Calm down Richard, I was not attacking you, just making a point, and I'm sorry that you missed it. Of course I was not advocating using a densitometer, I thought I had made this clear enough. Neither was I trying to lecture you in engine oil, what do I care, it was an analogy. I was just explaining two things: (1) That any plastic or gel filter is bound to fade in time, at a rate that is pretty unpredictable in real life. In any case, decades-old filters are an unsafe bet. (2) That, given the absence of practical ways to check the condition of your filters, it is reasonable to change them periodically. Now you have a known-good set to compare, and found that your old set had fared well. Then you conclude that it must the be the same for everyone (and apparently resent other people's advising to buy that new set). I did the same comparison, and found otherwise. Your personal experience is just as valid as anyone else's, no one denies it, but it does not set the rule - and neither does mine for that matter. No need for hard feelings.
Also, please try to understand how discussions work on a public forum. This is not a person to person conversation: when I say that colour head filters don't fade, it doesn't mean I want you to change how you make your filtering - what do I care, I don't even have a colour head to sell you. I say it because it is interesting information for all those reading the thread.
Also, please try to understand how discussions work on a public forum. This is not a person to person conversation: when I say that colour head filters don't fade, it doesn't mean I want you to change how you make your filtering - what do I care, I don't even have a colour head to sell you. I say it because it is interesting information for all those reading the thread.
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It's usually referred to as ISO(R), and it is described in "Way Beyond Monochrome", in the "Measuring paper contrast" chapter. Unfortunately the grades of the various manufacturers are not defined at the same value on the ISO(R) scale, as shown in the chapter. Ilford's grades span a wider contrast range than Kodak's, which is wider than Agfa's. ISO has proposed its own grading system, which doesn't match any manufacturer unfortunately.