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Old Dec 8, 2010, 10:45 AM   #1
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Default Teleconverter question

I have a basic question about how teleconverters work. Imagine that I have a 1.4x tc and a lens that is soft at f/2.8, but begins being sharp at f/4. Further, imagine that it is diffraction-limited by about f/16.
My understanding is that, using the tc, the lens will be diffraction limited by f/11 (with the effective f-stop using the tc being f/16). That is, diffraction limiting is not a property of the lens' aperture as such, but of the overall effecive aperture. Is that correct?
Second, I really have no idea whether the softness of the lens is a matter of the actual aperture or the effective aperture. Wide open, would the degradation from the tc apply to the soft f/2.8 resoluton, or would it be a reduction of quality of the f/4 aperture? That is, might I get better sharpness from the tc wide open (at the expense of the lower effective aperture) than I would have wide open without the tc present?
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Old Dec 8, 2010, 12:15 PM   #2
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A teleconverter that sits between the camera and lens, is really nothing more than a diverging lens. It spreads the light from the main lens over a wider area. Since the sensor doesn't grow along with it, you are throwing away some of the light, which accounts for the reduction in f/stop, and giving you a cropped view of the lens' output circle, accounting for the seeming increase in f/l. (interesting aside: this should allow use of 'digital' lenses with full frame cameras) Think of a slide projector or overhead projector that schools use. As the projector is moved farther from the screen, the image gets larger, but also less sharp.
Bottom line is that a t/c will never increase the sharpness of a lens, and the fact of adding another optical element, adds to the softness.

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Old Dec 8, 2010, 5:35 PM   #3
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Thanks, Brian. I pretty much expected that a tc had to be a net loss on IQ under any circumstance, but I just couldn't come up with a mental model that worked at both the wide-open and closed down ends. Your image of an overhead projector fills the bill nicely.
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Old Dec 9, 2010, 8:49 AM   #4
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Quote:
Originally Posted by tclune View Post
... it is diffraction-limited by about f/16.
My understanding is that, using the tc, the lens will be diffraction limited by f/11 (with the effective f-stop using the tc being f/16). That is, diffraction limiting is not a property of the lens' aperture as such, but of the overall effecive aperture. Is that correct? ..
No. The diffraction limiting factor is the size of the aperture, not the ratio to focal length. If you want to think in terms of ratios, it is the ratio of the size of the aperture to the wavelength of the light that counts.
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Old Dec 10, 2010, 6:30 PM   #5
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Basic rule is TC were engineered for use with long primes. If you have glass that is soft or suffers in the IQ department those imperfections are only magnified.

I use both a 1.4 and a 2x and have even stacked them but only on supertele primes with great results. I cant tell any difference in IQ with the 1.4, the 2x does show some degradation but still very good results.
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