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tele-converter question


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#1 Eric Weindel

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Posted 12 February 2010 - 07:48 PM

I'm trying to settle a debate with a friend of mine.

He's convinced that using a teleconverter on a shorter fast prime gives you the equivalent DOF as that lenses F stop but with the teleconverters light loss as ND. So using a 50 f1.4 with a 2x teleconverter would be equal to a 100 f1.4 with an ND.09 in front.

I say its not a 100 f1.4 but actually wide open a 100 f4 since the light loss is from changing the aperture's relation to the rear/front element.

Can someone just say I'm right?
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#2 John Sprung

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Posted 12 February 2010 - 08:16 PM

OK, whether it's a 100mm lens or a 50mm with a teleconverter, what you have is some glass and a hole of adjustable diameter.

The f/stop is the focal length of the glass divided by the diameter of the hole. Big hole, little number. Little hole, big number.

When you combine the 50mm lens with the 2X converter, the iris hole stays the same size, but the focal length doubles. So, the f/stop number also doubles.

If the ring on the 50mm lens is set at f/1.4, with the 2X converter it gives you the same exposure and DOF as a 100mm at f/2.8.

Distance of the actual iris diaphragm from the front or back of the combined glass doesn't matter.






-- J.S.
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#3 Eric Weindel

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Posted 12 February 2010 - 08:49 PM

OK, whether it's a 100mm lens or a 50mm with a teleconverter, what you have is some glass and a hole of adjustable diameter.

The f/stop is the focal length of the glass divided by the diameter of the hole. Big hole, little number. Little hole, big number.

When you combine the 50mm lens with the 2X converter, the iris hole stays the same size, but the focal length doubles. So, the f/stop number also doubles.

If the ring on the 50mm lens is set at f/1.4, with the 2X converter it gives you the same exposure and DOF as a 100mm at f/2.8.

Distance of the actual iris diaphragm from the front or back of the combined glass doesn't matter.






-- J.S.



Ok so this is the equation to why I'm right.
But there's a weirdness here. Did a test with the teleconverter on and off shooting at a blank wall and taking a shot at each stop. According to this test the closest shot to the wide open "100mm" was the 50 at f4. Could this just be chalked up to element loss in the teleconverter... light transmission? So the F stop might be 2.8 but the T stop would be closer to f 4??? That's a pretty big jump usually a T stop is pretty close to the F stop right?

Thanks

-ERIC
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#4 John Sprung

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Posted 13 February 2010 - 02:01 AM

That is a lot to lose just in transmission. Test it again with, say f/5.6 on the 50's stop ring, which should equal f/11 thru the converter. If you still get the equivalent of three rather than two stops (equal to f/16), that would be interesting. If the issue is only at wide open, it could be the diameter of the converter rather than the iris in the 50mm lens is acting as the limit.



-- J.S.
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#5 Eric Weindel

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Posted 17 February 2010 - 01:17 PM

That is a lot to lose just in transmission. Test it again with, say f/5.6 on the 50's stop ring, which should equal f/11 thru the converter. If you still get the equivalent of three rather than two stops (equal to f/16), that would be interesting. If the issue is only at wide open, it could be the diameter of the converter rather than the iris in the 50mm lens is acting as the limit.



-- J.S.




Thanks again...

You were right. The element in the teleconverter is actually quite small, forcing the widest aperture to be probably just a tiny pinch wider than F4 on the 50mm.
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