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05-09-2010, 09:50 PM   #1
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TC DoF question

Does a TC truly convert the lens to 1.4x or 2x focal length, or does it just magnify it?

When standing in the same spot, a 280mm lens at f4 will have a less DoF than 200mm at f2.8, and a 400mm at f5.6 even moreso.

05-10-2010, 12:49 AM   #2
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QuoteOriginally posted by Eruditass Quote
Does a TC truly convert the lens to 1.4x or 2x focal length, or does it just magnify it?
What's the difference?

Focal length is irrelevant for DoF.
DoF depends on aperture and reproduction scale.
Since you magnify the image twice with a 2x TC, you have half the DoF.
You can check this in a DoF calculator:
without TC: 200mm f2.8
with 2x TC: 400mm f5.6 --> gives half the DoF.
05-10-2010, 01:22 AM   #3
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QuoteOriginally posted by blende8 Quote
What's the difference?

Focal length is irrelevant for DoF.
DoF depends on aperture and reproduction scale.
Since you magnify the image twice with a 2x TC, you have half the DoF.
You can check this in a DoF calculator:
without TC: 200mm f2.8
with 2x TC: 400mm f5.6 --> gives half the DoF.
Interesting, never heard it explained in that fashion.

DoF calculators don't agree exactly, but are pretty close at those focal lengths and apertures.

DoF calculators completely disagree at 10mm f2.8 to 20mm f5.6.
05-21-2010, 05:41 PM   #4
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Any other opinions?

05-21-2010, 05:57 PM   #5
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Mine is to say that while DOF might not scale linearly with focal length, that's neither here nor there. It's still the case that whether you achieve a given magnification via a lens of a certain focal length plus a TC or via a lens of a longer focal length, as long as you are achieving the same magnification, DOF is going to be the same according to any logic any I think of.
05-22-2010, 01:14 PM   #6
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QuoteOriginally posted by Marc Sabatella Quote
It's still the case that whether you achieve a given magnification via a lens of a certain focal length plus a TC or via a lens of a longer focal length, as long as you are achieving the same magnification, DOF is going to be the same according to any logic any I think of.
Yes, that's true. It is of no importance whether you arrive at a focal length with a naked lens or whether you add a tc.

You should not forget, that in modern tele lenses, the rear element is usually a tc - just as part of the lens and then called the tele-negative element. This integral tc explains the short built of these lenses and is basically really nothing els, but a shorter lens combined with an external tc.

Ben
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