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Zeiss 15/2.8 Distagon Q&A — Vignetting

ZF.2 15mm f/2.8 Distagon T*

This is one of several pages resulting from the March 13, 2012 discussion with Senior Scientist Dr. Hubert Nasse. See the original list of questions.

These pages are a summary of the discussion based on notes taken, and as reviewed by Dr. Nasse. Bracketed comments [ ] are editorial in nature.

Vignetting is one aspect of the 15/2.8 Distagon design which is less than ideal; corner brightness drops to a rather dark 20% when shot wide open at f/2.8.

With subject matter containing already dark corners, the vignetting can result in a too-dark image in those areas.

Stopping down to f/4 helps significantly, and stopping down to f/5.6 eliminates optical vignetting except for a tiny trace amount in the extreme corners.

ZEISS 15mm f/2.8 Distagon relative illuminance @ f/2.8, f/5.6, f/8

The chart below shows the illuminance relative to optical center at f/2.8 across a 36 X 24mm frame. A steeper curve means more falloff to the corners; a flatter curve (see black curve for f/8) means more even illumination across the frame.

This graph shows the brightness in the field in absolute terms related to the brightness on axis with the fully open lens.

Compare the red and the blue curve; they tells us that the iris is not yet effective at the edge of the frame and in the corners when we stop down by one stop. The scales are linear in %.

ZEISS 15mm f/2.8 Distagon relative illuminance @ f/2.8, f/4, f/5.6, f/8

The same for the first 4 stops, but now on the more photographic logarithmic scale,
expressed in f-stops (= EV exposure value)

ZEISS 15mm f/2.8 Distagon absolute % illuminance @ f/2.8, f/4, f/5.6, f/8
Logarithmic scale, EV

Aperture f/4 (blue line) provides a useful intermediate setting short of the (nearly) full correction of f/5.6.

Relative brightness in the field for the first 4 stops. At f/5.6 the lens is nearly free from artificial vignetting. Above f/8 things will not change any more. Measured data.

ZEISS 15mm f/2.8 Distagon relative illuminance @ f/2.8, f/4, f/5.6, f/8
Logarithmic scale, EV

 


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