Weird issue w/Ext. tube + M42 lens

Started Apr 20, 2012 | Discussions thread
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Re: Weird issue w/Ext. tube + M42 lens
In reply to Martin Ocando, Apr 29, 2012

I think, as others have said, this is a sensor reflection.

I suspect the reflection is always there, but is relatively weak, so is swamped by the level of light being captured when the aperture is wide open. As the aperture closes, the effect of the reflection becomes more evident, as the overall light level drops; or perhaps stopping down makes the reflection more focused.

The reason this doesn't occur without extension tubes is that the rear element of the lens has a curved reflective surface - at some distances this has the effect of focusing the reflection back onto the sensor, making the flare more evident.

Is the rear element flat (or nearly flat) or concave?

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