Sometimes aspects of the "look" of a photographic image are dictated by the limitations of the conditions and the equipment used. As one example, when I'm shooting wild birds, even those who have become accustomed to finding food that we've set out for them, I know that they will not allow me to approach them. Even for as brazen a visitor as a Blue jay, my only chance for a photo will be from a partially hidden position some distance away. That means using a long telephoto lens (600 mm in this case) to get reasonable magnification. That in turn means that any background more than a few inches behind the plane of focus will be very blurry.
This is not a bad thing generally speaking, and probably an advantage when the surroundings could be distracting rather than adding to the story of the photo. My only purpose in this image is the jay himself and the way that he managed to get two seeds in his beak. I would actually prefer to have blurred the pine straw even more, but it was simply too close. Okay, now that I've drawn your attention to it, ignore it! Pay attention to the whiskery facial feathers and the bright, tasty seeds!
In photographing this flower, I had two somewhat conflicting aims. I wanted to show the overall feeling of softness and subtle curves. Allied with that, I wanted to minimize any blemishes in the leaves around it. On the other hand, I also wanted to capture the details of petal texture, and even the tiny grains of pollen. To suit all the aims, I used a 135 mm telephoto and shot at a very wide, f/2 aperture. Getting much closer than I could with the jay, I managed a nice perspective, as well as tack sharp pollen grains and buttery soft leaves. Horses for courses, lenses for looks.
With the same 135 mm lens at the same aperture, I caught this little doggie challenging my presence (she's actually quite friendly). At the greater distance, the lens can render her quite distinctly, yet separate her from the blurred background so effectively that she almost pops off the screen in a three dimensional way. It's a very effective illusion in the full-resolution image, which I hope is somewhat apparent even in the reduced size seen here.
That lens was on my camera and in my hands as a result of having just shot the flower above, so the canine portrait certainly represents a bit of serendipity.
Here's a case where the lens focal length and aperture were chosen deliberately to suit the situation. I wanted to create a dreamy atmosphere in this portrait of our dreaming pet. I feel that the clear details of fur around her eye, her ear tips, and the rug and blanket textures are sufficient to anchor us to reality, while the rest of Photon's soft form can recede into her fantasy land, perhaps remembrances of her youthful days filled with action.