It really depends on what you want your other settings to be. By lowering your ISO each step, you are halving the amount of light each time. You don't have to worry about noise at all with ISO settings of 200 or 100, so that isn't a concern here. Each of the camera settings (ISO, Aperture, and Shutter Speed) controls a different aspect of the shot. I'll try to explain what those aspects are as best as I can below.
ApertureAperture is responsible for Depth of Field (DOF), which is how much of your image is in focus. A small Aperture number (something like f/1.5) means a very small DOF, which means there is a very small range in your image that will be in focus. A larger Aperture number (something like f/22) means there is a very large DOF, which means almost all of your image will be in focus from front to back.
Here are some examples:
This first shot was shot with an Aperture of f/4.5
Notice how only the front bit is in focus and the rest of the image is completely blurry. Because of the small Aperture number, there is a smaller DOF which causes only a small plane within the image to be in focus.
In this image, an Aperture of f/5.6 was used
As you can see, more of the image is in focus when compared to the previous one because there is more DOF in this image.
Shutter SpeedShutter Speed controls movement. A slow Shutter Speed (usually 1/10 or less) will show any movement that is going on in the shot. These kinds of shots would be light trails of traffic, light pantings with a saber, blurred water effects, etc. A faster Shutter Speed (usually 1/30 and above) will capture less movement and show a still image. This is good if you want to freeze movement, like at a sporting event. A couple examples are shown below.
This first image is of a squirrel that kept moving around, so I had to use a faster Shutter Speed to freeze its movement. A Shutter Speed of 1/640th of a second was used.
This second shot is one you may have seen already, where I am doing a light painting with a saber staff. A Shutter Speed of 10 seconds was used here.
As you can see, a longer Shutter Speed will capture more motion in a shot, while a shorter Shutter Speed will freeze motion to capture the moment.
ISOISO is responsible for the sensitivity of the camera sensor and is also responsible for what is known as "noise" in a shot. Noise is the graininess you see in some shots. An example would be in the shot of the chairs previously shown above. I intentionally introduced noise into that shot to give it an older antique feel to it, but generally it is to be avoided when possible. With the right software, you can control the noise of a shot after it is taken, but typically it is best to avoid it from the beginning by not using high ISO settings (usually around 800+).
Each setting will control a different aspect of the shot and each setting will affect how the other settings work, depending on what shooting mode you're using. You can achieve different effects by changing one or more of the settings, which gives you lots of flexibility with your shots.
Here is a chart I found that will help explain how the settings work to affect the lighting of a shot:
You can use the settings to adjust the lighting or to achieve certain effects you want (more DOF, show movement, intentional noise, etc.) As an example, let's say you currently have your Aperture set to f/8 and your Shutter Speed set to 1/30th of a second, while your ISO is set to 200. If you want a slightly darker picture, you have three options. You can either increase your Aperture setting to f/11, reduce your Shutter Speed to 1/60, or decrease your ISO to 100. All of these will give the same result, which would be half the amount of light. Depending on the lighting you are shooting in, you may want to do more than one to reduce the light even more. Of course if you wanted more light, you can always do the reverse and reduce your Aperture to f/5.6, increase your Shutter Speed to 1/15, or increase your ISO to 400.
These are just examples mind you, but the best way to learn is to just play around with the settings in different lighting situations and see how the settings work together to create different effects. When taking your shots, if you want more DOF, then you can adjust your Aperture setting for that. You may also have to adjust either your Shutter Speed, your ISO, or both in order to balance out the lighting for the shot. If you want to capture motion, then you can set your Shutter Speed first and adjust your other settings from there. This is all assuming you use Manual Mode of course. The way I go about taking a shot is to first think what effect I'm after and what is more important to me. Do I want only one part of the picture in focus? Do I want everything in focus? Do I care about capturing movement or freezing it? By asking these questions before taking a shot, you can then change your highest priority setting first and adjust the others from there to balance out the light. As I've said before though, a good rule of thumb is to avoid high ISO settings (800+) due to the amount of noise that can creep into your shots.
I'm sorry if this has become too long, but hopefully it has served its purpose in helping you understand how your other settings effect your ISO and how you can achieve specific lighting effects with them.
