Originally posted by zapp After some thinking I have to disagree - in part. Diffraction kicks in at larger aperture numbers. With digital cameras diffraction often becomes visible at f/8 or even earlier. If f/8 is the sweet spot of your lens, a 2xTC will yield f/16, making diffraction effects more prominent. Therefore, I assume that Optimum aperture is not necessarily the same with or without TC. Diffraction has to be balanced with maximum performance, therefore opening up the aperture with TC installed can yield better results. It all depends on the lens, the camera sensor and the TC.
You need to test it.
Originally posted by Alex645 Counterpoint: Yes, the effective amount of light may become the equivalent of f/16, but the aperture is still f/8 on the lens. The TC does not convert the diffraction of the lens to what that lens would diffract at f/16. Any degradation of the image would be due to the quality of the TC itself. You'll see this if you compare an OEM TC with a third party equivalent.
Yes, completely agree with this. Theory is theory and is a helpful guide. But "the proof is in the pudding".
i think one needs to consider diffraction quite differently, Diffraction is always present, even wide open, diffraction is the bending of light around an edge, in this case the aperture. The issue with diffraction is that as you stop down the lens, the amount of light that is projected to the image which is distorted by difraction, increases as a percentage of the total light emitted from the lens, diffraction is much worse, on shorter lenses because the aperture is relatively small to begin with, and therefore plays a much more important role than on a long lens.
as a result i tend to discount it, for me there are a lot of other causes of lack of image sharpness that occur with very long lenses, especially due to vibration when on a tripod, shake when hand held, image movement due to low shutter speed, distortion due to air currents when shooting over long distances, and focus inaccuracy due to perhaps camera shake or movement to and away from the subject when hand held combined with narrow depth of field.
using long lenses, (beyond 300mm) requires a lot of work, the learning curve for 500mm+ lenses can be quite steep, and there are a lot of people out there who lack the patience and persistence to work outy all the issues. We should not get bent out of shape here discussing the theoretical optimum capabilities of a lens, before we tackle all the practical limits of its use. i for one, dont shoot MTF charts for a hobby