The Exposure Triangle

The purpose of this blog entry is to share with you some basic information about making photographs. It helps me to get it all down “on paper” and I hope you will also find it helpful.

 If you do much photography, you have heard of the “exposure triangle.”  This refers to three elements which determine what camera settings to use for a proper exposure.  They are 1) ISO   2) aperture and, 3) shutter speed. Let’s take each one of these in turn.

ISO) What does ISO stand for? Wikipedia says that "Although one might think ISO is an abbreviation for 'International Standardization Organization' or a similar title in another language, the letters do not officially represent an acronym or initialism. The organization provides this explanation of the name: 'Because "International Organization for Standardization" would have different acronyms in different languages (IOS in English, OIN in French), our founders decided to give it the short form ISO. ISO is derived from the Greek word isos...meaning "equal." Whatever the country, whatever the language, the short form of our name is always ISO.'"

See "https://en.wikipedia.org/wiki/International_Organization_for_Standardization

At any rate, ISO referred originally to the sensitivity of the film being used. The lower the number, the less sensitive to light the film was. At the same time, a film less sensitive to light also produced high quality negatives with little noise or graininess. The more sensitive the film, the more grain or noise.

If we start with ISO 100, the next ISO up would be ISO 200. ISO 200 captures double the amount of light as an ISO 100 film. Double the number again...which doubles the sensitivity to light...you get ISO 400 then ISO 800 and so on. You would make a decision about where you would be shooting and how much light you could count on and then choose the appropriate film.

With the advent of the digital era, this ISO language was still used, though of course you are dealing with a sensor and not with film. Here's a helpful explanation: "When digital cameras entered the scene, the ISO standards developed in the days of film were adapted to suit new technologies. To put it simply, ISO in digital sensors is just an arbitrary scale that describes the amount of amplification applied to the analog signal from the sensor before it is converted into a digital number."

Quoted from:

https://theauroraguy.com/blogs/blog/iso-is-not-what-you-think-it-is-what-is-iso-really?srsltid=AfmBOopLqqqBuzOfLGGSF9Jr3EGrEsbCCu66ZJy5gGwFSnr762EMvUaX

As with higher ISOs where film is concerned, one side effect of amplifying the signals, is noise. Fortunately there are programs which, in post-production, can help "denoise" high ISO captures. On to the next topic: aperture.

APERTURE) For my money, this is the most confusing part of the "exposure triangle" but here goes. Aperture refers to the opening in the lens which allows light to enter and hit the sensor. It helps to think of the human eye and its pupil. The pupil expands and contracts depending on the amount of light needed. On a bright day the pupils of the eye look like pin points so that a minimal amount of light can enter the eye. At night in a dark location, the pupils get very large to let as much light as possible to enter.

In similar fashion, a lens has a "pupil", or actually it has a diaphragm formed by a series of blades. When the blades are manipulated mechanically, you can open or close the aperture. Here's a picture.

Credit: By D-Kuru - Own work, CC BY-SA 4.0

https://commons.wikimedia.org/w/index.php?curid=98375898

Now we come to f stops, the way that the sizes of apertures are identified. The "f" in f stop refers to focal length. "Focal length is the distance (measured in millimeters) between the point of convergence of your lens and the sensor or film recording the image."

https://www.masterclass.com/articles/basic-photography-101-guide-to-understanding-focal-length#5XP7jSQMUmT6SfLBV3YkRo

Here's a picture illustrating that definition showing a 28mm lens.

https://expertphotography.com/understand-focal-length-4-easy-steps/

One way to think of f stops is as a ratio between the diameter of the aperture and the focal length of the lens. The formula is: f number = F/D where F is the focal length of the lens and D is the diameter of the aperture. If the diameter of the aperture is 25mm and the lens has a 50mm focal length, the f number would be 2.

Now it starts getting really fun! You see, if the aperture stays at 25mm but you are now dealing with a 100 mm lens, the f number would be 4 (100/25). Even though the aperture diameter is exactly the same, the f numbers are different...f2 in the first example and f4 in the second.

This allows us to compare apples to apples when we are dealing with lenses with different focal lengths. F2 needs to allow the same amount of light into the lens regardless of focal length. Thus an f2 aperture diameter for a 100mm lens would be 50mm (100/50).

