[APML] Stacking images in PS: detailed questions

To all,

Well, I'm new to this list but not to astro-photography; having dabbled in it for the past 35 years. Now armed with a scanner and diving into PhotoShop CS I'm looking at digitizing some old negs/slides and giving new life to them.

Some questions regarding stacking images in PhotoShop.

1) I've picked up and glanced through Jerry Lodriguss's great CD and he talks about stacking images, but I didn't see a lot about layer opacity. I found a reference in the APML archives (from Jerry himself) that says..

"Layer 1 (the background) stays at 100% (100/1)
Layer 2 should be at 50% (100/2)
Layer 3 should be at 33% (100/3)
Layer 4 should be at 25% (100/4)
etc."

Can someone explain why?

2) Maybe my understanding isn't clear, but if your 4th and 5th and even more layers are running at opacity of 25%, 20% and then even less; aren't you at risk of losing some information? Should this stacking be limited to an optimum number of layers?

3) If I'm trying to stack 4 or 5 layers is it better to do them all at once at decreasing opacities; OR should I stack two (100% and 50%) then flatten, then add another at 50% then flatten, add the next at 50% and flatten, etc.?

4) This stacking assumes images of same densities I would assume. I have lots of images where I've bracketed exposures (i.e. some great Comet Hyakutake shots done with 2, 2 1/2 , 3 , 4 min exposures on Ektachrome P1600). Densities are similar, but definitely not the same. What considerations should be given in opacities and stacking order where I have exposures that range from slightly under - optimum - slightly over?

Thanks in advance. As I get up to speed I'll post some images on a web site.

Calm and clear skies... Bill