Gah – ok. My point is that Multiply behaves differently in Photoshop. And you can see what I am trying to achieve. Imagine this in PS –
1) document 1920×768, background image fills document, photo of whatever – a field.
2) layer above it, only a small square, the rest transparent – layer set to Multiply mode.
So it’s just like putting a sheet of acetate onto a photo – doubling up the image where it counts but NOT adding black wherever it is transparent! Anyway – I’ve been trying all sorts with Masking – am pretty certain this is the key to the solution. Whatever that is..