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Blue spot in the middle of the FRAME (HELP!)


dwijip

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Guys need help, I shot this on the GH4 with a Mamiya 80 1.9, Cinelike D (with non speedbooster adapter) and ungraded. If you look at the middle of the frame there is a blue tint, I think I got this before with the GH4 with a 50mm 1.4 pentax on a cheap speedbooster off ebay. Anyone know what this is? 

 

vlcsnap-2015-02-13-12h41m04s31.png

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Looks like ambient edge flare.  If so, you might be able to reduce it when shooting by using an aggressive lens hood/matte box.

 

On the other hand, if you are using a medium format lens on a GH4 with a plain adapter, you might have all kinds of light bouncing around inside the GH4/adapter (especially with such a high-key image).  Is the adapter matte black on the inside?

 

I think that there have been reports on some of the focal reducers causing a similar problem.

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Yes I think adapting a medium format lens is the problem here, I definitely get it with a cheap focal reducer but I haven't seen this with any other lenses so I was worried that it was to do with the sensor.

With some more tests with other lenses and based other people's opinions, its not the sensor and its either the problem of adapting this particular lens with the GH4 or problem with the coating on the lens (which is old). The inside of the adapter I used it very matte black(fotodiox). Not many M645 to MFT adapter I could find other than this one. 

Anyway happy that its not the sensor(thank god) 

vlcsnap-2015-02-13-13h44m36s52.png

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Central hotspots are a ghost image of the aperture stop.  They are caused by a pair of reflections between two surfaces located between the aperture stop and the image plane.  Often this is a first reflection from the sensor followed by a reflection from one of the lens surfaces on the image-side of the aperture stop.  However, it can also be caused by pair of lens surface reflections.

Because its a ghost image of the aperture stop it gets smaller as you stop down.   If the ghost image is well focused and has low aberrations then it is possible to clearly make out the individual iris blades in the ghost image.  The surface brightness of the spot remains constant during stop-down, only the size changes.  But of course, as you stop down the overall image gets dimmer, so the relative brightness of the ghost image increases.  So, hotspots are much more problematic at small apertures because they are smaller and better-defined and also because they are bright relative to the image content.

Hotspots are typically blue because the anti-reflective coatings of the lens are less efficient in the blue portion of the spectrum.  Hotspots are also prominent in IR photography because coating performance is generally very poor in the IR.  Improving the lens coatings can minimize hotspotting, but can't eliminate it.  The best approach to avoiding hotspots is to design the lens in such a way that the pupil ghosts are very large and diffuse, and also to use the best possible lens coatings.

Hotspots were well known years before the Speed Booster first went on sale in 2013, and many lenses are well known to have hotspot problems.

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