Question: I want to stack x number of photos to incrase the exposure of the final image. But I need to do it manually, I mean, using Photoshop but without using wizards.

My approach would be using a layer for each photo and aplying an alpha for each layer. I want every photo has the same weight in the final image, so it must be the same alpha.

My question is this:
If I apply eg. 50% opacity to each one, every photo will have the same weight, but I'll can see the empty background because every one are semi-transparent. To avoid this, should I have a photo with 100% opacity? If so, that photo will have more weight on the final image, something I don't want...

Do you get my point? Any idea?

Thanks in advance!

Answer: 
When you set the opacity of a layer 50%, what you get is 50% of that layer, and 50% of all the layers underneath it combined.

For this reason you should set the bottom layer to 100%, then layer above that to 50%. Now you have a perfect balance of the two layers. The layer above that should be set to 33%, meaning your image is one-third that layer and two thirds of an even blend of the bottom two layers. The next layer above that should be set to 25% and so on.

Unfortunately you can only specify opacity in whole numbers so you start to introduce a bias that gets worse the more layers you add. One solution to this is to stack as many layers as you can, and then merge those layers and make the result 100%, then start again by placing the next layer on top of that at 50%.

Needless to say you should set your image to 16-bit mode when stacking to avoid rounding errors.
Do you mean like a multiple exposure? If so you want to not have a blank background layer but instead only have photo layers. To do that you can select your x images and open them as layers. I don't have LR or PS in front of me on this computer but in LR you can select multiple images and open them as layers in the same document. Similarly this can be done with a multiple selection from Bridge too. I found this tutorial on doing it directly from PS: [enter link description here](http://digitalchemy.wordpress.com/2010/10/08/photoshop-import-multiple-images-into-one-layered-document/ "Import Multiple Images into one Layered Document")

Once you have done that you will no longer have a blank white bottom layer. After that you can play with the opacity to make the effect you want. If you have more than 2 images 50% is probably not what you want but I am sure you can play around and get what you want once you have the images in layers.

The more I think about it the more I seem to remember that the bottom layer does not need a changes in opacity just the ones above it.