by ʇolɐǝz ǝɥʇ qoq

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This is on my 600D. I'm a noob so forgive if it's obvious. There's a clear moire-like interference pattern on the viewfinder on the 50mm which is not as obvious at all on the kit lens. It's especially obvious when looking at out-of-focus bokeh but doesn' show up in the photos.

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up vote 7 down vote accepted

It sounds like you're observing the laser etched glass focussing screen. A focusing screen requires a rough surface so an image can be formed for you to view through the viewfinder.

Ground glass used to be used for this purpose but now glass etched in a circular pattern with a laser is used, to improve the brightness of the viewfinder when used with slower lenses. This pattern is probably what you're interpreting as moire.

Here's what a standard DSLR focussing screen looks like close up:

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I'll add that the reason Mel is seeing it with this lens is that being a 1.8 its letting in a LOT more light than a kit lens, making the viewfinder much brighter and making the effect much more pronounced and noticeable. (I have seen a similar effect with the 50mm 1.4 on my D800) – Digital Lightcraft Oct 23 '13 at 16:15
Thanks. I thought it was something like this. I just don't get why it becomes so much more apparent when the lens is out of focus. – mel Oct 23 '13 at 16:22
its probably because of the DETAIL that is in-focus makes it less apparent. – Digital Lightcraft Oct 23 '13 at 16:23
The glass most likely isn't laser-etched, as laser polishing is an extremely new technology being researched at the Fraunhofer instutite in Germany. Rapid local heating a la a laser tends to cause glass to explode. – Brandon Dube Mar 7 at 0:16

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