Understanding Light in Photography – Beginner Level

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The word “photography” comes from the Greek, photos – light – and graphos – writing, delineation, or painting. In other words, photography is light painting. As humans, we don’t see color, we see light. The retina of our eyes contains two types of photoreceptors (light receptors) called rods and cones. The rods are more sensitive than cones but they’re not sensitive to color. The cones, much fewer in number, provide the eye’s color sensitivity and are much more concentrated in the center of the eye known as the macula. Light is made up of different wavelengths that the cones detect and transmit to the brain which allows us to see color. We see color because of the manner that an object reflects light and the mix of the wavelengths that are received by the cones from that object and, in turn, transmitted to the brain.

Okay, enough of the technical stuff and keep in mind that the above paragraph is somewhat of an over simplification. Its intent is to provide a general basis for going forward.

The principles of photography are the same as they’ve always been. However, in modern cameras, film has been replaced by the sensor. Like film, the sensor of a camera is not nearly as complex and adaptable as the human brain. As a result, the camera doesn’t see the same as the human eye.

Photos are made by “exposing” the film or sensor to light. No light equals no exposure. Too much light equals too much exposure. Therefore, in any given situation there is an amount of light that produces the “correct” exposure. This “correct” exposure is what we are trying to achieve when we press the shutter release on a camera. Point and Shoot cameras automatically adjust the amount of light passing through the lens and falling on the sensor. DSLR cameras allow you, the photographer, to adjust the amount of light that reaches the sensor which gives the photographer control over the exposure or amount of light falling on the sensor.

When you press the shutter release on a camera the shutter opens and allows light to hit the sensor. When the desired amount of time has passed, the shutter closes. This is true whether the shutter setting is 1/10,000 second or 30 seconds. So, setting the shutter speed on the camera determines how long the shutter is open when photographing any subject.

The lens of the camera contains a diaphragm that generally works like the pupil of your eye. In photography, this diaphragm is called the aperture. (The basic definition of aperture is opening.) When it’s opened wide, it allows relatively more light to reach the sensor and when it’s narrow, or stopped down, it restricts the amount of light that reaches the sensor.

In addition to shutter speed and aperture, you must also be aware of ISO. The ISO setting influences the light sensitivity of the sensor. It doesn’t control the amount of light reaching the sensor, rather the amount of light required to achieve “correct” exposure. Therefore, a change in the ISO requires a corresponding change in the shutter speed and f stop settings to get the same exposure as a different ISO setting. What the ISO does influence is the quality of the image. A low ISO setting will enable a sharp, noise, or grain, free image. A high ISO setting will, more than likely, produce an image with a considerable amount of noise. One more variable, the amount of noise resulting from a given ISO setting is heavily influenced by the camera you’re using and the sensor of that camera. A lower end camera will generally produce much more noise at high ISO setting than will a high end camera.

Shutter speed and aperture size, working in combination, determine the total amount of light reaching the sensor. You can probably figure out that if these two mechanical features of the camera work together, there must be different combinations of the two that will allow the exact same amount of light to reach the sensor and you would be correct. This relationship is called reciprocity. I’m going to assume that you already know that the shutter speed is measured in seconds or fractions of seconds and that the aperture is measured in a mathematical ratio, too involved to get into here, called f stops. The following table illustrates the different combinations of shutter speed and f stop that will provide the same exposure assuming an ISO of 125 on a bright, sunny day.

ISO 125            
Shutter Speed 1/2000 1/1000 1/500 1/250 1/125 1/60
Aperture f/4.0 f/5.6 f/8.0 f/11 f/16 f/32

The same combinations in a very low light situation that requires longer exposure time for the same f stop.

ISO 125            
Shutter Speed 1/60 1/30 1/15 1/8 1/4 1/2
Aperture f/4.0 f/5.6 f/8.0 f/11 f/16 f/32

With film there is something I’m not going to dwell on that’s called reciprocity failure.  It means that a color shift may occur with long exposures when using film and the relationship above won’t always work throughout the entire range.  With digital cameras, this is no longer an issue.  However, you may experience increased noise at longer exposures.

Here are some examples of this relationship.  All three photos are straight out of the camera with no processing at all except for converting the image from RAW to jpeg. Because of the difficult lighting situation (I was pointing directly at the sunrise) I captured these three exposures so I could create an HDR image.

The camera was set on aperture priority at f/16 and ISO 200. The camera automatically set the shutter speed at 1/125 second. So this is the metered exposure.

This is overexposed with a shutter speed of 1/50 second. Everything else is the same.

This is underexposed with a shutter speed of 1/320 second. All other settings the same.

In understanding light, the first challenge is fully understanding the amount of light and the mechanical functions of your camera.  Note that I said the first challenge.  Once you’ve mastered that part, the next step is understanding all the various kinds of light and sources of light and how they affect the image.

Small light sources provide harsh light with a lot of contrast.  Depending on where it’s located relative to your subject, small light sources can also reduce detail because it “flattens” the appearance of the subject.  For example, an on-camera flash is a small light source.  When you photograph a person who is looking directly at the camera, you lose detail because the flash “flattens” the natural curves of the face.  If you move the flash a little off to the side, you create harsh shadows that can be unappealing.

Large light sources provide soft light with less contrast.  It also, again depending somewhat on location, preserves the natural shape and curves of the subject.  Portrait photographers use soft-boxes and/or reflective umbrellas to soften the light coming from a strobe or a flash unit.  That’s why you’ll see photographers using a milky colored diffuser on their on-camera flash attachment.

Another example of a small light source is the sun.  I know the sun is very large, but it’s also a long distance from Earth which makes it smaller as a light source.  By photographing in the early morning or the late afternoon you get a much better quality light because it’s not as direct and the atmosphere diffuses the light and filters the light somewhat.

A cloud cover is a huge soft-box, so an overcast sky is a large light source.  Therefore, when photographing outdoors on an overcast day, not only do you have less light but also, the characteristics of the light are very different.

In the Intermediate Level, I’ll delve deeper into the sources of light, the control of light and the location of the light source.

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  • Daf

    Hi – beginner photographer here so I greatly appreciate these lessons!

    I’m confused tho – your image above labelled as over-exposed – with a shutter speed of 1/320 (all other settings the same) wouldn’t the faster shutter speed allow less light and thus have an underexposed photo? In the underexposed photo you have the shutter speed at 1/50 – wouldn’t that slower speed in fact allow more light to the sensor and cause an overexposure?


  • Steve Russell

    Daf, you’re absolutely correct.  That’s what I get for being in a rush to go watch a football game.  Thanks for pointing it out.

  • Daf

    Hi Steve – I’m actually grateful for the mistake in that I was able to reason that out. Maybe I’m getting it (photography basics,etc…) after all! ha ha! Thanks for the great posts.


  • Gemfoley

    also a beginner to this – thats the best explanation I’ve heard of aperture, shutter speed and especially ISO- thank you. much clearer now!

  • Gemfoley

    also a beginner to this – thats the best explanation I’ve heard of aperture, shutter speed and especially ISO- thank you. much clearer now!

  • Dreamjar

    Thanks, I shared this article in an article I wrote about creating a photography class for homeschoolers