Understanding Cheapo Diopter Set

[Class A]

I bought a cheap set of diopters and am a bit confused about their proper usage.

There are four elements: One is labeled "macro lens" and the others are "close up filters" labeled "+1", "+2" and "+4". I know what the "+x" diopters are supposed to do but what's the deal with the "macro lens"?

Here's an image of the set: images.trademe.co.nz/photoserver/tq/44/52180744.jpg

Can I use the "+x" diopters on their own?

I did a brief test and in particular when the "macro lens" is part of the stack then focus isn't in a plane at all. It not only degrades heavily towards the corners but also seems to be non-uniform / non-planar in the centre. It looks like there is a ring around the centre that is in a different focal plane then the rest (inner centre and corners). A bit like a doughnut-shaped focus plane. Am I'm seeing bad diopter optics or is the set not interacting well with my main lens (Tamron 18-250mm)?

I'm considering returning the set since the "macro lens" is damaged at the rim and misses three chips of glass at the perimeter. May not hurt image quality when using a high focal length(?) but the set was bought "new" and should have been defect free. The filters are of the single glass design and appear to be of low quality. I'm not too hopeful that these will allow me to do quality shots but they may be good for the odd gimmicky shot...

Any advice would be greatly appreciated.

[another view]

Am I'm seeing bad diopter optics

I'm afraid that's what it is. I have used a good one and a cheap one, and the cheap ones will be really bad around the edges. This actually might not be the end of the world because of a very small DOF with macro, but you'd certainly see it with a "brick wall test" - not that I'm a fan of those. However, the center should be sharp. I'd vote for returning the set.

Other fairly inexpensive Macro tricks are reversing rings (so you put the lens on backwards, something like a 50mm works great for this) or extension tubes. Neither of these have any optics in them, so there's nothing to degrade the image.