This has been a good discussion, and brings to the surface one of the main consideration in filmmaking and photography; filmspeeds and relative exposure. Anyhow....Not all things are as they appear. Filmspeed numbers are to help guide us to correct exposure, and the 1/3 Stop increment is useful for most considerations. I try not to be over technical, and keep things as simple as possible....however...I'll re-address the areas you have brought up.

> ...i wondered if this could be a typo or if my understanding of filmspeed is wrong...
>> Technically speaking yes....you would use an external daylight conversion filter intended for Type B films, of which EKTACHROME VNF 7240 is a Type B filmstock. When used with the filter, the effective filmspeed drops by 1/3 stop to ISO 80 due to the decreased light thru the filter.
> hmmm, i thought VNF has a nominal film speed of ASA 125 ... which means, the filmspeed would drop by 2/3 stops [125-100-80]

The filmspeed drop is about 1/3rd of a Stop. ISO 80 is 64% of ISO 125, and thus is approximately 2/3rds of ISO 125. This means that the drop from ISO 125 to ISO 80 has to be 1/3rd of a Stop of exposure.

>> On Super 8mm cameras that index via the cartridge's meter setting notch....most cameras will default to the ASA 160 Tungsten setting for indoor use or the ASA 100 Daylight rating. Since ASA 100 is only 1/5th of a stop different from the actual ASA 80 rating in daylight with the 85 filter.

KODAK made this determination initially when they were deciding which existing 16mm color reversal filmstock would be suitable as a replacement for the ASA 160 Ektachrome films. The filmstock would have to as close as possible....since the majority of Super 8mm cameras were designed to index their builtin light meters to ASA 40 Tunsgten/ASA 25 Daylight w/85 filter......and ASA 160 Tungsten/ASA 100 Daylight w/85 filter. ASA 80 is 80% of the nummerical value of ASA 100, thus is exactly 4/5ths of ASA 100, thus the exposure value drop is exactly 1/5th of a Stop. If the loss were anymore....then Kodak would Not have chosen VNF 7240 as a suitable replacement. In fact...it was/is the only current motion picture reversal filmstock that closely matches suitability in most existing Super 8mm cameras that were setup to film with the former Ektachrome 160 Type A filmstocks.

Nice chart....however....using a 1/3rd Stop exposure variation from setting to setting is most suitable for negative type films due to the long exposure latitude that negative filmstocks have. For reversal filmstocks....exposure has to be fine tuned a bit more (all things being relative) especially to not err on the side of Over-Exposure.....which would result in images that are too light. Your "0" beginning point....in theory should be zero...thus no exposure....not begin with ASA/ISO 25. However...for arguments sake.....you are correct here...the change in exposure from ASA 25 to ASA 50 is 1-Stop, and then is 2-Stops from ASA 25 to ASA 100. The fact remains though, that the true exposure drop from ASA 100 to ASA 80 (as in the original example) is 1/5th of a Stop, not 1/3rd. In practice....1/5th of a Stop makes little difference in visual density for most subject matter. And really, you can only adjust a given camera in 1/3rd stop increments which is enough to vary exposure density in the final image. Again...this is why VNF 7240 was chosen...since it was close enough to the former EK 160 Type A filmstock (exposure-wise) to be exposed nearly correctly in most Super 8mm movie cameras.

>> ....most will not note any significant difference in exposure. The only exposure variation you might notice would be in low light...since the ASA 125 is a bit slower than the previous ASA 160 Tungsten rating...and in low light...even that little bit more...can make a difference at times. > i agree that the slight underexposure is hardly worth bothering... if you go for the point and shoot filmmaking your results will be easily ½ stop off the optimal exposure and still look very well...

In very low lighting conditions (low light being a relative term....meaning low as in the range of a given filmstock's ability to record in that light level), as you approach the threshold of a filmstock's ability to record an image, those that were used to using the former Ektachrome 160 Type A filmstock will notice that this threshold point now requires more illumination that with the current Ektachrome VNF 7240 filmstock. This also applies exposure-wise to the former Ektachrome 160 Type G....however.....with the Type G filmstock.....it was NOT correctly balanced for Tungsten lighting...and was really more of a daylight filmstock (approximate color temperature rating of 4400°K versus Type A's color temp rating for Tungsten at 3400°K.......a B-I-G difference...but another topic altogether).

Best wishes,
Martin W. Baumgarten

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