can i get doubled when i use Canon -Extender EF 2x III to my cannon 600d 55-250mm to 500mm
from the canon usa website:
This lens is only compatible with fixed focal length L-series lenses 135mm and over, as well as the EF 70-200/2.8L, EF 70-200/2.8L IS, EF 70-200/4L, and EF 100-400/4.5-5.6L. Additionally, please see the lens and/or camera body manual for full compatibility information with the Extender EF 2X III.
Also the third party TCs that fit between camera and lens (Kenko, Tamron, ...) cannot be used either. No one makes a TC compatible with the EF-S mount.
There are teleconverters that will screw onto the front threads (58mm diameter) of the EF-S 55-250. However, these are intended for the small sensors of compact digital cameras, so are likely to vignette. Additionally, they are a quite heavy attachement to the end of the lens, stressing the zoom and AF mechanism.
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from Portrait Lens (around 80mm or equivalent - please check the full rules)