Manually Open and Close the Pupils of Your Eyes

Would you like to surprise people while at the same time encouraging them to look deeply into your eyes? With this little guide, you can.

Steps

Find yourself a nice and comfortable place where there are not too many distractions.
Look around and try to place yourself facing an object that is near (about arm's length) and in line with that an object (or scenery) that is far away.
Focus on the nearby object - at first you may have to do this by looking directly at it.
While moving your eyes as little as possible, focus your eyes and your attention on the far away object.
Repeat the process several times. After a while you can recognise the feeling of the focus of your eyes changing (can't really describe it but you'll know). Following on from that you will hopefully become familiar enough with it that you can do the focus change without the objects being there! As the irises of your eyes flex while your eye changes focus; you have learned to "isolate the muscle" that controls the focus of your eyes.

You could video yourself and see if you managed, or a less creepy idea is to just ask a buddy to watch your eyes (from the side, or they could mess with your focus) really closely and see if they see any change while you "exercise" - help them out by telling them what to look for.
It is impossible to check if you can do it in the mirror as you would be putting yourself out of focus (as well as ruining your concentration).
It's a lot easier to do if you hold one eye closed.
You can put a sticker on a window and use that as your near object and the scenery outside as your far object.

If you attempt this, work in brief sessions. Like any other muscle, the iris will grow fatigued when overworked. Stop if you get a headache or experience eye discomfort.
You can't actually control these muscles of your eye since they are innervated by your body's autonomic nervous system, which happens to be involuntary (meaning you can't tell it when to move, etc. unlike muscles in your arm or leg which are voluntary and innervated by somatic nerve fibers). So you are simply changing the effects of the sensory input that will in effect cause this contraction/dilation.