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

[sajeev_chacko]

hi,

 

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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.

 

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thnx

[jimstrutz]

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.

 

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It's all a compromise. The higher the degree of TC magnification,

the greater the aperture loss and the greater the image degradation.

[jeff_nakayama]

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?

[isaac sibson]

The relationship between magnification and light loss is not linear.

 

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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.

[jeff_nakayama]

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?

[geoff_doane]

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

 

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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%.

[pixseal]

Jeff, a 1.5X extender on a 2.8 lens would give you a maximum aperture

of 4.2

<UL>

<LI>200mm/f2.8 = 71.43mm aperture</LI>

<LI>300mm/71.43 = f4.2</LI>

</UL>

Ideally, a 1.4x extender would actually be a 1.4142135623731x

[sajeev_chacko]

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.

 

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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.

 

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

increment.

 

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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)

 

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and when one speaks of a 10% error, talking about number with the

accuracy of second decimals place is BULLSHIT.