The Na+ and K+ ions are very much alike except for size and since the cation is not incorporated into the crystal, the size has little if any effect. Most of the effect is in ionic strength and Van der Waal's force. They are used pretty much interchangeably and based more on cost or availability. Of course there is no such thing as NaI in the strictest sense.

Ian H;

Kodak film was consistent except for the caveat I noted above about the needs of different markets. I forgot to add though that the different markets also had to adhere to different laws regarding process chemistry and effluent from the manufacturing process.

Regarding color film, Kodak tolerance was far tighter than that of any other company and as such they tried to satisfy the most exacting professional. Thus, if the film was in tolerance but off by just 0.025 in color or even less, they put in a stuffer sheet to alert the discriminating professional. Fuji and Agfa did not do this.

And, BTW, this information is in the Kodak publication "Professional Color Films" in several editions. Those interested can certainly read up on this. The film was good but to the particular professionals they tried to make it even more perfect with exact instructions.

When Fuji E6 first came out, it was so variable that Fuji was forced to withdraw it from the market until they could correct the problem. To this day, a leftover of this is the fact that Fuji suggests a different time for the First Developer of their E6 films.