Difference in old and new ISO (ASA) and DIN equivalents

This is a question for a 'gray haired' expert on film speeds. I am

reviving my old interest in black and white photography (after

seeing the serious color deterioration after 20 years in my color

prints). So I just had a look at my old (1950's) Zeiss Ikophot

exposure meter and found that the ASA and DIN equivalents are

different from the ones published recently. Did the old DIN standard

use a different grey level saturation, or is this a Zeiss thing?

Does anybody perhaps have another brand vintage exposure meter (e.g.

Metraphot, Weston)with ASA and DIN scales, or an old text reference

to check this out? Here are some examples from the Ikophot dial: ASA

10 = 14.5 DIN, ASA 40 = 20.5 DIN, ASA 160 = 27 DIN, ASA 650 = 33

DIN.

 

Thanks for the help!

"The Amateur Photographer's Handbook" by Aaron Sussman, 7th Edition, 1965:

"In 1943, however, the American Standards Association arrived at a method of film rating which ultimately will become THE American Standard to be used by all exposure meter, film and flashbulb manufacturers..."

"The new ASA Exposure Index for this film (Plus X of the day) is 80.."

"(In 1961 this figure, with reduced safety factor, became 160)."

 

Same book, 8th edition, 1973:

"When the ASA system was first devised, the published speeds incorporated a safety factor of anywhere from 2.5 to 4x the minimum exposure required for a properly exposed negative. Because of this, a generation or more of amateurs suffered through the densest negatives you've ever seen .... Subsequently, Kodak and all the other film manufacturers adopted new ASA ratings with reduced safety factors for all monochrome film."

 

In other words, the ASA ratings were revised in 1961, and the conversion varies for meters made before then. Hope this helps!

There's one factor Steven didn't mention and that is the fact that the ASA and DIN ratings were not directly comparable, at least to the science nuts who devised them. The ASA rating was based on the straight line portion of the H&D curve. DIN was based on the exposure required to gain a particular density above base fog (the density of developed but unexposed film). In some cases these agreed more than others. Today we usually accept 100 ASA (now ISO) equals DIN 21. When ISO first came into use we used dual numbers for a few years like 100/21. In truth, according to the actual rating systems, ASA 100 might be DIN 20, 21 or maybe even 22. Before about 1961 DIN 21 equaled ASA 50, but all the books and magazines always pointed out the ASA safety factor and most serious photographers used the higher rating. In an era when few people had meters and larger negatives were the rule it made sense. Better to be overexposed than under; you could still get a decent print from a 6x9cm negative on 120 film.

The ASA standard in the fifties still used the factional gradient method with a safety factor sometimes referred to as 2.5. C.N. Nelson in "Safety Factors in Camera Exposures," Journal of Photographic Science and Engineering 1960 defined it as 2.35. DIN used a fixed density of 0.10 above Fb+f with the film developed to gamma infinity until 1957 when they adopted a developmental contrast that reflected normal usage.

 

I believe the old ASA standards had an EI rating calculator for light meters using the other or older systems. It's possible to obtain vintage standards from ANSI. Some of the earlier editions of Jack Dunn's Exposure Manual might cover the older standards too. There's also the "Theory of the Photographic Process" 2nd edtion. It was published in 1954.

Thanks folks, that really helped me out! Keep up the good work!