# Exposure Basics, lesson three?

Started Mar 18, 2013 | Discussions thread
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 Hantai. In reply to rrr_hhh, Mar 21, 2013

rrr_hhh wrote:

By the end of the course they would know that Light Exposure is the amount of light falling on the recording surface controlled by the Aperture and Shutter Speed and that ISO is a setting that controls how the camera processes that light into their image.

The exposure is the density of light falling on the sensor, which is all together different than the total amount of light falling on the sensor:

Total Light = Exposure x Effective Sensor Area

and different from the total amount of light used to create the photo:

Total Light Collected = Exposure x Effective Sensor Area x QE

where QE (Quantum Efficiency) is the proportion of light falling on the sensor that gets recorded.

For example, four times (two stops) more light falls on a FF sensor than an mFT sensor for a given exposure. A sensor with a QE of 50% records twice the light as a sensor with a QE of 25% for a given exposure.

It is the total amount of light used to make the photo, not the exposure itself, that is the relevant measure in terms of the IQ of the photo that has to do with exposure. In short, exposure is relevant only insofar as it is a component of the total amount of light that makes up the photo.

If we are working with a single camera, there is no need to make the distinction between exposure and total light, just as there is no need to make the distinction between mass and weight when in the same acceleration field.

However, if we are comparing different formats and/or sensors with different QEs, then the distinction is rather central.

And that's why techies should't try to teach beginners.

Probably best if beginners are taught by people who know and understand what they are doing.

If I may, you are really full of yourself, treating all others as ignorants or inferiors, which of course they aren't.

I'm thinking rather the opposite.  Bob seems to go to great efforts to teach people how things work.  The people who are "really full of yourself, treating all others as ignorants or inferiors" are the people "protecting" people from this knowledge that is "too complicated" and "unnecessary", and fight against things that are true, seeking to "teach" instead things that are not true, and lead to misunderstandings, but are "good enough" in many situations to pass (like saying the Sun goes around the Earth).

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