Very fast lenses are prone to an array of aberrations wide open - in this case ghosting.

Q: I’m using a very fast lens in an external application looking across water and we are experiencing unusually shaped blooming around lights near and far, as well as ghosts adjacent to distant lights. Is this down to the lens or is it the camera sensor?

THIS sounds like saggital coma, a lens aberration which causes light from off-axis to create a trailing comet tail away from the optical axis. This aberration is more common towards the edge of a lens – we’ve observed it with hemispheric cameras. You also seem to have some ghosting from internal reflections. Blooming is more consistent in shape around a point of light – it happens when a poorly designed sensor spills signal into adjacent pixels.

None of this is uncommon with a very fast lens wide open, even with a very slow shutter speed. It doesn’t mean the image isn’t sharp or worthwhile in areas of interest, so you’ll need to decide whether or not the enhanced images after dark you get with a fast lens are worth the price of sagittal coma around parts of the scene that may be superfluous to your operational requirements.

The simple fixes are worth trying first – get bright points of light out of the scene as far as possible by tweaking your angle of view – try panning, tilting and zooming – use shades or screens if you can. Stop down a couple of F-stops. Think about replacing the lens if you must.

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