posted on Mar 31, 2010 at 4:08AM
Head-2-Head Lens Review: Canon EF 85mm f/1.2L II USM vs. Canon EF 85mm f/1.8 USM
FalloffBy Ted Dillard
Falloff, again an unfortunate reality of any optical system, is the tendency for the frame to be unevenly exposed -- usually brighter in the middle of the lens -- with a gradual darkening towards the edges in all directions. Again, it’s more of an issue with wide-angle lenses in general because of the great difference in angular dispersion from the center of the lens outward. Typically, we’re also going to see the effects of falloff decrease as we stop the lens down from full open, simply because we’re using more of the “sweet spot” of the lens -- the center of the glass.
The f/1.2 shows this fairly dramatically, with the comparisons of the two contour maps from the EF 85mm f/1.8 lens above.
Compared to the performance of the f/1.8 lens shot at f/1.8, we’re seeing some very distinct differences that will be fairly obvious shooting any subject with an evenly toned background. You’ll notice the f/1.2, even at full-open f/1.2, has a more oval-shaped, less dramatic dropoff than the f/1.8 also at full-open. The EF 85mm f/1.8 has a pronounced circular pattern, a typical falloff from a lower-quality lens.
The following charts of the falloff performance echo this result.
These are just slightly different ways of looking at the same data. The f/1.2 lens, shot wide open, shows a fairly pronounced falloff, similar to the f/1.8. The f/1.2 shot at f/1.8, however, shows superior edge-to edge values.
Stopped down to f/7.1, the two lenses show dramatic improvements over their wide-open performance, yet the f1/.2 does still hold a slight edge.
We’re still seeing more falloff towards the edges, but now at a far-reduced scale (note the units of the RGBY levels, on the left axis- we’re seeing a variation of approximately .02, as opposed to the variations of .10 in the wide-open samples.)
For a lens used primarily for portraiture and some more general uses, this becomes an interesting detail. Often photographers will introduce a degree of vignetting, or darkening the edges of the frame to control composition and distractions from the subject. Depending on your tastes, the falloff of the f/1.8 may actually be desirable, especially with the lens at wide-open. If this isn’t your style, or if you’re shooting copy work or landscapes where you’re looking for even edge-to-edge tones, then the f/1.2 would be the choice.