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 …

Set your camera to Manual exposure mode (“M”) . Turn off Auto-ISO (if your camera has this feature, not all cameras have it). Set the shutter speed to 1/60, the aperture to f/5.6, and the ISO to 200. Now take a picture of something which catches your eye. Have a look at the back of your camera. Chances are that the picture is either too bright or too dark. That is okay. Now comes the fun part.

Change the shutter speed to 1/125 (leave ISO and aperture as they are) and take another picture. The picture should half the brightness as the previous one. Now change the shutter speed back to 1/60, change the aperture to f/8, and take another picture. The picture should have the same brightness at the previous one, since you have taken an image with the same Exposure Value (combination of shutter speed, aperture, and ISO). Now change the shutter speed to 1/60, the aperture to f/5.6, and the ISO to 400. The image should be four times as bright as the previous two images and twice as bright as the first image.

Play with these three settings for a while. Try to go crazy and set the shutter speed to 1 second (1″) or to 1/4000 and see what happens with the pictures. Change settings back and forth until you get an image which looks properly exposed to you. When you have reached a proper exposure, you are ready to move on.

Severely underexposed flower

Underexposed flower

Properly exposed flower

Overexposed flower

Severely overexposed flowed

Change the exposure mode to Aperture Priority (“A” or “Av” on most cameras). Change the aperture to f/5.6 and note the ISO setting and shutter speed. Now the camera is trying to calculate a proper exposure for you. By setting the camera in Aperture Priority mode, you have told the camera to use the aperture value you have picked but to change shutter speed to obtain proper exposure. The ISO should not chance unless the camera is in Auto-ISO mode. Take the picture and compare it to the one you took in Manual mode which you thought was properly exposed. How close are they?

Now change the exposure mode to Shutter Priority (“S” or “Tv” on most cameras). Change the shutter speed to 1/60 and note the ISO setting and aperture. Again, the camera is trying to calculate a proper exposure for you. By setting the camera in Shutter Priority mode, you have told the camera to use the shutter speed you have picked but to change aperture to obtain proper exposure. Again, the ISO should stay fixed as well.

I recommend you keep your camera in Manual exposure mode even when you take pictures of friends and family. Yes, you will get many bad shots to begin with but soon you will learn how to see which settings are appropriate in any given situation. If you pick for example Aperture Priority you put your brain in the back seat and the let the camera drive. It is good to know that the camera can drive, but for now you should take control and choose settings manually.

One last thing. Now go inside and repeat the exercise. What do you expect will change? Why?

When you have completed the exercise read the post More about exposure.

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.

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.