I can't understand how to use the manual mode on the D3100?

I've just purchased a Nikon D3100. I was experimenting with the manual mode. Even though I rotated the command dial to change the shutter speed, I can't understand how I'm supposed to chnge the aperture. Help please?

Added (1). Neither the manual book nor the PDF say anything about HOW TO use the manual mode.

Read the tutorial. Manual mode is tedious in any camera; you have to regulate independently aperture and shutter speed. It's a good way for learning though

Dig out the manual from the box of paper to be recycled. Open it, read!

Section two on page 77 explains exactly how to set shutter speed an aperture. Section 3 on the same page explains how to use the exposure indicator. That's all you really need. Page 78 explains how to set it to long exposure mode which is useful for the very trendy "light painting or writing.

Did you look on page 77? Explains how to change aperture.

As for how to use the manual mode to get correct exposure, that will require more than just the owner's manual. "Understanding exposure", by Bryan Peterson is a good place to start.

My suggestion is to begin on the auto or program mode. But look at what settings the camera is choosing. Then progress to the shutter priority (S) and aperture priority (A) mode, where you choose one and the camera decides the other. The shutter speed and aperture work together with the ISO to regulate the amount of light falling on the sensor. Too much light, and the image will be too bright and have washed out areas. Too little, and the image will be too dark. Your camera has a light meter that likes a middle grey tone, called 18% grey. The meter looks at the scene in front of you, and decides what settings will get close to it's nirvana of 18% grey, and sets a shutter speed and aperture to suit. Often, the exposure it will create will be in the ballpark of what you need. If it is a little too bright or dim, you can also adjust the exposure using the + and - compensation button. The ISO setting is also important. A higher ISO means you need less light for a proper exposure. But higher ISO settings have more noise than lower. Usually use the lowest ISO you can to get the shot.

Manual exposure requires some understanding of what exposure is and how the factors work together. The aperture is the opening in the center of the lens that lets light through. A bigger opening means more light will come through the lens. Wide apertures also have less depth of field. Narrow apertures have more DoF. Some lenses have wider maximum openings than others, these are called: fast" lenses. The f/stop is a ratio of lens length to opening diameter. They may be f/1.4 or f/1.8 for non-zooms (or prime), or f/2.8 for a good zoom lens. If you are using the kit lens, it probably has a variable maximum aperture, meaning the maximum opening is smaller at the long end of the zoom. So at 200mm, you will have less light at maximum aperture than at 28mm.

The shutter speed is how long the curtain in front of the sensor stays open. Measured in seconds and fractions of seconds, they are anywhere from the B setting, where the shutter stays open as long as it is pressed, to 1/4000s which will freeze water droplets in a splash. Faster shutter speeds like 1/250 or 1/500 can stop action on the football field or freeze a running horse. Long shutter speeds can be used to capture landscapes in the minutes before night, but longer shutter speeds require a tripod to stop the camera from shaking. Most people can't handhold a shot from a wide angle lens any slower than 1/30s without causing some camera shake. If you are using a longer lens, you will have to have a much faster shutter speed to handhold. So a 200mm lens should be at least 1/200 to handhold, a 300mm at 1/300 and so on. Also, with a slower shutter speed, if the subject moves you will have blur from movement.

Here is a guide to exposure:

http://www.fredparker.com/ultexp1.htm

Plan to read it several times before it starts to sink in, then try some experiments on your own. Have fun with your new camera.