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.
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 1DS Mark III you can expect fairly strong corner shading when shooting wide open, though how much varies considerably from lens to lens (the 16-35mm, for example, has some shading at 16mm even at F11. Perhaps unsurprisingly the Mark III has almost identical fall-off characteristics as the EOS 5D. We've included some thumbnails from other lenses further down the page.
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, and in most situations simply won't be an issue.
Lens: Canon 24-70mm F2.8 L EF
EF 24-70mm F2.8 L fall off thumbnails
Canon 50mm F1.4 EF Fall off thumbnails (F1.4-F2.5)
Canon EF 16-35mm F2.8 L II Fall off thumbnails
Compared to: Nikon D3
Just for interest here's a graph showing the fall-off characteristics of the 1DS Mark III and Nikon's D3 using similar pro-grade ultrawide zooms. There's not a huge difference (remember that the Nikon D3 is working with a shorter focal length here), though it's clear that - with these body/lens combinations at least - the Canon suffers from slightly stronger fall-off across the range.
East Coast Photo
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