Judging from my tattered collection of old issues of Popular Photography, the transition in popular usage occurred around 1983.
Judging from my tattered collection of old issues of Popular Photography, the transition in popular usage occurred around 1983.
It should be noted that both the ASA and the ISO curves are log-log. That is the density (vertical axis) is the logarithm of the inverse of transmissivity and the horizontal axis is the logarithm of exposure. For data on the same film that curves are identical. This curve is referred to as the gamma curve, the G-bar curve, the G curve, as well as the characteristic curve.
What is different is how the speed point is determined.
ASA took the Average Gradient (AG) which more or less corresponds to the straight-line portion of the curve (over a log exposure interval of 1.5). Then it computes a line with a slope of .3 times the slope of the AG and finds the tangent point between this line and the toe of the curve. "The speed is computed by taking the reciprocal of the exposure value E [the exposure where the point of tangency occurs], in meter-candle-seconds, and rounding the result to the nearest step on a scale in which the steps differ by a factor of the cube root of 2, as 100, 125, 160, 200, etc."
This is from the Kodak Data Book, Kodak Films, 6th edition, published in 1954. It is interesting that in the 7th edition, published two years later, Kodak explains the gamma curve again, but leaves out the description of how the speed point is calculated.
The ISO calculation of the "Speed Point" is entirely different. Again, ISO works with the gamma curve. First, they measure "base plus fog" by developing unexposed film. This is because the film base reduces transmissivity and the unexposed emulsion will also produce a slight residual fog. The starting point adds a density of .1 [an arbitrary convention] to the density of base plus fog. "The film is then developed so that another point, which has received 1.3 log units more exposure than the starting point, is 0.8 density units (2 2/3 stops) darker than the base point. Then by dividing .8 by the exposure at the base point, you determine the arithmetic speed of the film ..."
This description is available in a number of sources, I simply took the liberty of quoting "The Black & White Handbook", 2000, Hicks and Schultz.
It is said by many photographers that the old ASA is more honest because it determines the speed point on the curve where the rate of density vs exposure is significant while films with a shorter than normal or a longer than normal toe can distort the true value of the film.
Unfortunately, the ASA values were difficult to calculate back in the days before computers were common. ISO is less complex to calculate, the method was used by the Europeans long before it was adopted by the Americans, and the resulting film speeds are usually similar. The differences are miniscule considering that we round the film speeds to the nearest 100, 125, 160, 200, 250, 320, or whatever, anyway.
It is interesting to note that the ASA system was devised by couple of photogrammetrists (?) Jones and Condit based on research by Hurter and Driffield a couple of English photogrammetrists who worked for a British subsidiary Kodak's back in 1895 (the gamma curve was often also called the D&H curve). Jones seems to get more credit than Condit for devising the ASA index.
In any event, the gamma curves for ASA and ISO are identical, they are simply plots of the density vs log exposure. The difference is in the methods for determining a film's speed index.
A couple of quick corrections from further research:
The characteristic/gamma/H&D/G-bar curve is also referred to in some quarters as the D/log E curve.
As a historical note, my date of 1895 was wrong, from further research I find Ferdinand Hurter, who was Swiss, and Vero Driffield, English, originated the science of photographic sensitometry in 1890, not 1895.
ISO colour negative speeds are determined by simply taking the exposure for Status M 0.15 above fb+f in each of R, G and B then using
Hmean= (Hgreen x Hslowest)^0.5.
Best,
Helen
Last edited by Helen Bach; 20-Sep-2006 at 14:11.
My head aches
DG
Geez Louise...! I musta been in some kind of time bubble. Though I remember ASA being used in 1987 - so I'm not so sure about that. But the error in my post (& thinking) was that I was getting ISO and DIN mixed up. DIN was the log scale. I appreciate the info. It was something I paid attention to once upon a time - and since then, I'd just used my old standards...Originally Posted by Kevin Crisp
As for granularity - well, I'd been looking at some sort of 'objective' measurement to be able to figure out a good film stock to use for scanning. Surely - scanning must be the great equalizer, regardless of channel-dependent granularity indices. Anybody know of a good experiment published online looking at detail and film grain comparing various neg & pos. films?
Last edited by JW Dewdney; 20-Sep-2006 at 15:13.
I wonder if there are too many variables for a useful objective assessment. Scanner type, samples-per-inch, type and degree of grain reduction and sharpening (the interaction between the scanned appearance of the grain and that software), varying contrast, varying placement on the characteristic curve, etc.
I just try a few films for myself with the equipment and processes I have available and am happy with using. I find that Pro 160S is effectively grain-free when scanned at 2000 ppi on a (rented) Imacon 949 and printed to 20x24 from 4x5 with no grain reduction applied.
Though I did it as an example of B&W conversion, here is an example of Pro 160S scanned at 2050 ppi. The full 4x5 frame, and about 1/300th of the full frame. The detail is a compressed JPEG at the original scanned pixel dimensions - approx 500 x 500. It is of the little plant tag/plant tombstone in the centre of the upper right quadrant.
Best,
Helen
Last edited by Helen Bach; 21-Sep-2006 at 01:46.
I think there might be a legal firm named ASA, ISO, & DIN.
"I would feel more optimistic about a bright future for man if he spent less time proving that he can outwit Nature and more time tasting her sweetness and respecting her seniority"---EB White
That's interesting how they're calculated, actually. I have a question for someone. Why do motion picture films use EI (exposure index) instead of ISO? It's the same number but on motion picture products, they print it as EI. I'm a cinematographer so I'm curious why they are doing it different when it's effectively exactly the same.
Christopher,
I don't think that there is an ISO standard for the speed of motion picture negative films. ISO 2240 can be used for colour reversal films - though only if they are intended for amateur use.
In practice, I guess that mid-tone methods (eg LAD) are more relevant to cinematography than toe speed methods - but that is just my very selective observation. I use 18% aim density for establishing speed. Out of interest, what do you use?
Best,
Helen
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