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Did you do the Exercise — Understanding exposure? Good. Now lets talk about what you saw. When playing with the shutter speed and aperture settings you took some pictures which were underexposed (not enough light) and overexposed (too much light). Going back to the analogy with the glasses of water, lets say that a properly exposed image is the same as a properly filled glass of water. Now, we don’t want to fill a glass completely, so lets say a properly filled glass looks like this image.

Properly exposed (filled)

Such a glass of water could be served to a guest or at a restaurant without complaints.

Underexposed. Too little water in the glass.

If you were to serve such a glass of water to a guest you would get a remark that the glass is not full.

Overexposed. The water spilled out over the edge of the glass.

If you were to serve such a glass of water to a guest you would get a remark, that the glass is way too full and that water is spilling over the edge.

The digital sensor can be viewed as a glass of water, or more commonly thought of a bucket, with a capacity to hold water without spilling over the edge. Without going into a discussion about raw vs. JPG, lets just say that the bucket can hold up to 255 drops of water. If you pour more into the bucket the water will spill and get lost. An empty bucket equals complete black (value of 0) whereas a completely full (right to the edge) bucket equals pure white (a value of 255).

Proper exposure is usually somewhere in between. I say usual because sometimes you want the image to be darker, moodier or more dramatic, and sometimes you want the image to be brighter, lighter or romantic. Likewise, sometimes you want to fill the glass with less water, e.g. if you give the glass to your 4 year old son. Maybe he shouldn’t get a completely full glass of water as he might spill it.

For now go out and practice the little you have learned so far. We are going to take baby steps. Play with Aperture and Shutter Speed in Manual mode and take some images. They will for sure not be great but hopefully you will learn many things about your camera and the relationship between aperture and shutter speed.

Next up is some dull theory …

Do you want to get the best out of your dSLR? Then you have to understand the basic concept exposure. It is really not that hard. There are only three variables and they are easily explained.

Imagine you have to fill up a glass of water from a water tap. If you open the tap to its fullest you’ll have the glass filled in no time. However, if you only open the tap a little, so the water is only dripping out, it will take quite some time to fill up the glass. But either way in the end the glass will be full, right? So, the two factors in play here are the time the tap is open and how wide open the tap is. That’s the same with a camera!

The water tap represents the camera shutter and the lens aperture. The shutter speed is the amount of time the shutter is open that is, the amount of time light is collected onto the digital sensor (or film); longer shutter speed equals more light collected. The aperture is the size of the opening of the lens; the larger the aperture the more light is collected.

Lots of water coming from the water tap. This represents a large aperture.

Less water coming from the water tap. This represents a smaller aperture.

Shutter Speed

Shutter speed is expressed in fractions of a second, for example 1/4 of a second or 1/125 of a second. Of course, more light is collected a 1/4 s compared to 1/125 s. A doubling of the shutter speed will double the amount of light collected. This is very intuitive for most people.

Aperture

Aperture is expressed somewhat differently. It is expressed as fractions of the focal length of the lens, for example f/4 or f/32 where f is the focal length of the lens. Fortunately, you do not need to know the focal length, as the aperture is a relative number; both a 24mm lens and a 200mm lens have f/4 and f/32 apertures.

Just like shutter speed, f/4 collects more light than f/32. However, unlike shutter speed a doubling of the aperture will not result in a doubling of the light collected. One only needs to multiply by a factor of 1.4 (actually the square root of 2) in order to get a doubling of the light collected. This is because the aperture is the diameter of a circle and by multiplying the diameter by 1.4 we get twice the area.

The concept of aperture is harder for most people to grasp. But fear not, it is really not hard at all. I’ll show you some more practical examples along the way.

ISO

The third and last variable to exposure is called ISO. In the days of film this was called film speed and expressed in ASA. ISO is expressed as a number, for example 100 or 400. The ISO is a measure of the sensitivity of the digital sensor. In essence it is an amplifier so that the higher the ISO the brighter the image will be. However, this is as digital effect! The ISO setting does not change the amount of the light that is collected on the sensor. The ISO setting will amplify the amount of light. A doubling of the ISO results in a doubling of the brightness of the image.

In our analogy with the water tap, think of ISO as the number of people filling up glasses of water. If two people were filling up glasses of the water with the same settings (time and opening), you would get twice the amount of water. If four people were filling up glasses of water you would get four times the amount of water.

One glass of water. Representing ISO 100.

Twice the number of glasses. Representing ISO 200.

Four times the number of glasses. Representing ISO 400.

Whats next?
Now I think you need a little exercise.

One advantage of a dSLR over a point-and-shoot is all the controls available. I feel it is essential you spend time to get familiar with your camera and the controls it offers. For learning basic photography skills one of the best controls on the dSLR is the manual exposure mode (“M”).

All the brands of cameras have this ability so whether you shoot Canon, Nikon, Pentax, Sony or whatever other brand available go set your camera to Manual exposure mode. On a Nikon D7000 (my current camera body) the dial which controls the exposure mode is show in the image below.

Setting a Nikon D7000 to Manual exposure mode ("M")

In a series of posts I’ll take you through some theory and exercises beginning with understanding exposure.