Noob developer question

What does it mean when a developer reduces the effective film speed ? Like for example perceptol has a speed loss. Does that mean that a film i would expose at 400iso and develop in D76 need to be rated at 200iso to get the same results in perceptol ?

Could someone give a simple explanation to this ?

It has all to do with densities: for some developers you must expose more (set a lower iso value) to have the same densities than another.

-M-

You've got it right, to achieve finer grain Perceptol uses a silver solvent, in this case Sodium Chloride, because this reduces the effectiveness of the developer you need to increase the exposure to compensate.

Ilford published a Data sheet P10 Fine Grain Development around 1965 where they advocate adding Ammonium Chloride:

ID-11 Add ammonium chloride to ID-11 in the proportion of 20 g. per 500 cc working solution.
Camera exposures should be increased by about 50 per cent and the development times are double those specified for ID-11.

ID11 is the same as D76.

Ian

It's all about compromises. A Porsche 911 can't corner like it does and ride like a Cadillac. A developer is hard pressed to give you fine grain AND full speed, although, I believe Kodak was trying to do just that with their Xtol. Film speeds are not a fixed number but can vary greatly depending on developer and time in that developer. Extended development = very slight speed increase. Reduced development can result in substantial speed loss.