Rokinon Tilt-Shift 24mm F3.5

Vignetting

In simple terms, vignetting is the darkening of the corners of an image that occurs at wider apertures. Every lens, wide open, will exhibit some. Most of the time, it need not be a problem for modern photographers. First, it is often possible to shoot at smaller apertures, mitigating the effect. Second, cameras released in the last few years often have built-in tools to remove vignetting in a way that's invisible to the user. Advanced software also often incorporates lens profiles which can seamlessly correct vignetting. Last, it is always possible to purchase higher-quality lenses that will show milder vignetting.

Sadly, users of tilt-shift cannot benefit from all this (except closing down the lens). There are simply too many variables, too many possible configurations involved, to be able to build lens profiles to correct for vignetting. In addition, since tilt-shift lenses are rare, the option to purchase a better product rarely exists. Manual correction of vignetting via software is still possible, but must be managed on a case-by-case basis.

This section presents how the Rokinon 24mm tilt-shift performs in regards to vignetting.

Vignetting Test

The test was performed by pointing the lens at a blank wall. Since there is no in-camera corrections, we shot each image in JPEG. Files were scaled down, converted to grayscale for improved visibility, then exported. We tested the lens in its straight position, fully shifted down, and fully tilted up. The following settings were used:

  • Tripod
  • 2s Timer
  • Focal Length: 24mm
  • Camera: K-1
  • Camera Mode: Manual mode
  • Shutter Speed: Set relative to aperture
  • ISO: 100

APS-C cameras will benefit from a sweet spot when compared to full frame results.

The following images show what to expect from the lens, wide open, for straight and displaced positions, at F3.5.

Straight position

Lens straight F3.5

Shifted Down

Lens shifted down F3.5

Tilted up

Lens tilted up F3.5

The following chart illustrates the darkening in the corners in terms of the exposure difference relative to the center, for each of the movements, as well as the straight position.

Vignetting

The chart and the sample images clearly illustrate the effect of each movement on the lens' performances. Even in its regular position, the lens does show significant vignetting. Closing down the aperture only partially mitigates the effect, with vignetting never getting much lower than 0.8 EV.

Tilting the lens is only marginally worse. This can easily be explained: except on the front element, light rays pass through approximately the same path, whether the lens is tilted or straight. The center of the lens is where most of the light will travel.

Shifting the lens, however, is immediately visible. Shift lenses need to have a larger diameter than regular lenses, to accommodate the possible movements along each axis. This means that light rays will pass through the extreme borders of the optical elements, showing stronger vignetting with an asymmetrical shape.

Verdict

Vignetting will be visible at all apertures. This was to be expected for the displaced position, however the amount of vignetting is surprising and disappointing for the straight orientation. A simple slider adjustment in most photo editing programs will take care of vignetting in the straight and tilted positions, but the asymmetrical nature of the shifted vignetting means that more care will be needed to correct it.


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