A question about developers that raise or lower the ISO

RalphLambrecht said:

Actually and confusingly;it does both

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Not that confusing. How much of a change depends on the method used, and there it starts to become a little more complicated.

Film speed basically depends on how many Silver atoms need to be in a latent image center such that the grain is rendered developable. Sensitization techniques gave us emulsions which need latent image centers consisting of just three Silver atoms. The fewer atoms form a latent image center, the higher reduction potential the developer needs to have, and not every development agent has the same ability to discriminate between image and fog. And then there are developers which will actively bleach away small latent image centers but develop larger ones. Remember that a small latent image center is not only harder to develop, but also easier to bleach, and it depends on reduction potential of developer whether a latent image center is developed or bleached.

So yes, it is absolutely possible to formulate a developer which creates a fully developed negative but loses one or more stops of speed. The reason why it's not possible to gain a few stops: because we are a bunch of spoiled brats, taking advantage of >120 years of research into photographic emulsions and developers. Almost every single commercial developer sold today will give more or less full speed.

georg16nik said:

yet, when we place the useful zones on the straight line portion of the H&D curve, the “box speed” as per ISO becomes meaningless and some reach out for ISO raising developers.

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Now your talking personal taste in exposure, not film speed let alone the ISO standard. If you want to place the exposure in the straight-line portion of the curve and like what you are getting, great. That's called exposure, and exposure is not film speed. To place the exposure on the straight-line portion of the curve or anywhere on the curve, it helps to know the film speed value. So ISO is still relevant because it gives information about the film from which exposure can be determined.