This exercise is a repeat of the first exercise, with a small but vital variation.
Find somewhere with motion; people, cars, birds, whatever moving past the lens. Ideally, find somewhere different than you did the first time you did the exercise. Adjust the Shutter Speed to 1/30 as before, and center the meter by adjusting Aperture and ISO. Try to get the Aperture setting to somewhere in the middle of its range, 8.0 or so, by manipulating the ISO. Now take a picture. Click the shutter speed a couple of clicks in one direction or another,
and now make a complementary adjustment to Aperture to re-center the meter.
Take another picture.
Proceeed as before, moving the Shutter Speed first in one direction, but continuing to re-center the meter with complementary adjustments to Aperture. Then return to the 1/30 setting, and move in the other direction, always centering the meter with complementary adjustments to Aperture.
Your pictures should all come out about the same brightness now, but the combined effects of your Shutter Speed and complementary Aperture adjustments should be visible, making each picture qualitatively different. If there is very little depth to the scene, the effects of Aperture may be hard to see, but keep an eye on the background.
Find somewhere with near and far objects, and something to focus on in the middle. Try to find somewhere new, different from the last time you did this exercise. Perhaps a view looking down a street.
Focus somewhere in the middle, and set the Aperture to 8.0, and center the meter with adjustments to Shutter Speed.
Adjusting the Aperture first in one direction and then the other, while making complementary adjustments to Shutter Speed to keep the meter centered make a series of pictures.
Again, all the pictures should be about the same brightness, but the combined results of the Aperture and the complementary adjustments should be visible. If nothing much is moving, the shutter speed setting may not make much difference, but as it gets quite slow (1/15, 1/8 and so on) the pictures will at any rate get softer/fuzzier simply because it's nearly impossible to hold the camera still enough.
You can combine these two exercises by finding a location with near and far objects, as well as objects in motion. Make a series of photographs, adjusting Shutter Speed and Aperture in a complementary fashion.
This is the same deal, but with ISO.
As we've seen, the effects of changing ISO are somewhat more subtle than the other two settings.
In a darker situation, say, indoors, we might change the ISO up and down, and make complementary changes to the Shutter Speed. As you may have already observed in Exercise 1, a faster shutter speed will generally produce sharper pictures, even if nothing in the frame is moving. So, altering the ISO up to larger numerical values can be helpful here.
Experiment indoors, in a room with average-to-dim light. Center the meter, and then take a series of pictures with various ISO settings and various complementary Shutter Speeds.
Congratulations! You now know how to take "correctly exposed" photographs in Manual Mode, whatever "correctly exposed" means to your camera's meter. You can also adjust the various settings to various combinations of settings which will produce that same exposure (that same degree of brightness in the picture) but with various different visual effects.
What sorts of things does a slow shutter speed like 1/15 do, versus a faster one like 1/500?
What sorts of things does a numerically small Aperture, like 4.0, do versus a numerically large one like 16.0?
Why would I use a high ISO like 3200, if I didn't particularly want a grainy/noisy picture?