How does the numbers in a TC work?


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dRebelXT

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May 14, 2005
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#1
I was confused with teleconverters.
Let's say with a 1.4X TC attached to my lens, the same subject will be magnified by
1.4 times(?). The loss of light will be reduced by 1.4 stops, for example from F/2.8 to
somewhere arround F/5 (between F/4 & F/5.6). If it is a fictitious 1X TC, it will not
magnify my subject, but it will change the light admission of the aperture from F/2.8
to F/4.0, while DOF remain unchanged?

Is my understanding correct?
:confused:
 

Jan 23, 2005
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#3
dRebelXT said:
I was confused with teleconverters.
Let's say with a 1.4X TC attached to my lens, the same subject will be magnified by
1.4 times(?). The loss of light will be reduced by 1.4 stops, for example from F/2.8 to
somewhere arround F/5 (between F/4 & F/5.6). If it is a fictitious 1X TC, it will not
magnify my subject, but it will change the light admission of the aperture from F/2.8
to F/4.0, while DOF remain unchanged?

Is my understanding correct?
:confused:
The reduced intensity has nothing to do with "loss" of light. Rather, an x-times teleconverter enlarges lengths in the picture x-fold, and areas x^2-fold. The light captured by the lens therefore gets diluted over an x^2-fold area, so the intensity goes down by a factor of 1/x^2. A 1.4-times teleconverter makes the image one f-stop darker (since 1.4 is approximately sqrt(2)), a 2-times teleconverter reduces the intensity by a factor of 4 (or two f-stops). A 1-time teleconverter will not change the intensity.

Of course there are additional losses (absorption/reflection), but those have a much weaker effect.
 

dRebelXT

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LittleWolf said:
The reduced intensity has nothing to do with "loss" of light. Rather, an x-times teleconverter enlarges lengths in the picture x-fold, and areas x^2-fold. The light captured by the lens therefore gets diluted over an x^2-fold area, so the intensity goes down by a factor of 1/x^2. A 1.4-times teleconverter makes the image one f-stop darker (since 1.4 is approximately sqrt(2)), a 2-times teleconverter reduces the intensity by a factor of 4 (or two f-stops). A 1-time teleconverter will not change the intensity.

Of course there are additional losses (absorption/reflection), but those have a much weaker effect.
I see, Thank you for the explanation.
It's good to say the light intensity remain unchanged (reflection etc negligible), the TC changes
the area which emits light thus the F stops. Will a TC change the DOF also?
 

espn

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#5
dRebelXT said:
I see, Thank you for the explanation.
It's good to say the light intensity remain unchanged (reflection etc negligible), the TC changes
the area which emits light thus the F stops. Will a TC change the DOF also?
The TC reduces the aperture, which definitely will change DOF.

1.4X reduces by 1 stop
1.7X reduces by 1.5 stops
2X reduces by 2 stops.
 

Jan 23, 2005
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#6
dRebelXT said:
It's good to say the light intensity remain unchanged (reflection etc negligible),
To say it really correctly, one has to distinguish technical terms (I'm also guilty of being sloppy at times). What remains the same is the light flux (equivalent to power = energy/time). The intensity is the flux per unit area, so the intensity is reduced as the area gets larger.

the TC changes
the area which emits light thus the F stops.
The aperture that captures the light stays the same. E.g. f/2 (where f=50mm) is the same aperture as f/4 (where f=100mm), since 50mm/2=100mm/4=25mm (which would be the diameter of the aperture). (Outside of optics, e.g. for radio antennas, apertures are frequently given by their area, which makes more sense from a physical point of view.)

However, most people don't understand what the little f means, and it has become common practice to refer to say "1/2.8" instead of "f/2.8", or even worse, just "2.8". The "aperture number" changes - but not because the aperture is any different, but because the focal length is different.

The depth of field does change when using a teleconverter. I haven't done the math, but off-hand I'd guess that using a teleconverter is equivalent to cropping and enlarging the picture. The depth of field is determined by what amount of blurriness (circle of confusion) is "acceptable". Since the bluriness gets enlarged along with the picture, it hits the "acceptable" limit sooner, and the DOF is reduced.
 

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