Here'a a small experiment showing the changes in depth of field (in and out of focus areas) at a close focusing distance through a range of apertures. I set the stainless steel watch 25 cm from the camera sensor and the black watch behind it at 42 cm from the sensor. I locked a single focus point on the rim of the stainless steel watch and varied the aperture from f/1.4 to f/16 in steps. I used this lens because it's the only f/1.4 lens I own and there seems to be a lot of buzz around lenses with these very wide apertures.
Here's the first image using f/1.4. Note the extremely narrow zone of focus. Just the rim area is focused. Even the digits on the watch face are out of focus. This demonstrates the challenges of using very wide apertures at close range. If this were a person you could imagine the nose being in focus and the eyes blurred.
Stopping the lens down to f/1.8 broadens the in focus zone a little. The watch face digits come a little more into focus. In both cases the background reamins very out of focus.
Stooping down to f/2 continues this trend. Notice how the rim and watch face are now more in focus.
Lets continue to f/2.8. the trend continues. Also note there is still a pretty narrow depth of field even at this aperture. The slight angle at which the watch rests can be seen in that the upper part of the watch close to the focus point being slightly more in focus than the lower half of the watch.
Now at f/4 the entire watch face is in focus as the in focus zone expands.
Here at f/5.6 the entire watch face becomes sharply focused. Also the background becomes much less blurry.
At f/8 the watch face is razor sharp. The second watch come more into focus as well.
On to f/11 the same trend continues. The second watch and background, notice the photo on the wall, continue to coem into focus.
Now at f/14.
Finally at f/16
So there it is. Nothing too exciting but a useful comparison if you've been wondering about such things. Make note here that because I set the focus point just 25 cm from the sensor the background never comes completely into focus even at very narrow apertures such as f/16. If I set the focus point further from the camera this would of course change. Maybe that's an idea for another post.
As an aside I'll mention that the Fujifilm XF 23mm f/1.4 is an optically superb lens with a very wide maximum aperture. It's a fantastic everyday general lens as its focal length is neither too wide nor too narrow on the crop sensor Fujifilm X series cameras. My only issues with this lens are its too easily moved-by-accident aperture ring and its size. Like many mirrorless camera lenses this one is quire large relative to the size of the camera body and negates somewhat the size advantage of using a mirrorless camera relative to a dSLR.