I have both the Minolta white converters, x1.4 and x2. They are only compatible with the white lenses as the rear element sticks out too far for all other lenses. However they are rated the best TCs for these lenses. The x1.4 Is great and I seldom use my x2 as the quality impact, although quite small, bugs me.
Here is some information I copied from a website (years ago) that is no longer in existence. It covers all types of convertors.
Tele-convertors can be an excellent way to increase the range of a telephoto or zoom lens, at relatively little financial cost. There is however a price to be paid for using them. That price is a reduction in image quality. This is a guide that tries to help you to minimize that loss in IQ.
Tele-converters come mostly in 1.4x and 2x versions, with a few 1.5x, 1.7x and 3x. I will ignore the 3x TCs as they are not very practical. Any user of a TC or potential user of a TC will want to know how good or bad a particular TC is when used with a particular lens, Prior to making a purchasing decision. In other words, how much quality loss can one get by adding a particular TC to a lens.
The short answer is from 5-10% loss to 20-30% quality lost depending on the following factors:
1) OEM Matched or Dedicated TC (between 5-10%)
Minolta APO, KM APO or Sony APO or Sigma EX
2) Pro series TC (Kenko Pro 300, Pro 300 DG or Tamron SP) (5-15%)
Image quality will always be better than generic TCs
3) 1.4X = 5-10%, 2X = 10-20%
1.4X TC will always give better image quality than 2X
4) F stop being used (Wide open vs. Stop down)
Shooting stopped down will always yield better image quality than shooting wide open. This is especially critical for a lens that has better performance when stopped down.
5) MC7 (7 Elements) vs. MC4 (4 Elements)
7 Element TCs in most cases will give better image quality than 4 Element versions.
6) Prime vs. Zoom.
Using a prime lens will in most cases yield a better image quality than a zoom lens.
7) AF vs. MF
The TC and lens combination may not be able to AF if the lenses maximum aperture at it’s longest focal length is greater than F4.5 when using a 1.4x TC or F5.6 for a 2x TC.
In practice a 1.4x TC will add 1 stop to a lens. An f2.8 lens will become f4 and will AF. An f4 lens will become f5.6 and should AF. An f5.6 lens will become an f 8 lens and may not AF although it often does in good light. It will certainly hunt a lot and may not lock in poor light.
A 2x TC will add 2 stop to a lens. An f2.8 lens will become f5.6 and should AF. An f4 lens will become f8 and in theory should not AF although it often does in good light. It will certainly Hunt in poor light and may not lock focus. Much will depend on the quality of the lens.
The above factors or combination of factors will directly affect the image quality.