Exposure Basics, lesson three?

Started Mar 18, 2013 | Discussions thread

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.

Alternatively, it's why beginners often stay beginners, more often than not, at least so far as technical understanding goes. They start out learning something wrong that seemed to make sense, and leads to comments such as "Total Light = Total BS", "f/2 = f/2 = f/2", "larger pixels means less noise", etc., etc., etc., that, try as one might, they cannot let go of the dumbed down incorrect version they "learned".

Do you honestly believe that the concept of exposure as the density of light falling on the sensor is too hard for a beginner to grasp? Do you honestly believe that the concept that the more light a photo is made from the less noisy it will be is too hard for a beginner to grasp? Do you honestly believe that the idea of sensor efficiency, in terms of the proportion of light falling on it that is recorded, and the additional noise added by the sensor and supporting hardware, is a concept too difficult for a beginner to grasp?

Myself, I tend to think that the correct understanding is the more simple understanding:

• Exposure is the density of light falling on the sensor.
• Wider aperture and/or longer shutter speed means more exposure.
• Greater exposure, larger sensor, and/or more efficient sensor means more light recorded.
• More light recorded means less noise.
• Sensors can absorb only so much light, beyond which you will get blown highlights.
• Camera's ISO setting adjusts the brightness of the LCD playback and/or OOC jpg, as well as influencing the camera's choice of f-ratio, shutter speed, and/or flash power depending on the AE (auto exposure) mode you are using.
• Image files have a limited bit-depth, so increasing the ISO may push portions of the photo outside the range of the image file, resulting in blown highlights.

Is it really that confusing?

And in what will this help a beginners who is complainng that his action shots are blurred ?

The shutter speed is not fast enough to freeze motion.

That his night shots are so grainy/noisy ?

There is not enough light falling on the sensor.

That he can't isolate his wife against he background ?

The aperture is too narrow and/or the subject-camera distance is too great.

Or that he isn't able to get the whole rose sharp from the front petal to the back ?

The aperture is too wide, and/or the subject-camera distance is too close.

BTW with respect to other post you wrote : one doesn't teach photography like one teach programming. For programming you use a language, whole for photography you use a tool. The second doesn't imply the same level of abstraction, so you can proceed in a more pragmatically way.

I'm a beginner.  Tell me about aperture.

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