Started Jul 9, 2013 | Questions thread
Re: surface area

You got three things and a triangle has three sides.  Peanut butter and jelly.  Each one of the three exposure parameters occupies its own corner of the triangle.  Each exposure parameter sits at the end of an arm of our triangle.

The amount of light you want to get to your sensor is the surface area of the triangle.  Say the surface area of the triangle is defined by: f/8 , 1/60 , ISO 400    These settings make a good picture for you.

Say you want to open up to f/4

This larger aperture lets in more light so the triangle on the f-stop corner gets bigger.  Bigger triangle = more light = too bright image = less good than the first image that makes you happy.

We know that first image was a good exposure; we like the surface area defined by that first triangle.  If we make part of the triangle bigger, we got to make some other bit smaller so we have the same surface area / exposure.

The corners are measured in stops.

If I make my f-stop corner bigger by going from f/8 to f4, I have made it longer by 2 stops.  My surface area is greater and that gives me an overexposed image.

To counter the extra surface area, I need to make one of the other two corners smaller by 2 stops.  I can make the shutter speed faster so less light enters the camera.  I can decrease the duration of the shutter 2 stops from 1/60 to 1/250.  The shutter speed arm gets 2 stops shorter and offsets the longer aperture arm.  The area of my triangle is now equal to that of my first triangle and I get a good picture.

Another option is shorten both the shutter speed and ISO arms by 1 stop.  Together, they add up to 2 stops shorter in the arms and this offsets the 2 stops longer aperture arm.  So, Shutter = 1/125 (1/60 - 1 stop) & ISO = 200 (400 - 1 stop) offset my increased aperture (from f/8 to f/4) and I get a good picture.

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