Canon 1.4x-A extender
Construction: 3 groups 4 elements
I always find my lenses not long enough, but I am also always not rich enough to buy another lens. If you are like me, an extender is your ideal solution.
An extender extends the focal length of a lens by its factor while keeping its closest focusing distance. In plain language, it makes your lens longer. But it does this by trading off the amount of light reaching the camera (darker) and degrading the image quality (more glasses). For example, if one puts a 2x extender on a 200mm f4 lens, it would then become a 400mm lens but lose 2-f/stops of light throughout the aperture range.
The resulting image quality of an extender depends on how well it matches with the host lens. That is why Canon made three different FD extenders - FD 2x-A, FD 2x-B, and FD 1.4x-A.
FD 2x-A is designed for lenses (including zoom lens) of 300mm focal length or longer. It trades off 2-f/stops of light.
FD 2x-B is for lenses (including zoom lens) of less than 300mm. It trades off 2-f/stops of light.
FD 1.4x-A is for lenses of 300mm or longer. It trades off 1-f/stops of light.
There is only a very small difference in size without and with the 1.4x extender added on the FD 300mm 2.8L.
I got the FD 1.4x extender exclusively for my FD 300mm f2.8 L lens (Review here). And they pair up very well together. For the price of the extender, I now have another FD 420mm f4 (L) lens. And hey, I could get even more reach by putting them on a APS-C camera! The image quality degrade is almost unnoticeable, and the extra reach of 120mm costs only 1 f-stop of light. The image quality difference with and without the 1.4x extender is so insignificant that I had a hard time telling which photos were taken with 1.4x extender. It is a really good deal.
300mm + 1.4x at infinity. Although this is an edited, sharpened (not much) and cropped image, I include it to show the possibility of the combination.
300mm + 1.4x at close distance. Everything is still so smooth and sharp.
300mm + 1.4x at harsh situation. Color fringing becomes noticeable, but still very acceptable.
I haven't had the chance to try out the 2x extenders yet. So I cannot tell how well they perform. Again, it depends on how well the host lens matches with the extender. Even if the 1.4x-A performs well here, doesn't necessarily mean that it would do the same with lenses.
But if you have a FD 300mm f2.8 L lens, I highly recommend the 1.4x-A Extender.