In part 1 of exposing exposure, I wrote of the three ways one can change the amount of light (or sensitivity to light) that reaches a photo sensor.

But that only leads to the next question, that is that’s nice Aaron, but how does that help me understand my D40? Even a basic DSLR like the D40 has a lot of functionality in it, so what follows here is only a very basic introduction (and is applicable to nearly any camera, not just the 40).

Because they change the exposure in different ways, it is important to separate ISO from the shutter speed and aperture. As I previously wrote, ISO changes the sensitivity of the sensor to light, while aperture and shutter speed change the amount of light that reaches the sensor.

For the purposes of this article, I am going to assume to automatic modes (everything but P, M, A, and S modes) don’t exist. I never used them, so I don’t have a clue how they work.

Since I’ve previously written that boosting ISO introduces noise and should be considered a last resort option, I’m not going to cover it in this article.

Instead, I’ll focus only on shutter speed and aperture, and how they apply to each of the four modes.

Before we start, however, one last thing must be mentioned: exposure compensation. The very first thing I wrote in part one was that a good picture must have the perfect amount of light. The D40 has a very intelligent, advanced metering sensor that is pretty good at picking the right amount of light. It is not perfect. Heck, it gets it wrong quite a bit. Fortunately, Nikon realized this and gave us a way to compensate, using the (aptly named) exposure compensation button. This button sits right next to the shutter release and is labelled with a “+/-“ symbol. Hold this down while looking through the viewfinder and scroll the wheel. You’ll see a little bar symbol with “-“ on one side, “+” on the other, and “0” in the center. That is an indication of exposure compensation. If the picture is too bright (overexposed), adjust the compensation a little towards the - side. If it is too dim (underexposed), adjust to the + side.

Exposure compensation is handled in different ways for each of the four mode, but always applying the same basic principle: adjust the aperture and shutter speed to let in “just the right amount” of light.

Program-AE (P mode)

Program AE is the fully auto mode, or close to it. Unlike the green “Auto” mode, P mode will not automatically pop the flash. Otherwise, the camera makes all the decisions and takes a picture. If you decide to use the flash, the camera will use it, but you as the photographer must choose to use it. In P mode, the camera chooses what it feels is the best option for a picture. It will display the shutter speed and aperture in the info screen and in the viewfinder.

Effect of the wheel: the camera chooses what it thinks is the best option. It might be wrong. Turn the wheel and the camera chooses a different option while maintaining the same exposure. If the shutter speed is faster in the new mode, the aperture will be larger (smaller F number).

Effect of exposure compensation: the camera still chooses what it thinks is the best option, but the option (and alternatives) presented are now the best choices for the compensated exposure level.

Shutter Priority (S mode)

In shutter priority mode, you as the photographer set the shutter speed. The camera will then try to correctly expose the image for the shutter speed you’ve selected. If it can’t set an aperture that will work for the current shutter speed, you’ll get an over or under-exposed image (depending on the situation). Shutter priority mode is very useful when taking pictures of waterfalls (where you want a very slow shutter speed) or sports (where you want a shutter speed fast enough to stop the action).

Effect of the wheel: the wheel selects the current shutter speed, visible in the viewfinder. The camera chooses an aperture to correctly expose when using the current shutter speed, if possible.

Effect of exposure compensation: the choice of aperture is adjusted to expose to the compensated level.

Aperture Priority (A mode)

In aperture priority mode, you as the photographer set the aperture. The camera then will set the shutter speed to correctly expose the image (if possible). Aperture priority is useful to control Depth of Field, either to highlight a subject and blur the surrounding background (wide aperture, low F number), or attempt to maintain focus on a large range of subject distances, such as when taking pictures of scenery (narrow aperture, larger F number).

Effect of the wheel: the wheel selects the current aperture, visible in the viewfinder. The camera chooses a shutter speed to correctly expose the image, when possible.

Effect of exposure compensation: the shutter speed is adjusted based on the compensation value.

Manual (M mode)

In manual mode, the camera makes no decisions. Everything is up to you. If the settings are right, you get a picture. If wrong, you get something that is over or under-exposed.

Effect of the wheel: turning the wheel in manual mode changes the shutter speed.

Effect of exposure compensation: Exposure compensation does not exist in manual mode. However, the routine that normally changes exposure compensation (hold down the EC button, turn the wheel) changes the aperture.

Fortunately, while the camera doesn’t do anything, the meter is still running and will tell you if you have bad settings. That little bar in the viewfinder that displays the Exposure compensation value when the EC button is held is also an indication of the current exposure. If your settings are way off, the camera will tell you by indicating the image will be over/under exposed with that little meter bar. The meter is always available, but is that much more important in manual mode.