Are there any downsides to use a 2x extender?

Question

What are the downsides?

Answer

In general this is what you can expect for using a 2x extender:

1. Lower max aperture — which may cause your camera's AF system to stop working, or at least perform worse.
2. Darker view finder
3. Loss of sharpness, especially at the corners
4. Loss of color contrast
5. Increase in color fringing and chromatic aberrations.

The effects of the optical degradation will depend on the lenses you pair the 2x with. Also, some Canon lenses aren't directly compatible with the 2x TC, though there are "mods" that will allow it to work with many lenses.

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