AndreasT,

To illustrate how you can gain confidence by acquiring knowledge, while at first feeling confused. Here's a thought that is confusing me lately.

You probably have no trouble thinking of filter factors as simple as 1 stop for yellow, 2 stops for green, 3 stops for red. But lately I've been reading a chapter from LP Clerc which explains the common sense that the factor you use depends on the effect you want.

If you want to make a gray card gray those factors may be correct.

But a green filter can be used to lighten greens and darken reds. Really brighten the greens while leaving the reds natural. Or really darkening the reds while leaving the greens natural. You would want to use three different settings for the three different effects.

I had to think through those last few sentences, because the idea's still sinking in. But it's pretty obvious.

In sensitometry there is plenty talk of making sure that you use the correct illumination. What if the light source of your enlarger is tungsten and you shoot daylight? Does it invalidate your tests? My opinion is that it matters, but not much. Suppose you develop your film to 0.6 Contrast Index (you think) but in reality you truly obtain 0.5 Contrast Index. You will still get great negatives. So long as you are consistent, you will make adjustments as needed.