Fasten your seatbelt because we are now going to start talking NOT about the diameter of the aperture, but the AREA of the circle created by the aperture. The list of f stops is all about the area of the aperture. Typically f stops run like this:

1 1.4 2 2.8 4 5.6 8 11 16 22

These numbers represent what is referred to as "full stops." If you go up the numbers from 1 to 22 each stop along the way represents a halving of the area of the aperture circle, which means that half the amount of light is let in at each stop. If you go down the numbers from 22 to 1 each stop along the way represents a doubling of area of the aperture circle which means that the amount of light let in is doubled with each stop.

Why the weird set of numbers? Because, going up the scale, you can determine the next stop by multiplying by the square root of two (√2).and going down the scale you divide by √2. The square root of two is basically 1.4. So 1 times 1.4 is 1.4. 1.4 times 1.4 is 2 (rounded up for ease of use). And 2 times 1.4 is 2.8. Then 2.8 times 1.4 is 4 (rounded up). And on it goes. You can do the same thing in reverse, that is, you can start with f22 and move down. In this instance you divide by √2 rather than multiply.

The reason that we multiply or divide by √2 has to do with the formula for determining the area of a circle. That formula is Area = π times r2, where π equals 3.14 and r2 refers to the radius of the circle squared. Here is the rest of the answer borrowed from a great web site.

https://www.outdoorphotographyschool.com/aperture-and-f-stops-explained/

Ok then! And now, for your edification, here is a picture of different f stops on a 50mm lens.

By KoeppiK - Own work, CC BY-SA 4.0. https://commons.wikimedia.org/w/index.php?curid=78136658

I will end this section with a few take-aways. The sequence of f stop numbers I gave you represent "full stops"...that is a halving or a doubling of the light allowed to enter. There are also partial f stops. So your camera may have additional stops, say, where a third less or a third more light is allowed.

Regardless, the basic numbers are these:

1 1.4 2 2.8 4 5.6 8 11 16 22

Can't remember the whole list? You don't need to. Just remember the first two numbers - 1 and 1.4. You can generate the whole list by multiplying the first two numbers by two (if you are going up the scale). So 1 times 2 gives you the third full stop of f2. Then two times 1.4 gives you the fourth full stop of 2.8 and so on. By remembering the first two numbers and multiplying by two, you can leap frog your way up the scale.

One final word, unless you know the "backstory" about f stops it is easy to get confused about what the numbers mean in terms of light. It is natural to think that the higher the number the more light you would get. But of course, the opposite is true. That might seem counter-intuitive if you don't know the full story. Now on to the third and final area, shutter speed.

SHUTTER SPEED). This is much simpler than aperture! Shutter speed simply refers to the amount of time that the film or sensor is exposed to the light coming into the lens. It is measured in seconds or fractions of a second. The exposure happens when a shutter or a curtain with a gap in it moves across the sensor so light can hit the sensor.

"The agreed standards for shutter speeds are: 1/1000 S, 1/500 S, 1/25 S, 1/125 S,

1/60 S, 1/30 S, 1/15 S, 1/8 S, 1/4 S, 1/2 S and 1 S/ "

See: https://en.wikipedia.org/wiki/Shutter_speed

If you are observant, you have picked up on a trend. When we were talking about ISO we noted that ISO 200 picked up twice as much light as an ISO 100 film. That can be referred to as a one stop difference. When we were talking about apertures we noted that f 1 lets in twice as much light as f 1.4. Now with shutter speeds, we are still basically talking about doubling or halving the amount of light.

So a 1 S shutter speed lets in twice as much light as a 1/2 S shutter speed. That difference can also be referred to as one stop, by the way. And a 1/2 S exposure allows twice as much light as a 1/4 S exposure and so on. The longer the exposure, the greater the amount of light.

And of course, long exposures can generate blurry pictures of objects that are moving in front of the lens. Here is a helpful graphic:

https://photographygloves.com/blogs/vallerret-blog/shutter-speed

So there you have it. By using ISO, f stops, and shutter speed, you can get the perfect exposure. Each step of the way you can increase the amount of light or decrease the amount of light by adjusting ISO, aperture, or shutter speed. You could also make changes in all three areas, depending on your needs. There is of course much more to consider such as depth of field, but this will do for now! Thanks for reading.