Exposure

In the world of photography, the word exposure has many definitions...

1) It refers to light reaching either the electronic image sensor (digital) or the light-sensitive emulsion (film).

2) 35mm film is sold in 24 and 36 exposure rolls, where each frame is considered to be an exposure.

3) It also refers to the balancing act concerning the values of ISO, Aperture and Shutter-Speed, with our objective being to get an image which looks pleasing to the eye - where we can see details in the shadows, as well as the bright highlight areas. In such a case, a photograph is said to have a good exposure.


This page deals with the third definition, exposure in the context of balance between the 3 properties. 

Think of a typical two-tray balance scale.

Now, add an extra tray to the scale.

In each tray will go the ISO, Shutter-Speed and Aperture.

Just like with using a balance scale, our objective is to get all 3 trays to balance properly.

For me, determining the proper exposure for a photograph begins with setting the ISO.

With film cameras, we insert a roll of film in the camera and with digital, we can set it to whatever we want. Let's say we set the ISO to 100 for daylight. This causes one tray to drop due to the weight.

As a matter of personal preference, I almost always set the Aperture next, in order to either control the depth-of-field OR to control the light passing through the lens. If I want a limited depth-of-field, where my subject is in focus but the background is blurred, I will set a low f/stop number (f/1.8 to 5.6). I will also set a low f/stop if I am photographing a sporting event and wish to freeze the action.

With my second property set, the tray will drop, while the ISO tray comes up a little.

Finally, I set the Shutter-Speed to whatever will cause all 3 trays to perfectly balance.

Under most circumstances, this will give me a good exposure. Not too dark and not too bright.

It doesn't matter what property I set first, or in which order I set the others, as long as the scale balances.

Of course, there is no balance scale in your camera, but there IS something called a light-meter. With an older film camera, we may have a swinging-needle, while with digital cameras, we have a visual light-bar to show how our light-meter is reading a scene.

ISO 200 | 0.5 seconds | f/29

ISO 200 | 0.5 seconds | f/29

Take a good look at ALL the examples in this slideshow.

While each image has the same balanced exposure, they all LOOK different.

In this case, it's the water that has the most impact.

The ISO is unchanged (at ISO 200), but what does change is the Shutter-Speed and the Aperture. If I were to change the Shutter-Speed (so it stays open longer to blur the water), I must close the Aperture down in equal proportion.

Conversely, if I wanted a fast Shutter-Speed to freeze the water, I must open up the Aperture to allow more light to pass through the lens.

If I were to open up the Shutter-Speed without adjusting the Aperture to match, I end up with an image that is lighter.

When shooting on film, we will not know whether we achieved a good quality image or not, until it gets developed. That's just how it is.With digital though, most cameras (and apps) will have something called a histogram and without a doubt, this is your best friend for determining image quality.

The histogram is a graph that shows how the image stacks up, literally.

A digital camera's LCD display allows for instant feedback of our photos, that's all.

NEVER trust it to determine if the exposure is good or not.

Instead, use the camera's Histogram display.

The brightness value of each pixel is measured, organized and stacked one-upon-another.

The graph it creates goes horizontally across the bottom, from left (black) to right (white) and covers the entire tonal range in-between.

Pixels are stacked vertically, and the more pixels there are at any given point, the taller the graph. Our aim is for a balanced exposure, something resembling a mountain range, where the pixels are concentrated in the middle of the graph.

Much like what is seen in this graphic.

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