Micro Four Thirds Focal Reducer Shootout

Started Feb 20, 2014 | Discussions thread
OP brian Senior Member • Posts: 1,195
Shootout Part 2: Hotspotting

In the second part of the focal reducer shootout I shot images of a back-illuminated soft box with a square patch of black gaffers tape taped to the front.  The lens used was the Sigma 18-35mm f/1.8 set to f/16.  The reason I used this lens is that its exit pupil varies substantially during zoom, ranging from 70.3mm at the 35mm focal length to 149.4mm at the 18mm focal length.  This is useful because hotspotting behavior is strongly dependent on the exit pupil distance.  Note that 149.4mm is an unusually long exit pupil distance, and most SLR lenses have an exit pupil distance less than 100mm.

Combined with the Sigma lens are 1) Vizelex Light Cannon; 2) Zhongyi Lens Turbo; 3) R.J. Focal Reducer; 4) Metabones Speed Booster; and 5) Sigma lens by itself with a plain adapter (no focal reducer).

In the case of the Lens Turbo and R.J. Focal Reducer the intermediate focal length position was chosen to show the worst instance of hotspotting.  This occurs when there is a sharp ghost image of the aperture stop.  For the m4/3 Speed Booster the worst hotspottiing occurs at the 18mm focal length position (exit pupil distance = 149.4mm).  Hotspots like this always occur in the middle of the frame because they are ghost images of the iris opening, which lies on the optical axis.

The Sigma 18-35mm lens by itself has very low ghosting, as shown by the two images in the bottom row.  Among the focal reducers, the Vizelex Light Cannon performs best because it has only a single optical element.  Unfortunately, this simple design also means that image sharpness is extremely poor, as can be seen in Part 1.

DISCLAIMER:  I'm the designer of the Speed Booster optics.

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Brian Caldwell

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