Sensor flare, tamron SP90

Please excuse the length of this post, but i am wondering if anyone is able to help.

I have an adaptall SP90 2.5, a lens that I love using. Perhaps I am sad, but even looking through this lens gives me great pleasure, never mind the images it is capable of capturing.
I purchased the lens predominantly for macro, but I am increasingly using it as a portrait lens and for candid shots. It is for this purpose that I am having problems with a well documented property of this lens...Sensor flare.

The rear element of the lens is flat and in certain higher contrast situations, light reflected from the surface of the sensor is reflected back onto the sensor and produces a blueish haze spot in the centre of the image. This spot is evident when the lens is stopped down. wide open it isn't there, but shows from about f8.
There is rarely (if ever) a problem in the macro range, but it becomes an annoyance at greater distances. The particular pain of the problem is that since it is sensor flare, rather than lens flare, it is never obvious throught the view finder that there will be an issue on the final image. Lens hoods, direction of light etc have little impact.

There are several ways to reduce or eliminate the problem. Using the 2 x teleconverter prevents the sensor flare as does using wider apetures. It also seems that only certain levels of contrast in an image will cause a problem, in particular if the centre of the image is darker with lighter areas towards the edge. Obviously a darker region in the centre of an image will show the flare more readily, but it is more than this. A property of the problem seems to be ( again well documented else where) that with a lighter or mid-tone centre to the image the flare is non existant rather than less noticeable.

Thats a poor summary of the problem, now for my question. The rear element is flat on the surface facing the sensor, but convex on the other face. What would happen if this element was turned around, ie surface 1 from the sensor was the convex face and surface 2 was the flat surface? Would this affect image quality? More specifically:-
1. Does switching the orientation of one element impact on the optical properties of the lens.
2. Would having the flat face lens as surface 2, reduce reflection back to the sensor and hence reduce sensor flare?

An obvious answer might be to try it and see, but I do not have a lens spanner or the will to try if it is pointless.
As far as I can see, appart from any change in basic optical properties that might occur, there are other reasons why this might not help.

A. Presumably the rear element is coated. Would a coating on surface 2 be a problem?
B. Suface 2 would still be flat, would it still reflect internally in the element and cause the same problem? ( I am aware that some have experienced sensor flare cuased by reflections from the flat face of a filter on certain lenses/conditions)

I appologise for the ramble, but are looking to you for your collective expertise and perhaps the experience of a lens tinkerer who may have played with a problem like this.
thanks in anticipation.
neville

I would imaging that reversing the Element would have a big impact on the optics. The convex shape is surely set in the correct place to bend the light to a given path, moving it would move the light path.

Would it help using the Tamron extension tube (it has no glass) rather than a 2x converter?

That is the drawback of that lens. I have only noticed it once when I was at f18 and even then it didn't show on all photos at f18. I mostly use the lens wider so had never noticed. The lens produces wonderful warm colours and a smooth look, I think it has real character and the 1:2 I find very useful for flowers and fungi. It's also a cracking portrait lens. So my advice would be to use it knowing its limitations.

Barrie, you share my first thoughts, but then I thought about spectacle lenses. In most ophthalmic glass the two faces are different, so could be regarded as having a right and wrong side. Yet in extremely rough and ready observations with my wife's specs, there appeared to be no noticeable difference which side I viewed from. I am fully aware of the flaws in my 'test', but it was this that set me wondering.

I do have the matched extension tube and there is no flare with this, but I never have problems with sensor flare when used in the macro range anyway. flare only presents a problem for portraiture and regular focusing distances.

Dr O. In the close focusing and macro range, flare only manifests at very small apertures and I tend to shoot wider than this too, so it is no problem. Here the flare normally only appears at apertures f16 or below. Yet at longer focusing distances I can get the flare at wider apertures and can be very noticeable by f8.
I echo all your comments about it's qualities and will continue to use it despite its limitations. However, 'knowing' this lens is my problem, as(like you report)flare doesn't show in all images even when lighting, contrast and other factors seem very similar.
For macro I feel I know the lens, but for portraits etc it seems less predictable unless I shoot f5.6 or wider.

thank you to both of you for your thoughts.

Have a look at Fig. 2.9 link