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Vignetting & Light Falloff

Vignetting and light falloff aren't something we would normally test in our digital SLR reviews, primarily because cropped sensor digital SLR's and don't exhibit much falloff. With full frame sensors, however, there's a far more real risk of corner shadowing (especially with wider lenses), though it's unusual for it to be an issue in everyday photography.

Technically vignetting refers to a darkening of the corners of the frame due to a physical obstruction such as the rim of the lens barrel or a filter, light falloff refers to a reduction in the amount of light reaching the far corners of the frame due to the angle of incidence of the light reaching there. Light falloff is sometimes referred to as cos4 vignetting. In this section of the review we will refer to this effect as falloff for simplicity (and more likely accuracy) however it could well be either or both vignetting / light falloff.

Testing


Measurement Areas
We aim the camera at a white wall (about 0.5 m away) which is evenly lit by two soft boxes (producing about 10 EV across the entire wall), and a heavy diffuser placed over the front of the lens. A sequence of shots are now taken at every aperture from maximum to F11 with a variety of lenses.

These images are then processed by our own analysis software which derives an average luminance (Lum) for the four corners of the frame (5% each) as well as the center (10%), the corners are averaged and the difference between this and the center of the frame is recorded. This value can then be plotted (see graphs below) as a representation of the approximate amount of falloff.

Hence falloff of -30% would mean that if the luminance center of the frame was at exactly 100% (pure white) the average luminance of the corners would be 70%. Anything more than -20% may well be visible in everyday shots, although this depends on the framing of the shot and the exposure.

Range of falloff

The chart below demonstrates the difference that these figures above can make, we took the blank wall luminance value of 75 (about 190,190,190 RGB) as our normal level. Remember that these patches are solid and the actual effect of shading is a softer gradual roll-off which would never be so obvious. The thumbnails are created by breaking the fall off into the same bands for clarity, so the same comment applies to them.

Fall off results

As you can see, with wide zoom lenses on the Alpha 900 you can expect fairly strong corner shading when shooting wide open, though how much varies considerably from lens to lens. We found with this particular lens (the 24-70mm ZA) that the amount of fall-off at the wide end remains fairly constant throughout the aperture range, and overall the amount of fall-off is slightly higher than the equivalent Canon body/lens combo.

Although you can see fall off in some shots with some lenses, whether or not it's seen as a 'problem' (one that's common to all larger format sensors) will vary from user to user. It's easy to remove (the new version of Sony's raw coverter has a corner shading removal option), and in most situations simply won't be an issue.

Lenses: Sony 24-70mm F2.8 ZA and Canon 24-70mm F2.8 L EF

Sony 24-70mm F2.8 ZA fall off thumbnails

 
@ 24mm
@ 70mm
F2.8
F3.5
F4.5
F5.6
F8.0
F14
F22

Sony 50mm F1.4 Fall off thumbnails (F1.4-F2.5)

F1.4 F1.7
F2.0 F2.8
F3.5 F4.0

Compared to: Canon EOS 5D (50mm F1.4)

As the graph shows the amount of fall-off is broadly the same for the Alpha 900 and EOS 5D when used with their respective 50mm F1.4 primes. Stopping down to around F2.8 removes pretty much all the visible corner shading.

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