Why you shouldn't use cheap UV lens filters

I've read about the occurrence of weird flare appearing in photos when you have a cheap UV filter on your  lens, but this is the first time I'm able to produce it.

First, a list of equipment used:

• Nikon D300 digital SLR camera
• My brother-in-law's Tamron 90mm f/2.8 macro lens
• Cheap, no-name UV filter on the macro lens

So, I'm indoors with my back facing the doorway.

There's a bright source of light, which is actually strong mid-day sunlight reflecting off the walls of a neighboring house.

I attempt to shoot a reflection of myself in the mirror.

Here's what I get.

Looking at the flare a little closer now, it might be caused by the reflection of the bright spot in the mirror on the filter.

I now wonder if the same problem would occur with more expensive filters.

Anyway, I removed the filter, and re-shot the photo, and no matter how hard I tried, I couldn't get any flare in the photo.

None of my lenses have a filter on them, a decision which was influenced by Thom Hogan's article on filters.

In a recent post, Thom points out the following:

Nikon used to use flat glass on the front of their exotic telephotos for protective reasons, the same as people use UV and other filters for. They've moved to curved protective glass and nano coating the back side of the prime elements for a reason.