Exposure

what does exposure mean?

When photographers talk about exposure, they actually talk about image brightness.

The goal is a correctly exposed image. An image that is too dark is called underexposed, and an image that is too bright, is called overexposed.

what is exposed, and what is it exposed to?

Since the “exposure” contains the word expose, we need to ask ourselves, what it is that is exposed. 
That’s the sensor of our camera, or for film photographers, the film.

And what is it exposed to?
To the scene you want to photograph, or in other words to light.

So light is the base of it all. Photography is Greek. Some people say it means painting with light, but it really means the light paints so the light is the artist and the source of our images.

Let me repeat that, because it is so important:
LIGHT (or in other words the brightness of your scene) IS THE BASE COMPONENT OF EXPOSURE

the components of exposure

1. Scene Luminance

So the base component of every image is light, or in other words the brightness of the scene you want to photograph.

Photographers call that scene luminance.

That scene luminance can vary greatly. For example an outdoor scene in bright sunlight is 1000 times brighter, than an indoor scene lit by a regular light bulb.

Obviously that has a massive influence on the brightness of our image.

For us humans, that is often a bit hard to understand, because our eyes, or better said our brain usually does a great job in providing a pretty constant level of brightness.

2. Aperture

The scene, that can be brighter or darker as we already know, is projected onto the sensor of our camera with the help of a lens.

The opening of that lens is called aperture and can be made bigger or smaller.

Quite logically, the sensor will be exposed to more light, if that opening is bigger, and less light if it is smaller. So the aperture also has quite a big influence on exposure.

Compare that to a window with blinds – if you close blinds just like you close the aperture, less light will enter the room.

3. Shutter Speed (Exposure Time)

Now the light has travelled through the aperture and you would assume that it reached the sensor.

Yes and no. Before you start taking an image, the sensor is covered by a shutter – a door if you will. Only when you press the shutter button of your camera, the door will open and start exposing the sensor to the incoming light.

The longer that shutter is open, the longer the sensor is exposed to the light, and the brighter the image will get.

The term shutter speed is a bit misleading. The better and also correct term is exposure time, but in photography you will mostly hear shutter speed.

After the shutter has closed again, the camera sensor is no longer exposed to the light, so per definition, exposure has ended.

So the three basic factors of exposure are scene luminance, aperture and shutter speed.

After the exposure, the image is being processed and that includes one more process that has an influence on image brightness – the gain.

the components of exposure

1. Scene Luminance

The scene that you want to photograph is projected onto the sensor with the help of a lens.
Within that lens, we can change the size of an opening. This opening is called aperture. Quite logically, the bigger the opening, the more light will get to your sensor.
Compare that to a window with blinds. Closing the blinds will make your room darker. Obviously closing the aperture, will make your image darker.

Alternative 1

Exposure Bars

"Exposure Bars" adds scene luminance as a fourth element

Exposure bars is a concept by Photographer and YouTuber Wolf Amri.
He even offers free cheat sheets on his website www.free-photography-course.com.
The addition of scene luminance as the first and primary component of exposure not only makes understanding much easier, but is most importantly a correct way of explaining.

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