Tele-Convertor: why 1.4x and not 1.5x or 1.2x

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hi,

i wonder why it there is a 1.4x TC and not 1.2x or 1.5x. is it because this is the maximum conversion possible with apperture stop down by 1 stop. and if there is any 1.5x TC, it will give stop the apperture by 2. then in this case, instead of something like 2x TC there should be bigger TC, may be 2.2x or 2.4x. anybody knows that reason.

thnx

-- sajeev (chack74@yahoo.co.in), January 26, 2002

Answers

With a 1.4X TC the aperture is reduced by 1 stop. They frequently use 1.4X because it's convenient to do 1 stop aperture calculations. A 2X TC will reduce the aperture by 2 stops. It's also frequently used for the same reason. But I have a 1.5X TC that reduces the effective aperture by 1.2 stops, and I have seen 1.7X TC's that reduce the effective aperture by 1.5 stops. And there are 3X TC's for approximately a 3 stop loss as well.

It's all a compromise. The higher the degree of TC magnification, the greater the aperture loss and the greater the image degradation.

-- Jim Strutz (j.strutz@gci.net), January 26, 2002.


technically, on an f/2.8 lens, wouldn't a 1.4x TC give you an f/3.92 max apeture? when then, would canon say the 1.4x TC gives you 1 stop less? wouldn't a 1.5x on an f/2.8 lens give you a simple f/4 max apeture? why 1.4x? why not 1.5?

-- Jeff Nakayama (moonduck22@hotmail.com), January 26, 2002.

The relationship between magnification and light loss is not linear.

A 2X TC looses you 2 stops. In order to loose 1 stop, you don't divide by two (loosing one stop on a 1X TC would be pretty pointless), but you take the square (ie ^2) root. Thus, 1.4X. If you were to stack two 1.4X TCs, you'd loose 2 stops, and you'd get 1.4x1.4 increase in magnification. 1.4 x 1.4 = 1.96 =~2. The relationship between the magnification factor of a TC and the loss of light is a quadratic equation. 1.4 and 2 are typical lengths because they give you integer losses of light, ie 1 and 2, whole numbers being easier to deal with.

-- Isaac Sibson (isibson@hotmail.com), January 27, 2002.


you know how the max apeture is a ratio of the focal length to the diameter of the diaphragm? according to that model, if you double the focal length for a given lens, the max apeture would be two stops less. when you put on a 1.4x TC, say on a 200mm f/2.8, the focal length would turn out to be 280mm. you can derive the diameter of the diaphragm by dividing 200mm by 2.8. which is about 71.4285714285714285714285714285714mm (200/71.4285714285714285714285714285714= 2.8). so on the 200mm f/2.8 with 1.4x TC to find the max apeture you would divide 280mm (new focal length) by 71.4285714285714285714285714285714mm (diameter of diaphragm), and get a new max apeture of 3.92. if you had just put on a 1.5x TC the new max apeture would have been a simple 4. i still don't understand why 1.4x?

-- Jeff Nakayama (moonduck22@hotmail.com), January 27, 2002.

1.4 is just the square root of two, rounded off. (1.414 ~ 1.4)

Focal lengths and aperture values typically get rounded off to "convenient" figures. Add people love that. That way a 30-180mm lens can be sold as a 28-200, and still be within 10%.

-- Geoff Doane (geoff_doane@cbc.ca), January 28, 2002.



Jeff, a 1.5X extender on a 2.8 lens would give you a maximum aperture of 4.2 Ideally, a 1.4x extender would actually be a 1.4142135623731x

-- Dave Herzstein (dherzstein@juno.com), January 31, 2002.

but dave, in that case the camera with just half or full stop apperture increment should shows 4.0 which is closer to 4.2 rather than 4.5. this means a drop of 1 and half stop and not 1 stop with 1.5x. this is fine according to what Geoff says that a 10% is acceptable. and i guess 0.2 is just 5% of 4.0.

also, i heard there are 3x T/C. for a f2.8 lens what is the drop in apperture. i guess it is f8.4, but with a camera with half stop increment, it would show just 8.0 because the next one is 9.5 which is quite far.

in short this is the case for a camera with half (or one) stop increment.

f2.8 + 1.4X = f3.92 ( approx f4.0 = 5 % less than 4.2) f2.8 + 1.5X = f4.2 ( approx f4.0 ) f2.8 + 3.0X = f8.4 ( approx f8.0 = 5 % less than 8.4)

and when one speaks of a 10% error, talking about number with the accuracy of second decimals place is BULLSHIT.

-- sajeev (chack74@yahoo.co.in), February 01, 2002.


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