real film ISO/ASA

[rui_marques]

Hi, some time ago I read (unfortunately can't remember where) that film

sensibility is always a litle lower than stated (e.g. real sensibility for a

400 ASA film is 320 ASA). Is it really like that? Assuming this is true, when

processing a 400 ASA film exposed for 320 ASA is any compensation in

processing times necessary? How to determine that compensation?

Thank's a lot (Sorry for my bad english, I hope my questions are

understandable...)

[ronaldo_r]

Not true. Depends on your camera's meter. Nonetheless, film can be tolerant of overexposure. In moderate amounts it can increase contrast - again entirely up to the way you and/or your camera meter the scene. My film camera EOS 30/Elan 7e was generally very good with exposure so I usually shot film at its official rating, i.e 400 at 400 etc

[rick_jones5]

The film manufacturer tests his film using specific ISO standards as a way to give you a starting point for obtaining a printable image. Bacause there are so many variables (different developers, meter sensitivity varies, metering methods vary, different users want different results) the assigned ISO number may or not produce the results you want. Only you can decide what is appropriate. Personally, I decide by evaluating shadow detail. I try to assign the highest E.I. which will give me the shadow detail that looks right to me. Again, for me, that can be as much as double the exposure the maker suggests (i.e. 200 for a 400 film). I have always looked at exposure and development as two separate issues. Exposure to set shadow detail and development to tailor contrast to the paper I use. Other will disagree as I have read numerous posts recommending shorter development times when a films speed is down rated. Regardless of your approach TRUST YOUR EYE. If shadow detail is skimpy in the negative try lowering the E.I. If a majority of your projection prints don't show the contrast you prefer adjust your development times. So, look carefully at your results and adjust accordingly.

[skygzr]

Exposing at 400 as opposed to 320 is only 1/3 of a stop. You won't notice much difference.

 

I routinely expose most films at less than what's on the box (Ilford FP4 at 50, for instance). You get better shadow detail and there's no reason to change processing times.

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00IW14

[nancy s.]

No. It is not true. I know that some people regularly over expose negative film, but if your camera meter is correct, this should not be necessary. If you over expose reversal film, you are all done.

[continuity]

You find out the appropriate Exposure Index by testing, testing, then more testing. Given

a consistent "workflow", same lens, camera, film, developer, photo, you bracket around

the ISO and come up with the one you like. (ISO is set my the MFG and is at best nominal,

EI is the setting you use to actually expose the film).

 

Different developers also compound the problem. i.e Rodinal can be "slower" (ISO 400 -->

EI 250) and Ilford DDX can be faster (i.e. ISO 400 -> EI 500)

 

The technical method can be found in Ansel Adam's The Negative. You're basically trying

to find out the minimum amount of exposure (shadow detail) that can be reliably recorded

and go from there.

[chris_waller]

The determination of film speed is according to strict scientific principles, and is based upon the exposure required to produce a certain density given a certain development. However, having said that, film can be exposed at an ASA other than its rated speed providing development is also changed. For example, I use Ilford FP4, rated at 125 ASA, but often expose it at as low as 50 ASA, cutting development accordingly, in order to handle scenes of high contrast without losing shadow detail or burning out the highlights.

[j.a.]

Hi Rumi,

 

Contrary. The ISO/ASA real value is higher than the initial told number, namely for the colour negatives having an ISO initial value like 200, 400, 800. Example, a 400 ISO film behaves itself like an 500/525 ISO film and it is rather tolerant when used 1,5 - 2 stops over.

 

Thank God, the b&w films are the most flexible to overexposure, consequently they are the "nec plus ultra" for art photographers or for the reporters, because they can reproduce a personal point of view. But you have to do your own film chemistry...

 

best

 

 

J.A.

[spanky]

Maybe I'm an exception but I noticed the few times I overexposed a roll and adjusted my development just as an experiment I noticed that while I did lower contrast (probably due mostly to the cut in development time) I also seemed to increase graininess and the resulting tones didn't look near as nice as when I rated the same films (from the same lot number) at the box speeds. Just my $.02

[kristian_skinner1]

Colour film, which some people seem to be talking about, is generally roundabout the stated iso, but the true film speed of bw film is frequently a full stop slower than the iso on the box when processed correctly, and it varies with the way one processes it, developer and especially the camera. Very accurate, recently tuned shutters give the film speed usually 2/3 stop or so slower than the box speed. If you compensate with the processing time, that's just pulling the film, which will decrease contrast. There's a very methodical way of determining the true iso, given on http://www.celluloidandsilver.com/zone-system-testing.htm if you're nice, a pro lab might let you use their densitometer, if not there's another visual, less accurate way of determining it, but I can't seem to find the link, sorry.

[larrydressler]

Like said A film speed is set by the manufactuer at a set tempature and time in a standuard Developer.... I am not sure if Kodak is still using D-76 but I do to test new films.

 

Diafine gives a 1 to2 stop increase in film speed with little or no increase in shadow loss or grain increase... and Microdol-X gives a 1/3 to full stop in film speed with a loss of grain but some detail... Rodinal shows film as it is... flaws and all.

 

A Film is only as good as it's mates ... Camera/ Meter/Developer and Photographer .. not to include processor Photographer and eye of caring...

 

Larry

[rui_marques]

Thank's all for your time. Helpful link Kristian. I guess that testing is the best (only?) way to find out what one is looking for. Thank's also for tips on that.

[fred_obturateur]

"I routinely expose most films at less than what's on the box (...) no

reason to change processing times".

After several tries I came to the same conclusions. You get easy to print negs which is the main goal to achieve, right? (along with *clean* negs) :-)

Good luck!

[josh_carroll]

This is a little bit of a combination betwen fact and fiction.

 

Your medium film speeds do tend to be rated about right, however people used to use meters which...weren't the best. They found their negaitves often underexposed, and so began to slightly overdevelop or overexpose them, people assumed it was their film rather than their meters and so it came about that people assumed that film was lower rating than they say on the box. Or something along those lines, I'm afraid I forget the details.

 

However, higher speeds do tend to be given with incorrect ASA ratings - Ilford Dleta 3200 is notorious for being a 1600 film with push times recommended as standard.

 

Some films are rumoured to be given with pushed times from somethign like 320 to 400 just to give a little more contrast on the negative... I find I personally prefer Tri-X 400 over HP5+ at 400 because it's slightly more contrasty, which would be explained if it was a 320 film exposed at 400 and slightly pushed for contrast.

 

But in the end this argument is a bit of a dead end as all negs are down to personal preferance, and whether they are wrongly rated or not it's the results that matter.