Thus if you want all the subject matter you are shooting, to be in focus, you need a large depth of field and as a result you use a large f-stop this means that the opening in the lens is small.
Yes, it is counter intuitive but just go with it or read this article on the Wikipedia which gets into the math of focal length and size of the pupil of the lens - aperture.
NOTE: ISO in the film days of photography, was an indicator of the film's sensitivity to light. Now, ISO, it is used to set the sensitivity of the DSLR sensor.
Today's self-imposed lesson was to leave the subject and camera position alone. The ISO was also fixed, auto-focus was turned on, the white balance was again custom based upon my lights and to then play with the f-stops to see how the depth of field changed. These images were taken with the 60mm f/3.2 macro lens.
I started again with an auto image. This time I used the setting of A-Dep which sets the camera to get the best depth of field while picking the ISO and the Sutter Speed. The meta data show the following - ISO: 400; Tv: 1/6; Av: 32. As you can see the cuff from the front to to the back are all in focus.
Here is my first attempt - ISO: 400; Tv: 1/30; Av: 32. Which of course because the shutter speed was faster, resulted in a darker image since not enough light was allowed in.
To prove this point, this first picture is ISO: 400; Tv: 1/30; Av: 18 and as you can see it is over exposed.