Baja, this is a really good thread, thanks for starting it.
Not really selective focus, but this is the most common way - for me at least - to use focus, and actually even more importantly, focal length to create an image which simply gives the flowers all the attention and gets the background so out of focus and undistracting as possible. Here I used a 300 mm lens at f/11. A widely used, easy and no-nonsense technique. Pretty much always works. The background is actually so out of focus, that most people don't even think about it. It really is nothing. However, the background is not so much out of focus that it feels plastic, it still has a slightly organic feel to it.
A photo from my garden in 2017, Crown Imperial 'Rubra' behind tulip 'Poppie'. In the background there are more Crown Imperials.
It was shot with the same 300 mm lens as above, but with the aperture wide open at f/2.8. Here I wanted to convey the feel of the garden so wanted both a little background and foreground, but I still wanted the Crown Imperials to stand out, so there is never any unclarity about what the real subject is.
The same 300 mm f/2.8 lens, but this time with an added 1.7 teleconverter and selective focus on the flower only. Shot at f/5 to give it a bit of a dreamy feel.
Striped Squill (Puschkinia scilloides)
The same 300mm lens at f/3.5. Here I take advantage of tall out of focus foreground grass to seemingly make the Striped Squill rise from nothing. You can also use other out of focus objects for similar effects, but grass is probably the easiest to start with. Many that are not familiar with photographic techniques mistake this effect to be something that is created by post processing, but actually it all happens in camera.
