Exposure triangle explanation please.

Started Jul 9, 2013 | Questions thread
Jeff Veteran Member • Posts: 5,087
Re: It's only an explanatory graphic ...

AnthonyL wrote:

Jeff wrote:

AnthonyL wrote:

and which part of your explanation did I say I did not understand?

Well, if you're comfortable with this explanation, then the triangle is obvious. Get yourself a piece of triangular graph paper -- label each corner with ISO, f-ratio, and shutter speed, and label each grid line moving away from the corner with a one-stop change in the corresponding value.

Constant exposure is maintained by moving along any grid line parallel to a side of the triangle.

That's the exposure triangle.

Did that answer your question?

The problem with that model is that it is very 2 dimensional. You can move along the ISO to Av points, or the Av to Tv points, but it doesn't show readily the three elements interacting. My model shows all three elements being pushed pulled, or if you hold the length of one element fixed, the other two must change.

With Auto ISO now becoming more prevalent a better visual model I think would be more helpful.

Let S be ISO, t be shutter duration, N be f-Number. We'll call image brightness B where

B = log2(N^2/t) - log2(S)

The first term is the definition of exposure value, the second describes the effect of ISO on image brightness. What we want is B = log2(L/k) where L is scene luminance and a meter calibration constant. Manufacturers using slightly different values of k. Taking a logarithm,

B = 2*log2(N) - log2(t) - log2(S)

This how these *four* elements interact, and why a triangular graph paper labeled by powers of two establishes an 'exposure triangle' for a given level of incident light.

If you're trying to incorporate the third dimension of incident light, then you really do have a three dimensional object and can't express it as a triangle in two dimensions.  You need an 'exposure pyramid'.

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