Thursday, December 18, 2008

UV filters can summon ghosts

I recently got my hands on Canon's new 5D Mark II, and it's a beauty. However, when testing low light video recording with a Canon EF 50mm f/1.8 II lens, I noticed something interesting:

Note the "ghosted" image of the Christmas tree's lights as the camera pans right. This is caused by using a UV filter on the lens that isn't coated to reduce reflectivity.

Light is reflecting, presumably from the mirror/sensor, back at the inside of the UV filter. That light then bounces back at the sensor, creating the ghosted image.

Here's the same shot, but without the UV filter.

The ghosts are gone!

I suppose that this could happen with any lens, but that it's aggravated in this case by the fact that the outer lens surface on the 50mm is recessed, making it further away from the filter.

So what to do? The 1.8 is pretty fragile, and I don't like the idea of using it without some protection, be it from fingerprints, scratches or worse yet, a good crack.

B. Weiner on agrees, and recommends a good multi-coated UV filter. According to Mr. Weiner, "[m]ulticoating cuts down the reflection by a factor of greater than 10 relative to an uncoated filter."

So there you go. If you want to avoid ghosting when using a lens with a recessed outer lens element, you should spring an extra few dollars for a multi-coated UV filter.

As a side note, one point that I find interesting is that this normally would have taken me a very long time to notice, let alone correct. The reason that the ghosts are so apparent is because the camera shutter stays open while recording video. It is far less noticeable when taking still pictures, as the shutter doesn't need to stay open nearly as long, minimizing the effect of the reflections on the shot.

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