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| p.1 #12 · Can someone explain to me lens magnification? |
Please note that extension tubes may adversely affect imaging quality, and/or autofocus performance, depending on the particular lens and the amount of extension.
When using an extension tube, you are operating the lens in a regime in which it is not designed to operate. Aberrations may increase, especially in the image periphery. This is mitigated by stopping down the aperture, or using the least amount of extension that is required.
Autofocus performance may be compromised when extending lenses with a slow aperture, because increasing the back focus distance makes the angle of incoming rays smaller, thus making phase detection more difficult. The decrease in incoming light corresponds to the effective f-number of the lens + extension, which for a lens whose maximum aperture is already f/5.6, may easily become f/8 or slower at magnifications it was not designed to achieve natively. Consequently, the AF system may hunt excessively, or worse, refuse to lock focus entirely, moving the focusing group back and forth rapidly about the position of sharpest focus. This is also in part due to the AF system not "knowing" that you have inserted extra distance between the lens and the camera--it can't move the focusing group in sufficiently fine increments.
In light of this, extension tubes should be considered a secondary resort when investigating options for achieving higher subject magnification. The ideal choice is to use a lens that natively possesses the desired magnification at the required subject distances. If this cannot be done, a teleconverter is often preferable, depending on the application. For birding, a teleconverter is almost always the better choice over an extension tube.