Darkness). There is only one problem with following this advice. It makes all negatives look like they were taken in the same sort of light conditions. There are times when the mood you want to convey in a print calls for a flat or contrasty negative. You really have to think about the final print to decide exactly how you want to expose and develop. When in doubt follow Steve's advice. As I said I do the same 90% of the time.

If you decide that you will just use higher or lower contrast paper when printing you will be in for a lot more work and expense.

 

[Vaughn]

I still don't understand why it's important to even know your film speed. Who cares? Why is it important to know what your film speed actually is? All that matters is how much you have to expose in order to get the pictures you want.

Understanding what film speed is, is what's important.

 

[ic-racer]

I still don't understand why it's important to even know your film speed. Who cares? Why is it important to know what your film speed actually is? All that matters is how much you have to expose in order to get the pictures you want.

Yes. But, unless you want to make some permanent mark on your meter, having a number is nice.

And sharing ones EI is like sharing ones shoes size. Like who cares.

 

[michaelbsc]

I believe it is one of Ted Orland's Photographic Truths that no two light meters will agree.

I have a box full of broken ones from eBay that all read the same.

My wife keeps threatening to throw them out, but I figure I'm saving them from hoarders.

I gonna fix them one day real soon now.

 

[BetterSense]

Yes. But, unless you want to make some permanent mark on your meter, having a number is nice.

Well, there you go. I don't use light meters, so I guess that's why I don't understand.

 

[Steve Smith]

The links by Steve are great.

I shouldn't really take the credit. It was just me regurgitating an almalgamation of things I had read on the subject put into a couple of paragraphs which I could refer to later.


Steve.

 

[Stephen Benskin]

iso/asa/box speed is determined in a lab not in "real life"
( as ian stated in his post )

This is a photographic myth. Instead of regurgitating talking points, why not provide an argument with facts and prove it?

 

[removed account4]

it's a myth, why do you say that ?

do you have first hand knowledge how film makers assign asa values to film ?


i have never seen the process, but Photo Engineer,
a retired emulsion engineer at kodak has referred to the process on occasion.
what was suggested is that in a lab setting, the film is exposed and processed
using D76 or another developer in a controlled setting, and the film is evaluated.

i have never heard that it was imaginary, or mythological.

 

[Photo Engineer]

Just a comment here!

Kodak has ISO certification for all of the "standards" of the industry. One ISO standard is for ISO speed! This speed is determined in the lab by direct calculations and is then VERIFIED by field experiments to be correct. All Kodak films are thus verified for speed 2 ways. If they are not, then they cannot claim the ISO label for speed. Remember that ISO is a lab defined methodology. Remember that Kodak does it BOTH ways.

When I was there, the release B&W developer was D-76 straight! All developers are tested though and posted with the data on the Kodak web site.

PE

 

[removed account4]

thanks ron,

i didn't think i imagined ..., well, maybe just part of it ...

- john