Understanding exposure

[TCampbell]

I've also always liked the water glass analogy... people can get their mind around needing enough water to fill the glass and they can understand how a tap can be opened to a trickle or wide open and gushing water (flow rates) for a short or long period of time to fill the glass.

When I started, we didn't introduce ISO into the equation because you can't change the ISO with a setting on the camera... you had to change the film loaded into the camera to do that.

So the equation was simply Ev = Tv + Av (Exposure value = Time value + Aperture value) and the table looked like this:

Av​

0​

1​

2​

3​

4​

5​

6​

7​

8​

9​

10​

Tv​

1.0​

1.4​

2.0​

2.8​

4.0​

5.6​

8.0​

11​

16​

22​

32​

0​

1s​

1​

2​

3​

4​

6​

7​

8​

9​

1​

1/2​

1​

2​

3​

4​

6​

7​

8​

9​

11​

2​

1/4​

2​

3​

4​

6​

7​

8​

9​

11​

12​

3​

1/8​

3​

4​

6​

7​

8​

9​

11​

12​

13​

4​

1/15​

4​

6​

7​

8​

9​

11​

12​

13​

14​

5​

1/30​

6​

7​

8​

9​

11​

12​

13​

14​

6​

1/60​

6​

7​

8​

9​

11​

12​

13​

14​

16​

7​

1/125​

7​

8​

9​

11​

12​

13​

14​

16​

17​

8​

1/250​

8​

9​

11​

12​

13​

14​

16​

17​

18​

9​

1/500​

9​

11​

12​

13​

14​

16​

17​

18​

19​

10​

1/1000​

11​

12​

13​

14​

16​

17​

18​

19​

[TD="bgcolor: #EBEBEB, align: right"]0[/TD]

[TD="bgcolor: #ebebeb"]

5​

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10​

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15​

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20​

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Notice the diagonal grey shaded stripes... every box with the same exposure value number (Ev) is an equivalent exposure. You could clearly see the relationship between changing f-stops and shutter speeds.

If a person can get their head around just the relationship between aperture and time, then introducing ISO is easy... it's just one more element that can be used as the trade-off when setting exposure.

 

[weepete]

I'm still rooting for the exposure tetrahedron! XD

 

[Ysarex87 81]

I've also always liked the water glass analogy... people can get their mind around needing enough water to fill the glass and they can understand how a tap can be opened to a trickle or wide open and gushing water (flow rates) for a short or long period of time to fill the glass.

When I started, we didn't introduce ISO into the equation because you can't change the ISO with a setting on the camera... you had to change the film loaded into the camera to do that.

So the equation was simply Ev = Tv + Av (Exposure value = Time value + Aperture value) and the table looked like this:

Av​

0​

1​

2​

3​

4​

5​

6​

7​

8​

9​

10​

Tv​

1.0​

1.4​

2.0​

2.8​

4.0​

5.6​

8.0​

11​

16​

22​

32​

0​

1s​

1​

2​

3​

4​

6​

7​

8​

9​

1​

1/2​

1​

2​

3​

4​

6​

7​

8​

9​

11​

2​

1/4​

2​

3​

4​

6​

7​

8​

9​

11​

12​

3​

1/8​

3​

4​

6​

7​

8​

9​

11​

12​

13​

4​

1/15​

4​

6​

7​

8​

9​

11​

12​

13​

14​

5​

1/30​

6​

7​

8​

9​

11​

12​

13​

14​

6​

1/60​

6​

7​

8​

9​

11​

12​

13​

14​

16​

7​

1/125​

7​

8​

9​

11​

12​

13​

14​

16​

17​

8​

1/250​

8​

9​

11​

12​

13​

14​

16​

17​

18​

9​

1/500​

9​

11​

12​

13​

14​

16​

17​

18​

19​

10​

1/1000​

11​

12​

13​

14​

16​

17​

18​

19​

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15​

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10​

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15​

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[TD="bgcolor: #ebebeb"]

20​

[/TD]

Notice the diagonal grey shaded stripes... every box with the same exposure value number (Ev) is an equivalent exposure. You could clearly see the relationship between changing f-stops and shutter speeds.

If a person can get their head around just the relationship between aperture and time, then introducing ISO is easy... it's just one more element that can be used as the trade-off when setting exposure.

:thumbup::thumbup: EV table! When cameras were intelligently designed we didn't have a problem:

Look at that shutter speed and aperture ring locked to EV 12. Turn one and the other turns with it. Change film (ISO) or the light intensity and just re-lock to the new EV value. I used to load one of these up with Tri-X, lock the EV ring to 16.5 and head out into the sunshine. A point in the evolution of photography when maximum clarity was achieved. We are now in the period I refer to as technological obfuscation.

Joe

 

[peter27]

And yet, for me at least, the triangle analogy works well because the internal angles always have a total of 180°, so if you change one of these at least one of the others has also to be adjusted to compensate.

And this, right here, is a great example of why I am filled with despair. This worthless bit of chartjunk seems to somehow crystallize things for people, and then they write things like this that almost make sense, but not quite.

If you had an exposure circle with three variable sized wedges or something, this sentence would make perfect sense (and, as an aside, you'd be back to the bridge, except now it's wrapped around in a circle). The "exposure triangle" is always drawn as an equilateral triangle, though, and the internal angles are always all 60 degrees. If you dig around slightly, you'll even find "explanations" that explain how you have to make your adjustments so as maintain the equilateral triangle shape.

However it is, it seems to work well enough to be this self-propagating object, and really, who the hell am I to judge? Apparently there's some bizarre alchemy that makes it work for some people.

You're right of course that the shape of the triangle should change, but I think it makes it easier to understand if the form remains constant - it's a teaching tool after all and ease of access is an important factor. The idea of a circle with three wedges inside is good; perhaps a semicircle representing the 180° divided into three parts, drawn over, under or inside the triangle, could help some people to visualise the relationship between the three parts a little better.

 

[vintagesnaps]

Technological obfu-something sounds about right Joe! I don't know that the triangle concept describes exposure that well, there are so many things perpetuated online that aren't necessarily accurate but just get passed around. I just learned (I think) basically how to use a meter to determine if the amount of light is what's needed for each photo you take, and how to understand how aperture and ISO and shutter speed work with that light coming in to your camera and how to adjust each one.

People learn in different ways so maybe there are various ways that could be used to show how those three factors work. I see where this is going with the analogy but the river concept doesn't seem like it would necessarily make sense to beginners. It isn't how I understand exposure and wouldn't work for me I don't think.

 

[EIngerson]

"Shutter speed, Aperture, ISO....SWIPER, NO SWIPING!"

 

[amolitor]

"Shutter speed, Aperture, ISO....SWIPER, NO SWIPING!"

Who's got kids, eh?

 

[480sparky41 40]

Well cripes ..... Lets toss white balance, focal length, depth of field, diffraction, chromatic aberration, distortion and subject/camera movement into the New Formula. We can all it the Photograph Octaterrahedron.

 

[LungFish]

I propose an alternative analogy - the exposure 'hole with a sensor behind it'.

To capture more light you open the hole bigger, leave it open for longer, or amplify what the sensor records. To capture less light you do the reverse, and making two equal and opposite changes will give you the same amount of light.

Or is that too literal? Do I need to describe something entirely unrelated to explain something which is actually pretty obvious? I firmly believe most people who struggle with exposure are actually struggling with the terminology rather than the concept.