Exposure triangle

“Here’s an idea: When you tell these little stories, have a point! It makes it SO much easier for the listener!”

-Planes, Trains and Automobiles


I’ve read a lot of blogs about photography. What I haven’t been able to find is someone explaining what they are learning while they are learning it.

I really believe the best way to learn is to attempt to teach someone else.  I don’t actually expect anyone to be particularly interested in my amateur explanations, but it is helping me figure a few things out.

Today’s explanation involves the dreaded Exposure Triangle.  I even made my very own diagram to make things a bit clearer, at least in my mind.

There are many analogies about the exposure triangle.

A popular one is the glass of water – the size of the glass is the ISO, the flow of the water is the aperture and how long you leave the tap on is the shutter speed.

Okay, not bad.  How about thinking about a window.  The aperture is the window size.  How long you leave the window open is the shutter speed.  And the ISO is…um…how long you look at the light?

Okay, maybe not as good.

For every picture, I have to make three decisions about my settings shooting in manual mode – ISO, shutter speed and aperture.  And it probably won’t be the same way twice.

The ISO is a measurement of the sensor’s sensitivity to light.  Under low lighting conditions (dark and gloomy), I need a higher ISO (on my Canon Rebel T3i the range is 100 to 6400).  ISO 6400 is much more sensitive to light than ISO 100.  When it’s bright and sunny, I need to dial it back.  In the old days of film, I remember being advised to always buy ISO 400 as a good middle-of-the-road film speed.  The real joy of digital cameras is that you can switch between settings, which is a lot easier than burning through a whole roll of film.

On the DSLR for outdoor photography, I use 100 or 200.  I have learned the hard way NOT to use auto ISO.  My duck pictures are a prime example.  ISO was on auto, so it cranked it up to 3200 without me noticing (because it’s clearly the camera’s fault that I didn’t look).  It looks very grainy.

Aperture controls how much light comes into the camera using a measurement system call f/stops.  F/stops are a crazy numbering system.   A low number means a really big opening, a big number means almost closed.  There is a much more complicated explanation involving focal length and pupil diameter, but that’s all I can really figure out right now.

A full stop is the effective doubling or halving of light. Why couldn’t they use full integers?  I don’t know.  I’m sure it’s a math question.  However, I felt really smart when I noticed this, so I thought I’d share.

1.4       2          2.8       4          5.6       8          11       16       22       32

I like patterns in numbers.  Every other full stop number is double the previous.   If you can remember the first two numbers (f/1.4 and f/2), you can figure out the next number in the sequence fairly easily (although I have been known to count it out on my fingers).

F/1.4 is twice as much light as f/2.  F/8 is half as much light as f/5.6.

Of course, the good people at Canon (and every other camera manufacturer in the world) needs us to have more and more buttons.  On my T3i, I have 1/3 stops.  Every three clicks up or down is a stop of light.

Shutter speed is the length of time the shutter is open. Fast shutter speeds are like blinking really fast.  You capture an instant in time.  A slower shutter speed captures motion blur.  Generally, your camera measures it in factions of a second.  I can’t take pictures slower than 1/60 of a second without a tripod – I shake too much.

Back to my arch nemesis, math, the shutter speeds on my camera usually double with each setting.

1/500      1/250      1/125      1/60      1/30      1/15     1/8

Yes, it doesn’t work out exactly in half, but close enough.

That’s what I’ve figured out this week.  I think my project this week is going to involve shutter speed to create motion blur.


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