The Correct Exposure

In our previous episodes, we covered the three factors in an exposure: the ISO, the Shutter and the Aperture. The combination of those three forms the Exposure Triangle. Photography is all about capturing light, and it is no surprise that the three components (ISO, Aperture and Shutter) regulate the amount of light the media is exposed to. One recurrent question I am often asked is “what should be my ISO settings, my Shutter speed and my Aperture?”. Unfortunately, the answer is always the same: It depends on . This short answer can be frustrated for unexperienced photographer, but that is the whole point of the Photography. There is not just one good value to stand by at all time, and we are going to see why. All Digital camera have a Light Metering System integrated that assesses the brightness level of the situation you are trying to capture. We will cover the different Light Metering option in a future episode. Based on the Light Metering system, if your camera is set to Full Automatic Mode (aka the green mode) the camera will use that information to set the ISO, the Shutter Speed and the Aperture appropriately, or should we say, appropriately according to itself (the camera) and then take the shot for you, leaving you only the power to decide whether it is a good photograph. Here is a sample exposure taken in full Auto Mode:
ISO 800 F5.6 1/20
ISO 800 F5.6 1/20

As you can see the photograph is well exposed.

What does “well exposed” mean? 

It means that the image brightness is well balanced, it’s neither underexposed (dominated by blacks, dark) nor overexposed (dominated by whites, blown out).

histogram showing the 3 extremes
Histogram showing the three extremes
Check the Exposure with the Histogram (we’ll cover this in a future episode) However, this is not the only combination of ISO setting, Shutter speed and Aperture setting that will give you a correct exposure for this situation. And here is how I prove it:
  • I look at the exposure settings the camera gave me previously: ISO: 800 | Aperture: 5.6 | Shutter: 1/20
  • I then decide to override my camera and switch it to Full Manual Mode and setting the same combination of ISO, Shutter and Aperture value as previously.
This is a shot taken in manual mode. It is not as complicated as you might have thought, correct? We will review the different shooting mode most Digital cameras offer today in a future episode.
ISO 800 F5.6 1/20
ISO 800 F5.6 1/20
As seen in the previous episodes we said that each time:
  • I’d increase the ISO by a full stop it’d increase by 2 the amount of light captured.
  • I’d increase the Shutter by a full stop it’d divide by 2 the amount of light captured.
  • I’d increase the Aperture (smaller f-number) by a full stop it’d increase by 2 the amount of light captured.
  • I’d reduce the ISO by a full stop it’d dived by 2 the amount of light captured.
  • I’d reduce the Shutter by a full stop it’d increase by 2 the amount of light captured.
  • I’d reduce the Aperture (bigger f-number) by a full stop it’d divide by 2 the amount of light captured.
The 6 lines above is really what the basics of Photography and correct exposure is about. So let’s apply those rules to find 3 correct exposures for this situation. I first increase the Aperture by 1 stop ( f5.6-f4) but subsequently, to not overexpose the initial shot I need to reduce the Shutter speed by a full stop (1/20-1/40):
ISO 800 F4 1/40 
ISO 800 F4 1/40
Then I decide to reduce the Aperture by another stop (f4-f2.8) therefore I reduce the Shutter speed by a stop as well (1/40-1/80):
ISO 800 F2.8 1/80
ISO 800 F2.8 1/80
Then I leave the Aperture (f2.8) but decide to reduce the sensibility of the sensor by 4 stops (ISO 800, ISO200)  which mean I need to reduce the Shutter speed by 4 stops, so we do not underexpose the shot:
ISO 200 F2.8 1/20
ISO 200 F2.8 1/20
I appreciate this might appear very confusing and very mathematical to some of you. You will get used to it as you try it with your camera If there is one thing you need to understand form this, it is there is not 1 single correct exposure. As a matter of a fact I would say there are about 7 possible combinations to achieve a correct exposure. While Photography is about capturing light, obtaining a correct exposure should not be the goal but a mean! Photography is an art, and as such it is meant to share with the viewer an idea, a concept, a feeling. It is a mean of expression. I took the following exposure in a different location and while the exposure is correct at all time the choice of ISO, Shutter and Aperture was first driven by the effect I wanted to give and only then used to balance any compromise: In the exposure below, I wanted to show the car light trails which required a long exposure (10sec), low noise (ISO 100) and big depth of field (DOF).
ISO 100 F22 10"
ISO 100 F22 10″
In this exposure I wanted to give a starry effect to the light poles, so I needed a small Aperture (F11), low noise (ISO 100) so longer exposure to capture enough light (30sec)
ISO 100 F11 30"
ISO 100 F11 30″
With those two exposures, we are reaching another milestone in our journey through the basics of Photography: Capturing a correct and creative exposure! In the first exposure, the creativity came from the Time factor while for the second exposure; it was the Aperture. In our next episodes, we will tackle the Shutter and the Aperture as creative factors.

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