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I was doing some macro work at a magnification of about 4:1 in a metal El Nikor 105mm f/5.6 and a Baader U, and I was disappointed with the results. The images were definitely soft compared with the same subject photographed in visible. Although the El Nik 105mm is rated as good for UV, I had noticed there was a significant focus shift when using my tri-colour UV filters (315BP25, 345BP25, 380BP20). Interestingly, when changing from 380BP20 to 345BP35 you have to rack the lens in (i.e. move it towards the camera) whereas when changing to the 315BP25 you have to rack out (i.e. move the lens away from the camera). Perhaps the problem was that the Baader U lets pretty well the whole of the UVA range through and the El Nik couldn't focus across the range because of its focus shift. So I did comparison shots with the 380BP20 and 345BP25, refocussing for each filter. This is the full image: And here are the comparisons, looking at the comma: You can see that the Baader U is soft compared with the visible image. And that the 380BP20 image is sharper than the Baader U image. But what really surprised me is how soft the 345BP25 image is: I thought that perhaps I hadn't focussed carefully enough so went and repeated it, and got the same result. So looks like I better use the 380BP20 for macro work rather than the Baader U. So it appears that the El Nik 105 performs OK for macro at the long wavelength end of UV, but image quality drops off noticeably at shorter wavelengths. (I couldn't try the 315BP25 filter because I don't have a light source that will let me focus with this filter.) And so the softness when using the Baader U is probably down to a combination of focus shift and the poorer IQ of the El Nik at shorter wavelengths.