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10-25-2008, 02:35 PM   #1
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Does use of a TC "drop" the lens's projected image from the edge of the lens?

I'm not sure I know the answer to this. I think I do, but I thought I'd ask as I'm not positive of the answer.

What I know:
Even the best lenses perform better in the center of the projected image circle over the sensor than at the edge (some far better than others, yes). This difference is alway more evident wide open regardless of lens (you can see in my sig I do have one of the rare ones that's close to the same @ any aperture; no need to diverge the thread on this point ).

What I'm wondering:
So given how light enters a lens and that the light rays that project in line with the FOV of the lens project onto the sensor, when I put a TC on a lens, is it true that some center x% of the image circle (value of x% based on the magnification of the TC) is magnified to cover the sensor, thus pushing the reduced 'edge' performance of the image circle farther out and off the sensor?

And if I have a lens that has fantastic center performance but only "ok" edge performance when wide open, am I making better use of the superior center resolution of a lens when I have a TC is attached? For an example... So, if a lens's front element is 50mm diameter, and I use a 1.5x TC, is it that only the light that passes through the center 33.3mm of that front element making it to the sensor? (yes, I know a full frame lens's edge is already lost on a digital sensor; i.e. the cropping factor. only citing these numbers as a talking point example)

Thanx much for your informed replies,

10-25-2008, 05:23 PM   #2
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I would speculate yes but only if you have a theoretical/magical TC that has no impact on image quality itself and the magnified ratio doesn't go beyond the resolution limits of the lens. Finding this magical TC is a problem.

One way of thinking about it is say you wear glasses and your perscription is some value X. Unfortunately, your optometrist is all out of X lenses and you're in a rush so he stacks two Y lenses that are the equivalent of X. Do you think the additional glass/plastic would affect your view of the world (looking straight ahead)?

Again, wild speculations here but since nobody has replied .
10-25-2008, 05:53 PM   #3
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Original Poster
Yes, the loss through the add'l elements is a known & a definite. I have never seen a "TC test" though to know how adding a high quality TC affects the contrast/mtf tho. I guess in retrospect the fact that a 1.4x loses 1 stop and 2x loses two stops of light, it's moreso due to the lowered area of light through the lens making it to the sensor, than the extra glass the light is passing through. And with that, I'm answer my own question...?

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