Well, I’ve spent a little time now playing with the Diopter filters on the Tamron 28-200m Ashperical Zoom lens, and I have to say that they definately have a place in the camera bag, but for my requirements the value of buying the set is very limited.
The set I purchased was admittedly, low quality, the entire set of four diopters cost less than £20, so the results may not be of the quality you would get from a more expensive set.
The optical quality is pretty much what you’d expect for the price, essentially a set of magnifying glasses for your camera lens.
The most useful seems to be the 1x diopter. It allows for a much closer minimum focus distance, without too much distortion.
The lens used has a standard minimum focusing distance of 210cms (6.9ft) @ 200mm. With the 1x dioper added, the minimum focus distance is reduced to 63cms (25 inches), with the maximum distance reduced to 104cms (41 inches). This gives you a total focus range at 200mm of 41cms (16 inches), so for close-up photography this is sufficient ( Just don’t forget to take it off the lens afterwards ).
The images were shot at f6 and 1/500 sec shutter speed, and you can see the depth of field is shallow even at this distance.
The image below shows the same composition , but from the lens without a diopter present.
This is the composition with the 4x diopter added to the lens at a distance of 40cm (15 inches), the minimum focus distance for this diopter @ 200mm.
You can see that even at this level of magnification, the majority of the image is very soft, and there is a rather interesting glow over most of the shot.
This image was taken at with the 1x diopter set at f11 and the depth of field is acceptable.
If all else fails, and a diopter set doesn’t quite do what you originally thought they would, then you could always take some images similar to these — http://bit.ly/pdxenT.