Double exposure 'transparency' calculation.
I have something in mind for a double exposure with a portrait and landscape image.
I've tried to emulate the desired result in photoshop with digital images, by adjusting the transparency of the top layer - obviously achieving the same result on film will be quite different. Transparency in the case of Photoshop doesn't effect the dynamic range of the image like exposure would with film - the highlights being more apparent - but rather Photoshop considers the image a 'global tonality', adjusting the image in a linear fashion, shadows and highlights alike.
Basically the landscape image, which I'd like to be only just visible over the portrait, is at 15% in Photoshop. This is as much as I'd like the image to be visible on film, the portrait of course being fully exposed.
How exactly would I go about achieving this result on film?
Hope that makes sense!
Here's a test image of the kind of result I'm after:
As you can see the blades of grass need to be a higher tonality than the dark background for the effect to work. Is this even possible without overexposing and therefore that image being most dominant? I can't get my head around it.