I've got an eos 550d and the Tamron 18-270 mm lens. Is there a speedlight flash that will work with this lens

SOURCE: Does the Tamron 18-270 f3.5-6.3 Di VC LD Lens fit

If it has the correct Canon mount then it will fit, but is still unsuitable.

The lens is designed for APS-C sized sensors, so the image it produces will not be big enough for the full-frame sensor on the EOS 5D. If you use it then there will be unacceptable vignetting in the corners of the images.

The angle of view of the lens will also be different to what you may expect: although it is an 18-270mm focal length it will have the same angle of view as a 28-420mm on a 35mm full frame equivalent camera.

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SOURCE: What is the focal length

It's not the lens but the camera that determines the focal length multiplier. On some cameras the 18-270mm lens is an 18-270mm. The XSi's imaging sensor applies a 1.6x multiplier on any lens used with the camera, so the 18-270mm will be approximately 29-430mm.

SOURCE: Hi I'm looking for a

Have you tried the Tamron website?