@inkjunkie wrote:
Having been looking at the 1.4 II extender, wish it didn't take a full stop out as I will be doing a lot of lower light shooting.
Hmmm....
That's really just math. Also the choice of "1.4" has mathematical significance.
The focal ratio of any lens is simply the focal length of the lens (in millimeters) divided by the clear aperture (in millimeters).
So if you had a 100mm lens and it's widest possible aperture opening was 25mm then that would be an "f/4" lens because 100 ÷ 25 = 4.
If you then add a 1.4x tele-converter, it multiplies the focal length by 1.4x... so the 100mm lens effectively becomes a 140mm lens.
However... the 100mm still only has a 25mm aperture opening... the tele-converter can make the focal lenght longer, but it can't make the lens physically wider. So now when you divide 140mm (the new effective focal length) by the 25mm aperture opening (because that part didn't change) you get 140 ÷ 25 = 5.6.
Notice how the lens dropped from an f/4 to an f/5.6 (which is exactly one full f-stop) by adding a 1.4x extender. That only happens because the multiplier is 1.4.
It turns out 1.4 (the real value is 1.41421...) is the square root of 2. Whenever you change the size of a circle by the square root of 2 (either multiply the diameter by the square root of 2 to make it larger ... or divide the diameter by the square root of 2 to make it smaller) you will EXACTLY double or halve the area of that circle.
And since this particular circle is the aperture opening which allows the light to flow through the lens, you exactly double or halve the amount of light that can arrive at the sensor (assuming the shutter speed wasn't changed).
So the 1.4x multiplier was selected because it happens to be the square root of 2 (well... a rounded off value for the square root of 2) and it changes the exposure by exactly one full f-stop so you don't have to worry about fractional f-stop changes.
A 2x change to a circle's diameter will quadruple it's area (if making it bigger) or reduce it to one quarter (if making it smaller). So a 2x extender changes the camera's effective f-stop by 2 full stops.
Basically you muliply BOTH the focal length and the focal ratio by the multiplier of the tele-converter.
This is true of all teleconverters.
Tim Campbell
5D III, 5D IV, 60Da
