I know that f/1.8 gives more background blur than f/8 and I use it constantly, but I have never understood the mechanism. A wider opening lets in more light, and I cannot see why that should also change what is in focus.
What is the connection between the size of the hole and depth of field?
Accepted Answer
Only one distance is ever truly in focus. Everything else forms a small disc rather than a point, and depth of field is the range where that disc is small enough to look sharp.
The size of that disc depends on the cone of light converging on the sensor. A wide aperture makes a fat cone, so a point at the wrong distance spreads into a large disc quickly. A narrow aperture makes a thin cone, and the same misplaced point spreads much more slowly.
So it is the angle of the cone doing the work, and the extra light is a side effect of the same opening rather than the cause.
This is also why a pinhole camera has effectively infinite depth of field, and why it needs such long exposures.
Worth correcting the usual shorthand that a fast lens gives you blur, because it leads people to buy the wrong thing.
Background blur depends on aperture, focal length, subject distance and how far the background is behind the subject. Aperture is only one of four, and often not the strongest.
An 85mm at f/4 with the subject close and the background far will out blur a 35mm at f/1.8 in most rooms. If your backgrounds are not separating, moving closer to the subject and further from the background usually does more than a wider lens will.
Sensor size enters through the same door. A smaller sensor needs a shorter focal length for the same framing, and shorter focal lengths have more depth of field, which is why phone cameras have to fake it in software.
The discussion on each answer is open to members.
Join free to read the rest