> Subtracting details from background without adding details to subject ...
Not sure what you mean by this. Blur is only due to focus distances and aperture sizes. Making the depth of field narrower (making the OOF regions more blurry) does not add detail to the areas that are in focus. Usually, it's even the other way around.
Example:
Say we shoot a portrait of a person at 5m with a forest 30m away in the background, at 3 different apertures: f/1.4, f./11 and f/20
At the largest aperture (f/1.4) the background will be completely out of focus and the face of the subject will have sharpness at "80%" of what my lens/sensor combo can do in terms of resolution. The less-than-excellent subject sharpness is because lenses aren't perfect and using the largest aperture will reveal this. Even if you use an expensive professional lens, it will have it's maximum sharpness at some aperture that is smaller than the largest. What does happen in the shallow DOF shot is that we have a form of perceived sharpness (usually referred to as "pop") which is an effect that is simply due to the fact that the subject is so distinct from the background.
At f/11 the subject sharpness is better than at f/4. It is now probably near 100% of the maximum resolution the sensor/lens combination can deliver. The background is significantly more discernible/focused now. If it was a green blur in the f/1.4 shot is now a forest of very slightly blurred trees.
At f/20 the subject's sharpness is again less (e.g. 90%), this time due to the physical limitation known as diffraction that occurs for very small apertures compared to the wavelength. This shot has completely focused trees in the background.
To put it another way: when you take the f/11 portrait and go to f/1.4 you take away almost ALL of the background information, and SOME of the foreground information, while adding NO new information.
The entire shot will be less focused when you do.
But there are still advantages. With a larger aperture the sensor/film is receiving more light which means the shutter speed can be increased. If the subject isn't perfectly still this can result in decreased blurriness. It's been a while since my HS photography class but I think that's correct.
Not sure what you mean by this. Blur is only due to focus distances and aperture sizes. Making the depth of field narrower (making the OOF regions more blurry) does not add detail to the areas that are in focus. Usually, it's even the other way around.
Example:
Say we shoot a portrait of a person at 5m with a forest 30m away in the background, at 3 different apertures: f/1.4, f./11 and f/20
At the largest aperture (f/1.4) the background will be completely out of focus and the face of the subject will have sharpness at "80%" of what my lens/sensor combo can do in terms of resolution. The less-than-excellent subject sharpness is because lenses aren't perfect and using the largest aperture will reveal this. Even if you use an expensive professional lens, it will have it's maximum sharpness at some aperture that is smaller than the largest. What does happen in the shallow DOF shot is that we have a form of perceived sharpness (usually referred to as "pop") which is an effect that is simply due to the fact that the subject is so distinct from the background.
At f/11 the subject sharpness is better than at f/4. It is now probably near 100% of the maximum resolution the sensor/lens combination can deliver. The background is significantly more discernible/focused now. If it was a green blur in the f/1.4 shot is now a forest of very slightly blurred trees.
At f/20 the subject's sharpness is again less (e.g. 90%), this time due to the physical limitation known as diffraction that occurs for very small apertures compared to the wavelength. This shot has completely focused trees in the background.
To put it another way: when you take the f/11 portrait and go to f/1.4 you take away almost ALL of the background information, and SOME of the foreground information, while adding NO new information. The entire shot will be less focused when you do.