Film ASA- Followup

[Mike Panczenko]

A few days ago I asked how film ASA/ISO numbers are assigned. I don't think I was as clear as I could have been, sorry. I was wondering, and am assuming that equations are used on results obtained from certain tests (if I am wrong, please correct me), what equations are used and what tests are conducted to find the film speed. How is the number 500 arrived at, versus the number 50? I haven't been able to find this out anywhere. Thanks!

[John Pytlak RIP]

Exposure index:

 

http://www.kodak.com/US/en/motion/support/faq/exposure.shtml

 

Calculation of ISO film speed for still film color negatives:

 

http://webstore.ansi.org/ansidocstore/prod...ISO+5800%3A1987

 

B&W still film speed:

 

http://webstore.ansi.org/ansidocstore/prod...ku=ISO+6%3A1993

 

Color reversal still films:

 

http://webstore.ansi.org/ansidocstore/prod...IT2%2E21%2D1994

[Dirk DeJonghe]

May I suggest the reading of the classic " The negative" by Ansel Adams, available from most on-line bookshops. It explains how to set up B&W processing and how to use the zone system for exposure control.

 

This is all for still photography but very applicable to motion picture. I recommended this book to a new-to-film DOP (he had a lot of video experience). He shot about 3 or 4 rolls of 5218 reloaded in 36 exposure cartridges and made phenomenal progress as a film DOP.

 

Basically Adams recommends to do exposure tests and find where zone 1 (just above D-Min) falls. If you understand this then you will also understand that push processing a negative will not really increase the speed of the film, just add some density and give higher print numbers.

[John Pytlak RIP]

The blackest black you will ever get is from the minimum density (D-min) of the negative. So unless you have enough exposure to have some shadow area image densities above the minimum density of the negative, you will not capture the information.

 

With Kodak VISION Premier Color Print Film (2393), the blackest black you can get on the film is over 5.0 in density, or more than a 100,000:1 brightness ratio. Projection lens flare and ambient light in the theatre reduce this somewhat, but you still can achieve remarkably rich blacks on the screen.

[Mike Panczenko]

If you do have time, would you mind explaining so I don't have to buy the book? I'd understand if it's too involved,though. Thanks for the links.

[David Mullen ASC]

Does the actual ASA, ISO, EI number mean anything -- i.e. why is 100 ASA that number and not another? I understand why 200 is twice as fast as 100, etc. But how did that particular numbering system begin?

[Mike Panczenko]

I found out from a search, that the ASA number is defined as 10/H, where H is the exposure in lux seconds, at which certain image perameters are reached. The question still remains, though, what these image parameters are.

[John Pytlak RIP]

A very good discussion of film speed parameters is in the Kodak technical paper "Technical Basis of Photographic Speed Determination, or What is a Normal Exposure" by Daan M. Zwick, SMPTE Journal, August 1979, Volume 88, page 533.

[Laurent Andrieux]

1) The asa settings don't fit to film neither video

2) you ahve different methods to determine one film sensitivity

3) The general formula is S=K/H where S is sensitivity, K a figure depending on the film, H the necessary lumination for obtaining a certain reference density this reference density depending upon the method :

for instance, for b and w : K = 0.8, Rd (refence density) is 0.1 over the toe -