Changing semitransparent image background

[foolsparadise13]

Hello,

I am a fish researcher and an avid photoshop user. I am trying to figure out how to change background of a photo I took from white to black, which sounds simple, but fish fins are semi-transparent and my usual tricks look terrible. I thought I would try and reach out here to see if anyone had any tips, tricks, or ideas, on how to change the background while making the fins look as they should. The photo I am trying to change (white background) and a photo of a similar fish from the internet on a black background (for comparison) are posted below.

Thanks to anyone who has any input.

 

[Eggy]

One way would be:
- Select/color range and select the white color with maximum fuzziness (200)
- Inverse the selection so the fish is selected.
- Use the selection tool carefully to clear the unwanted selection patches in the fish.
- Press CTRL+J to put the selection on his own layer.
- Add a new black layer UNDER the new fish layer.



You'll need to darken the top of the fish since the white is a reflection of the white background.
The edge of the top and botton fin could be soften a bit but I see that there's a hard edge even on the white background.

Maybe someone else has a better idea.

 

[Rich54]

Here's another way, using the Blend-If sliders. I added a black layer above the fish and activated Blend-If on the black layer (see tutorial below). I adjusted the sliders as shown where the red arrow is pointing to get the tail fin where I wanted it. As Eggy says, the top edge of the fish needs to be darkened a bit because of white reflections, and both the top and bottom edges need to be softened. Also, with Blend-If, some of the new black layer is showing within the body of the fish, but that is very easily masked away. The tail fin is the important area.