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Author Topic: Some Basic questions on a 2x TC.  (Read 707 times)

K-amps

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Some Basic questions on a 2x TC.
« on: May 10, 2012, 10:18:00 AM »
I have some basic questions on a TC. In this case lets say a 2x iii being used on the 100-400L.

 1.   Putting the 2x at 400mm bumps up the F from 5.6 to f11. I can see this affects the amount of light that comes in… but what is the effect on diffraction? Ergo if with the TC, I got sharp images at f8, do I now set the combo to f16 for same sharpness/ diffraction limitations?

2.   What is the effect on the “amount” of OOF blur? With or without the TC. So again if I use the lens at 400mm f5.6 for same framing (just using my feet to get same framing) vs. 400mm + 2x TC (and taking a few steps back) which scenario (for same framing) will give me less or more OOF blur?

Thanks!
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Some Basic questions on a 2x TC.
« on: May 10, 2012, 10:18:00 AM »

neuroanatomist

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Re: Some Basic questions on a 2x TC.
« Reply #1 on: May 10, 2012, 11:26:32 AM »
1) Diffraction will be technically unaffected - the effective aperture is reduced because light from the central portion of the lens' image circle is spread over a larger area.  But, the physical aperture (iris disphragm diameter) of the lens is unchanged, and that's what determines the amount of diffraction.  But...in practice, the effect of diffraction will be more evident for the same reason - the light is spread over a larger area.  So, in practice, you can think of the TC as affecting diffraction - in other words, if adding a 2x TC to an f/5.6 lens on a 7D, the resulting image will be softer due to the effects of diffraction since f/11 is well over the DLA for an 18 MP APS-C sensor.

2) DoF will deeper with the TC (for the same framing), because you're comparing f/5.6 vs. f/11 at the same framing.  If you didn't move, you'd have a shallower DoF with the TC due to the longer focal length (and a smaller FoV, obviously).
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K-amps

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Re: Some Basic questions on a 2x TC.
« Reply #2 on: May 10, 2012, 04:52:51 PM »
1) Diffraction will be technically unaffected - the effective aperture is reduced because light from the central portion of the lens' image circle is spread over a larger area.  But, the physical aperture (iris disphragm diameter) of the lens is unchanged, and that's what determines the amount of diffraction.  But...in practice, the effect of diffraction will be more evident for the same reason - the light is spread over a larger area.  So, in practice, you can think of the TC as affecting diffraction - in other words, if adding a 2x TC to an f/5.6 lens on a 7D, the resulting image will be softer due to the effects of diffraction since f/11 is well over the DLA for an 18 MP APS-C sensor.

2) DoF will deeper with the TC (for the same framing), because you're comparing f/5.6 vs. f/11 at the same framing.  If you didn't move, you'd have a shallower DoF with the TC due to the longer focal length (and a smaller FoV, obviously).

Thanks John, I was hoping you'd take a stab at the questions!

On #2: I misstated the question, the question should have been 400mm naked vs 200+2x (also 400). The thing is, the width of the aperture stays the same, i.e. this is what determines OOF blur, not the amount of light stopped by the added TC. (Lets assume the 100-400 is f5/6 natively at both 200mm and 400mm). What i think is, amount of OOF blur = same but light reduced by TC assuming both are effective 400mm focal length... IS this right?

Thanks again!
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neuroanatomist

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Re: Some Basic questions on a 2x TC.
« Reply #3 on: May 10, 2012, 07:22:22 PM »
A 200/2.8 + 2x becomes 400/5.6, and that will give the exact same framing, DoF, and exposure as a native 400mm f/5.6 lens.
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Re: Some Basic questions on a 2x TC.
« Reply #3 on: May 10, 2012, 07:22:22 PM »