Exposure Basics, lesson three?

Started Mar 18, 2013 | Discussions thread
Great Bustard
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Explaining "aperture"
In reply to olliess, Mar 21, 2013

olliess wrote:

Great Bustard wrote:

So, I take it that your opinion is that saying that the total light collected is the product of the exposure, effective sensor area, and the QE is too much for a beginner.

It seems to me you could just take out the word "quantum" if that's the tripping point.

They could just say "QE" if the word "quantum" is holding them back. 

The reason for the "quantum" in "quantum efficiency" is because read noise is the other player in the sensor efficiency.

How would you teach "stops"? Explain to me, if you would, why f-ratios, in one stop increments, go: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. Would you just have the class memorize these numbers or explain the numbers to them? Or would you just tell them not to worry about the numbers all together, and just move the dial left or right?

Personally, I'd tell them something like this: The "aperture" refers to the size of the circle that passes light int the lens. You can control the aperture with an iris, like the one in your eye. Technically, the f number is the just focal length of the lens divided by the diameter of the hole, but the main thing to understand is that is that a circle twice as big across lets in four times the light. So, you know that f/8 is twice as wide across as f/16 (it's a fraction, remember), and it lets in four times as much light.

Traditionally, the apertures are marked at "stops" which each let in half as much, 1/4 as much, an 1/8 as much, etc. So they are numbered like 1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, ... they may sound like a strange sequence of numbers, but you'll get used to them soon enough. (You can alway remind yourself what the next one is by noticing that the square of each f-stop is twice as big as the square of the previous one).

The stops work together with the traditional shutter speeds, which are numbered exactly the same way, so 1/30s, 1/60, 1/125, 1/250, 1/500 (sometimes marked 30, 60, 125, 250, 500, etc.) are each twice as fast (half as long) as the last one.

And then teach the sunny/16 rule and equivalence of exposure in the traditional way, referring back to the fact that you can click-click-click with the knobs 1 stop at a time to keep the equivalence.

Would that be too confusing?

Myself, I would explain the three apertures (physical aperture, virtual aperture, and relative aperture) and their connection to the amount of light that passes through the lens onto the sensor.

Then I'd explain how increasing the aperture diameter by a factor of sqrt 2 (~1.4) results in doubling the aperture area, which will double the amount of light falling on the sensor for a given shutter speed.

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