Tip of the week: Photography basics - What is exposure?

Term "exposure" is often wrongly interpreted only as the time of exposing the sensor to the incoming light. And actually, that time is just one piece of the exposure puzzle.

Exposing the image correctly does not mean just choosing the right time of exposing the sensor, but also includes setting the correct aperture and chosen ISO value.

Thus we can say that the exposure consists of three parameters:


Shutter speed


How should we properly balance between those three different parameters? Maybe, one of the easiest ways to understand it is to imagine it as a faucet, a glass and some water. Let's say that the water coming from the faucet that is filling the glass is the light, and by filling the glass to the top means getting enough light to properly expose.

Closeup of shutter speed dial on a classic camera

Size of the glass we can identify with the ISO value. The bigger the glass, the smaller the ISO and thus the more water (light) we need to fill the glass to the top. If we raise the ISO, the glass gets smaller and less water (light) we need to fill it up. So the first step to properly expose is to set the size of the glass - choose the ISO value. We set it according to the amount of light we have available in the scene. If we have a dark scene with little light (a little amount of water available) we need to use the smaller glass (higher ISO) to be able to fill it up with our available amount of water.

Shutter Speed Dial

Shutter speed (time of exposing the sensor to the light) will be the amount of time we keep our faucet open for the water to flow. So the longer the water flows, the bigger the glass it can fill completely. If you run the water for too long, you will overfill the glass which in photographic terms means the image will be overexposed - meaning that too much light has fallen on the sensor. If you keep it open too short, you cannot fill the glass, and your image will be underexposed. Which means that the sensor on the camera did not receive the sufficient amount of light.

Macro of aperture scale

Aperture value is the size of the faucet. The bigger the faucet, the more the water flows per time so it will fill the glass more quickly. So if you set the aperture to large values (smaller f numbers) you will have bigger faucet and you can fill your glass more quickly. Thus you would need shorter shutter speed (shorter time of the water to flow), and vice versa.

Generally, after you set your ISO (glass size) you need to balance between shutter speed (time of the water flow) and aperture (size of the faucet) so to properly fill your glass to the top. And be careful not to spill the water, or leave the glass half empty.

Mountain Pirin from peak Polezan peak Polezan

Properly setting the exposure parameters is the crucial part of getting the properly exposed image. And the size of the glass is always better to be as big as you can, since higher ISO values can lead to noise, less dynamic range and generally lower image quality.

Photo credits: Dvmsimages, Milan Ljubisavljević, Payphoto, Thejipen.

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September 26, 2019


thank you William

September 25, 2019


I'm about to do a workshop at my daughter's photojournalism class. I'll have to forward them the link to your blog! Thanks for writing. William

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